PHYLUM PLATHYHELMINTHES. CLASS TREMATODA. CLASS CESTODA.PHYLUM NEMATHELMINTHES. CLASS NEMATHODA. LABORATORY DIAGNOSIS OF HELMINTIC INFECTION
Helminth Parasites
The word “worm” is used loosely to describe organisms with elongated bodies and a more or less creeping habit.
The word “Helminth” does mean “worm”, but in zoological terms it is more restricted to members of the phyla Platyhelminths, Nematoda and Acanthocephala.
There are three groups of medically important helminthes: Cestodes (tapeworms), Nematodes (roundworms) and Trematodes (flukes).
These parasites live in both the body spaces (gut lumen, bile ducts, lungs, oral cavity, etc.) and in tissues (blood, muscles and skin).
According to the way of development parasites are classificated into biohelminthes and geohelminthes.
Geohelminthes develop without intermediate hosts. Soil is the best environment for their egg’s development. Humans are infected through dirty fruits and vegetables which contain geohelminthe’s eggs (for example, Ascaris lumbricoideus).
Biohelminthes have complete life cycle with intermediate hosts. There are trophycal connections between definitive and intermediate hosts (for example, Taenia solium).
General characteristic of Flatworms (Phylum Plathelminthes).
The flatwotms (phylum Platyhelminthes) consists of some 12, 200 species. These ribbon-shaped, soft-bodied animals are also called because they are flattened dorsoventrally, from top to bottom. Flatworms are among the simplest of bilaterally symmetrical animals, but they have a definite head at the anterior end. Their bodies are solid: the only internal space consists of the digestive cavity. Flatworms are acoelomates (have no body cavity), usually hermaphroditic, and a digestive canal is present. Trematode eggs are excreted in the stool, urine, or sputum of the definitive host. All flukes require a mollusk as their first intermediate host. The larval stage that escapes from the mollusk may then enter a second intermediate host. The larval stage that escapes from the mollusk may then enter a second intermediate host (fish, crustacean), encyst on vegetation, or penetrate directly into the skin of the definitive host. Infection generally results from the ingestion of insufficiently cooked fish, crustaceans, and vegetation.
Phylum Plathelminthes includes classes:
1) Class Trematoda;
2) Class Cestoda.
General characteristic of Class Trematoda (Flukes):
1) Flattened dorsoventrally (leaf-like).
2) Unsegmented.
3) Body is covered by cuticle.
4) Organs of fixation: oral sucker, ventral sucker.
5) Organs and systems of Flatworms: digestive system, excretory system, nervous system, genital system: Trematodes are hermaphrodites except genus Schistosoma.
6) The life cycle is passed in two hosts (alternation of hosts) and has sexual and asexual stages.
Liver Flukes:
FASCIOLA HEPATICA – an agent of fascioliasis. It is a biliary (liver) fluke. Relatively common large fluke.
Distribution: endemic in Far East.
Localization: bile ducts, gallbladder, pancreas.
Morphology: large sizes (3-5 cm) and conical form of the body; possess sucking disks (oral and abdominal) that provide them motion. Multibranched uterus is situated under the abdominal sucking disk. Testis are branched too and situated in the middle part of the body;
Embryonated egg capsule of Fasciola hepatica.
Transmission: fecal-oral (ingestion of water, some non-water plants and vegetables, which contain adolescariae).
Invasive stage: adolescariae.
Clinical disease: Parasites obstruct bile ducts and lay eggs within them, leading to cholelithiasis (gallstones). Biliary obstruction can occur, sometimes causing biliary cirrhosis.
Life-cycle: Final host – herbivorous mammals (horses, pigs) and humans. Intermediate host — the snail Limnea truncatula. Eggs are excreted in feces of infected host. Egg hatches and forms a miracidium in water. It penetrates a snail host. In snail, after such stages as sporocyst and redia, free-swimming cercariaes are released to water. Cercariaes lose their tails and are covered by thick membrane and transform into adolescariae. They collect on watercress plants. Adolescariae are ingested by humans (final host), where they transform into juvenile and then — adult fluke.
Diagnosis: immature eggs in feces. An egg has large sizes, thin membrane, yellow color and small cover in one pole.
Prevention: involves not eating wild aquatic vegetables.
OPISTHORCHIS FELINEUS – small biliary fluke, causing Opisthorchiasis.
Distribution: Siberia.
Localization: bile ducts, gallbladder, liver.
Morphology: flat, the length of the body 4-13 mm. In the middle part of the body there is a branched uterus. Behind it there is a round ovary. There is a rosella-like testis in the back of the uterus – a diagnostic sign of the Opisthochis felineus.
Transmission: ingestion of contaminated raw, frozen, dried, pickled, and salted fish, which contains metacercariae.
Invasive stage: metacercariae cysts in fish muscles.
Clinical disease: cholecystitis and cholelithiasis, hepatic colic, cirhosis. Clinical picture is very similar to Clonorhis infection. Infection can lay dormant for several years before presenting clinically.
Life-cycle: Final host – carnivorous mammals and humans. First intermediate host – the snail Bithynia leachi genus, second – fish. Life cycle of Opisthochis felineus is the same as of Clonorshis sinensis.
Diagnosis: immature eggs in feces, in fluid from biliary drainage, or duodenal aspirate. Eggs are 15-30 mcm in sizes, have oval form and yellow color. The outer membrane is thick, and there is a cover in the front of the egg. The internal structure of the egg is microgranular.
Prevention involves not eating undercooked or contaminated raw, frozen, dried, pickled, and salted fish; eradication of snail hosts when possible.
DICROCOELIUM LANCEATUM – causes Dicrocoeliasis.
Distribution: worldwide.
Localization: bile ducts, gallbladder and liver of herbivorous mammals (cattle, horses). Very rare in humans.
Morphology: the worms are 1 centimeter long with lanceolate form of the body;
the intestine (gut) has two nonbranched channels which are situated in the lateral sides of the body. Two round testis are situated in the front of the body – the diagnostic sign of the Dicrocoelium lanceatum.
Transmission: ingestion of plants with the ants, which contain metacercariae.
Invasive stage: metacercariae.
Clinical disease: is similar to fascioliasis.
Life-cycle: Final host – herbivorous mammals (cattle, horses). First intermediate host — the snail of Zebrina and Helicela genus, second intermediate host — ants Fornica genus.
Diagnosis: immature eggs in feces. An egg have oval form, smooth membrane, brown color, a cover is present in the front end.
Prophylactics: eradication of the snails, ants when possible; dehelmithization of cattle.
Clonorchis sinensis
Clonorchis sinensis, also known as the Chinese liver fluke is a narrow elongate liver fluke found in the Far East, mainly Japan, Korea, China, Taiwan and Vietnam.
It belongs to the group of Oriental liver flukes where there are three main species, which commonly infect man.
Clonorchis sinensis
The other two species are Opisthorchis felineus and Opisthorchis viverrini. The three species are so similar in their morphology, life cycles and pathogenicity that they are very rarely discussed as separate species.
All members of this group are parasites of fish-eating mammals, particularly in Asia and Europe. Man is the definitive hosts and water snails and fish are the intermediate hosts. Infections can be easily avoided by maot eating raw fish since this is the only way that infection can be passed on.
Clonorchis sinenesis parasitise the biliary duct in humans who become infected by eating raw or undercooked fish. Dogs and cats are the most important reservoir hosts.
Definitive diagnosis is made by observing the characteristic ova in feces following concentration of feces or from duodenal aspirates when there is complete obstructive jaundice or from the Entero-Test.
Life cycle
Humans become infected by ingestion of insufficiently cooked crayfish or crabs containing metacecariae which excyst in the intestine, penetrate through the wall into the peritoneal cavity and make their way through the diaphragm and pleura into the lungs.
Morphology
The adult worm is an ovoid, reddish brown fluke about 12 mm long.
Clinical Signs of Disease
As the parasites grow in the lung cyst, inflammatory reaction and fever occurs. The cyst ruptures and cough develops resulting in the increase of sputum. The sputum is frequently blood tinged and may contaiumerous dark brown eggs and Charcot-Leyden crystals. Haemoptisis may occur after paroxysms of coughing. Dyspnoea and bronchitis develop with time. Bronchiectasis may occur and pleural effusion is sometimes seen. The disease resembles pulmonary tuberculosis. Cerebral calcification may also occur.
The Schistosomes
The Schistosomes are blood trematodes belonging to the Phylum Platyhelmintha. They differ from other trematodes in that they have separate sexes. The male worms resemble a rolled leaf where they bear the longer and more slender female in a ventral canal (the gynaecophoric canal). They require definitive and intermediate hosts to complete their life cycle. There are 5 species of Schistosomes responsible for human disease; S. mansoni, S. haematobium and S. japonicum with S. mekongi and S. intercalatum being less common.
They are the only trematodes that live in the blood stream of warm-blooded hosts. The blood stream is rich in glucose and amino acids, so along with the plasma and blood cells, it represents the environment, suitable for egg producing trematodes.
Over 200 million people are infected over at least 75 countries with 500 million or more people exposed to infection. Improved water supplies promote spread of disease, forming potentially new habits for snails.
The disease caused is called schistosomiasis or Bilharzia and is the most important of helminthic diseases.
Infection by the three most common species is the same in both sexes and in all age groups. Though, S. mansoni and S. haematobium is seen to occur more often and most heavily in teenagers especially males.
Life Cycle
Adult worms of S. mansoni live in the plexus of veins draining the rectum and colon, and in branches of the portal vein in the liver.
Adults of S. japonicum live in the anterior mesenteric blood vessels and in the portal vein in the liver. Whilst the adults of S. haematobium live in the vesical plexus draining the bladder.
Once the eggs are laid by the adult female worms, the majority of them first pass through the veins of the blood vessel in which the worm is living, and then into the lumen of the intestine and pass in feces (S. mansoni and S. japonicum), or into the lumen of the bladder, and are then excreted in the urine (S. haematobium). Those eggs that reach fresh water hatch, releasing a miracidium, which must infect a snail of the correct species within 24 hours. The eggs of each species are markedly different but each produce virtually identical miracidium.
Asexual multiplication takes place in the snail, and results in the release of cercariae (minute in size with forked tails, 200mm long) into the water about 3 – 6 weeks later. Cercariae actively swim around and when they have located, or come into contact with a definitive host, they actively penetrate the skin. They can stay active looking for a host for 24 – 48 hours after which, if they don’t find a host, they will die.
The head of the cercariae migrates to the liver and develops into either adult male or female worms (flukes), here they pair up and then migrate to their region of the venous blood system (species specific sites). The females leave the males and move to smaller venules closer to the lumen of the intestine or bladder to lay their eggs (about 6 weeks after infection). The majority of adult worms live from 2 – 4 years, but some can live considerably longer.
Schistosoma mansoni
Introduction
S. mansoni occurs in West and Central Africa, Egypt, Malaysia, the Arabian Peninsula, Brazil, Surinam, Venezuela and the West Indies. The intermediate host is an aquatic snail of the geuns Biomphalaria. Man is the most common definitive host; occasionally baboons and rats are infected.
The adult worms live in smaller branches of the inferior mesenteric vein in the lower colon.
Morphology
The adult males measure up to 15 millimetres in length and females up to 10 mm. The schistosomes remain in copula throughout their life span, the uxorious male surrounding the female with his gynaecophoric canal. The male is actually flat but the sides roll up forming the groove. The cuticle of the male is covered with minute papillae. The female only posses these at the anterior and posterior end as the middle section being covered by the male body. Oral and ventral suckers are present, with the ventral one being lager serving to hold the worms in place, preventing them being carried away by the circulatory flow.
The ova of S. mansoni are 114-175mm long and 45-68mm wide. They are light yellowish brown, elongate and possess a lateral spine. The shell is acid fast when stained with modified Ziehl-Neelsen Stain.
A non-viable egg is dark coloured and shows no internal structural detail or flame cell movement. Eggs can become calcified after treatment and are usually smaller, appear black and often distorted with a less distinct spine.
The schistosomes differ from other trematodes in that they are dioecious, digenetic, their eggs are not operculate and infection is acquired by penetration of cercaria through the skin.
Clinical Signs of Disease
The clinical sigs of disease are related to the stage of infection, previous host exposure, worm burden and host response. Cercarial dermatitis (swimmers itch) follows skin penetration and results in a maculopapular rash, which may last for 36 hours or more.
After mating, the mature flukes migrate to the venules draining the large intestine. Their eggs are laid and they penetrate the intestinal wall. They are then excreted in feces, often accompanied by blood and mucus.
It is the eggs and not the adult worms, which are responsible for the pathology, associated with S. mansoni infections. The adult flukes acquire host antigen, which protects them from the host’s immune response.
The host’s reaction to the eggs, which are lodged in the intestinal mucosa, leads to the formation of granulomata and ulceration of the intestinal wall. Some of the eggs reach the liver via the portal vein. The granulomatous response to these eggs can result in the enlargement of the liver with fibrosis, ultimately leading to portal hypertension and ascites. The spleen may also become enlarged. Other complications may arise as a result of deposition of the eggs in other organs e.g. lungs.
Katayama fever is associated with heavy primary infection and egg production. Clinical features include high fever, hepatosplenomegaly, lymphadenopathy, eosinophilia and dysentery. This syndrome occurs a few weeks after primary infection.
Laboratory Diagnosis. Microscopy
Laboratory confirmation of S. mansoni infection can be made by finding the eggs in feces. When eggs cannot be found in feces rectal biopsy can be indicated.
Schistosoma mansoni eggs
Serology
Serological tests are of value in the diagnosis of schistosomiasis when eggs cannot be found. An enzyme linked immunosorbent assay (ELISA) using soluble egg antigen, is employed at HTD.
Schistosoma japonicum
Schistosoma japonicum is found in China, Japan, the Philippines and Indonesia. It causes disease of the bowel with the eggs being passed out in feces.
It differs form S. mansoni and S. haematobium that is called zoonosis, in which a large number of mammals serve as reservoir hosts, cats, dogs and cattle play major roles in the transmission of the disease.
The life cycle is not very different from that of S. mansoni, the intermediate hosts are from the subspecies Oncomelania hupensis. Sexual maturity is reached in about 4 weeks and eggs may be seen in the feces as quickly as 5 weeks after.
They worms live coupled together in the superior, mesenteric veins and desposit 1500 – 3500 eggs per day in the vessels of the intestinal wall. The eggs infiltrate through the tissues and are passed in feces.
Morphology
The adult worms are longer and narrower than the S. mansoni worms. The ova are about 55 – 85mm and 40 – 60mm, oval with a minute lateral spine or knob.
Clinical Signs of Disease
The main lesions appear due to the presence of eggs, occurring in the intestine and liver. The eggs which are sequesters in the intestine mucosa or submucosa initiate granulomatous reactions, resulting in the formation of pseudotubercules.
Due to the number of eggs released by the females the infection is more severe than one with S. mansoni. This is also happens due to the better adaptation of the parasite to man, therefore, the circumoval granuloma is very large. The initial illness can be prolonged and sometimes fatal.
Laboratory diagnosis. Microscopy
Laboratory confirmation of S. japonicum infection can be made by finding the eggs in feces. When eggs cannot be found in feces rectal biopsy can be indicated.
Schistosoma japonicum egg
Other Schistosome species, which are responsible for human disease, are
S. mekongi and S. intercalatum. These two species cause similar symptoms to that of S. mansoni and can be summarised in the table.
Schistosoma haematobium
Schistosoma haematobium differs from the other two species previously mentioned and that it causes urinary schistosomiasis. It occurs in Africa, India and the Middle East. The intermediate host is the Bulinus snail.
Just like S. mansoni, its distribution runs parallel to the irrigation projects and in areas, which favour the intermediate hosts. They are exclusively parasites of man.
The mature worms live in copula mainly in the inferior mesenteric veins and the females deposit their eggs in the walls of the bladder and finally making their way into the urine. The life cycle is very similar to that of S. mansoni, with sexual maturity being reached within 4 – 5 weeks, but eggs may not appear in the urine until 10 – 12 weeks or even later.
Morphology
The adult worms are longer than those of S. mansoni. The ova are relatively large, measuring 110mm – 170mm in length and 40mm – 70mm in width. They have an elongated ellipsoid shape with a prominent terminal spine.
Schistosoma haematobium egg
Clinical Signs of Disease
The clinical signs of disease are related to the stage of infection, previous host exposure, worm burden and host response. Cercarial dermatitis (Swimmer’s Itch) following skin penetration results in a maculo-papular rash and can last for36 hours or more. The mature flukes of S. haematobium migrate to the veins surrounding the bladder. After mating, the eggs are laid in the venules of the bladder and many penetrate through the mucosa, enter the lumen of the bladder and are excreted in the urine accompanied by blood. Thus, haematuria and proteinuria are characteristic, though not invariable features of urinary schistosomiasis.
As with all Schistosoma species, it is the eggs and not the adult worms which are responsible for the pathology associated with S. haematobium. In chronic disease, eggs become trapped in the bladder wall resulting in the formation of granulomata. Following prolonged infection, the ureters may become obstructed and the bladder becomes thickened resulting in abnormal bladder function, urinary infection and kidney damage. Chronic urinary schistosomiasis is associated with squamous cell bladder cancer. Heavy infections in males may involve the penis resulting in scrotal lymphatics being blocked by the eggs.
Laboratory diagnosis
The definitive diagnosis of urinary schistosomiasis is made by finding the characteristic ova of S. haematobium in urine. Terminal urine should be collected as the terminal drops contain a large proportion of the eggs. The urine can either be centrifuged and the deposit examined microscopically for ova. Eggs can sometimes be found in seminal fluid in males.
A bladder biopsy is seldom necessary to make the diagnosis. A rectal snip may show the presence of ova as they sometimes pass into the rectal mucosa.
Serological tests can be of value when eggs cannot be found in clinical samples. An enzyme linked immunosorbent assay using soluble egg antigen to detect antischistosome antibody is most sensitive.
There is a marked periodicity associated with the time when most eggs are passed out. Higher numbers of eggs are encountered in urine specimens passed between 10 am and 2 pm, presumably as a result of changes in the host’s metabolic and physical activities.
Prevention: involves proper disposal of human waste and eradication of the snail host when possible. Swimming in endemic areas should be avoided.
Tapeworms (Cestoda)
Tapeworms (Cestoda) consist of a rounded head, called a scolex, neck and long strobila or chain of proglottids (multiple segments) of varying stages of maturity.
The scolex has specialized means of attaching to the intestinal wall, namely suckers, hooks, or sucking grooves.
The worm grows by adding new proglottids from its germinal center next to the scolex. The oldest proglottids at the distal end are gravid and produce many eggs, which are excreted in the feces and transmitted to various intermediate hosts. Humans usually acquire the infection when undercooked flesh containing the larvae is ingested. In certain instances, eg, cysticercosis and hydatid disease, the eggs are ingested and the resulting larvae cause the disease. All cestodes have stage of larva and stage of oncosphere in the life cycle.
Adult Parasites of class Cestoda are hermaphroditic and live in the intestinal lumen of vertebrate hosts. Those that can infect man include Diphyllobothrium latum, Taenia saginata, Taenia solium, Hymenolepis nana.
Larval stages of Echinococcus granulosus and Taenia solium (hydatid, cysticercus) may be found in various organs and tissues of man and other intermediate hosts.
The cestodes (or tapeworms) form a group of worms, exhibiting two unmistakable morphological features; they all possess flat, ribbon like bodies and lack an alimentary canal.
Adult tapeworms usually inhabit the alimentary canal of their hosts (though they occasionally are found in the bile or pancreatic ducts) and attach themselves to the mucosa by means of a scolex. Despite the lack of a digestive system they do absorb food from the hosts intestine; thereby providing the tapeworms a habitat that is associated with high nutritional levels, feeding the tapeworms high growth rate.
Larvae on the other hand show a wide range of habitat preferences, being found in almost any organ of both vertebrate and invertebrate hosts. Though most larval species show a preference for a particular site.
This lack of an alimentary canal markedly separates tapeworms from nematodes and trematodes. The outer tegument of the body must serve not only as a protective coating but also as a metabolically active layer through which nutritive material can be absorbed, along with secretions and waste material to be transported out of the body.
The body consists of a chain of segments or proglottids, which can be immature, mature or gravid; the latter of which contain a fully developed uterus packed with eggs. Therefore, each tapeworm is made up of a ‘string of individuals’ having a complete set of reproductive organs in progressive degrees of sexual maturity and budding off from a body attached to the host tissue by a head or scolex.
Except for Hymenolepis nana, which can develop directly in the same host, the lifecycle of tapeworms involves both an intermediate and definitive host.
Taenia species
Taenia species are the most common cestode parasites of humans. More than 60 million people are infected with T. saginata (“beef” tapeworm) world wide and about 4 million are infected with T. solium (“pork” tapeworm).
T. saginata has a comsmopolitan distribution, but is more common in developing countries where hygiene is poor and the inhabitants have a tendency of eating raw or insufficiently cooked meat. T saginata is the most common adult tapeworm found in man. T solium is virtually extinct in Europe and the USA. \
The adults of both species live in the small intestine of man, the definitive host The gravid segments are very active and escape through the anus, releasing large numbers of eggs in the perianal region or on the ground where they can survive for long periods. When ingested by pigs or catle, the eggs hatch, each releasing an oncosphere, which migrates through the intestinal wall and blood vessels to reach striated muscle within which it encysts, forming cysticerci.
When inadequately cooked meat containing the cysts is eaten by man, the oncospheres excyst, settle in the small intestine and develop there into adult cestodes over the next 3 months or so. The segments of T. solium are somewhat less active than those of the beef tapeworm but its eggs, if released in the upper intestine, can invade the host (auto-infection), setting up the potentially dangerous larval infection known as cysticercosis in muscle of any other site.
Both humans and cattle or pigs are necessary to the complete life cycle of Taenia species. (In Europe and the USA cattle are the normal intermediate hosts, but in the tropics several other ruminants, e.g. goat, sheep llama and giraffe, may serve as the intermediate hosts.) Eggs ingested by the intermediate hosts usually contain oncospheres. The oncospheres then hatch out in the duodenum, pass into the intestine where they penetrate the intestinal wall and are then carried by the circulation to be deposited in tissues (usually muscle). There they develop into cysticerci larva, which are white and ovoid, measuring approximately 8 x 5mm.
Humans become infected by ingesting inadequately, cooked beef or pork with cysticerci, containing an invaginated protoscolex.
The protoscolexes evaginate and pass into the small intestine where they attach themselves to the mucosa and develop into adult worms.
Eggs and proglottids are passed out in the faeces, and are then eaten by the intermediate host, thus, perpetuating the life cycle.
Larval Cestodes, which Infect Man
Infections in man with Echinococcus granulosus, Echinococcus multilocularis and Multiceps multiceps are caused by an accidental ingestion of eggs, which are excreted by the definitive animal host. The disease that developes due to the invasion of these parasites is caused by the larval stages or hydatid cyst, and is known as hydatid disease or hydatidosis.
Each cestode possesses an elongated tape-like body, which lacks an alimentary canal. The adult tapeworms are strings of individuals having a complete set of reproductive organs (proglottids) in progressive degrees of sexual maturity and budding off from a body attached to the host tissue by a head or scolex.
The larval stage, show a wide variation being found in almost any organ of both vertebrate and invertebrate hosts.
Echinococcus granulosus
Echinococcosis or Hydatid disease in man is caused by the larval stage of the dog tapeworm, Echinococcus granulosus. Hydatid disease is most extensively found in East Africa, North Africa, South Africa, the Middle East and parts of South America and Australia. The intermediate hosts are cattle, sheep, pigs, goats or camels and the definitive host for this disease is the dog or other canids.
Life cycle of the cestode
Larval infection in man causes hydatid disease.
Adult worms are only seen in the definitive hosts, dogs, they cannot develop in man. Man is an accidental intermediate host of hydatid disease. When the ova are ingested by a suitable intermediate host, they hatch in the duodenum and the oncosphere migrates to the blood stream where it is carried to the liver, lungs and other organs of the body. Here it develops into a hydatid cyst, which consists of an outer thick laminated cyst wall, and an inner, thiucleated germinal layer. From the inner layer brood capsules are produced which contain protoscoleces. The brood capsules detach from the germinal layer, releasing free protoscoleces. Hydatid sand is the name given to the fluid in the cysts, which consists of protoscoleces, tissue debris, and sometimes free hooklets. Here, the life cycle stops in humans, but is continued when a hydatid cyst containing protoscoleces egg in sheep liver, is ingested by a suitable canine host where the protoscoleces develop into adult worms.
Morphology
The adult worm measures approximately 3 – 8.5 mm long. The scolex has 4 suckers and a rostellum with hooks, the latter becoming tightly inserted into the crypts of Lieberkühn. The mature strobila has only 3 – 4 proglottids, one is immature, one is mature and the final one is gravid; when gravid the eggs are expelled in the faeces.
Due to the close similarity of the eggs to other Taenia species found in dogs they were until recently thought to be morphologically indistinguishable.
The larvae in man develop into a unilocular cyst which gives rise to unilocular hydatid disease.
This is characterised as having only one bladder or many completely isolated bladders, each enclosed in its own well-developed envelope. The latter consists of several layers, the most prominent being the laminated layer. Within this again is the germinal membrane from which the brood capsules arise inside and develop thousands of larvae or protoscoleces, the whole being suspended in a hydatid fluid.
Clinical Signs of Disease
Hydatid disease in humans is potentially dangerous depending on the location of the cyst. During first period of disease is asymptomatic, but liver cysts may cause hepatic dysfunction. Some cysts may remain undetected for many years until they become large enough to affect other organs. Symptoms are then of a space occupying lesion. Lung cysts are usually asymptomatic until there is a cough, shortness of breath or chest pain. Hepatic cysts result in pressure on the major bile ducts or blood vessels. Expanding hydatid cysts cause necrosis of the surrounding tissue.
Slow leakage of the hydatid fluid results in eosinophilia and rupture of an abdominal hydatid cyst results in severe allergic symptoms.
Symptoms may not manifest themselves for 5 – 20 years after the infection.
Cysts in the lungs can erode into a bronchus, causing bloody sputum; end cerebral cysts can cause headache and focal neurologic sings. If the cysts rupture spontaneously or during trauma or surgical removal, life-threatening anaphylaxis can occur (the cyst fluid contains parasite antigens, which can sensitize the host).
Laboratory Diagnosis
1. Imaging and serodiagnosis are the mainstay of diagnosis. Serological tests include Enzyme linked immunosorbent assay (ELISA), an indirect haemagglutination test a complement fixation test and a Western Blot system.
2. Microscopic examination of the cyst fluid to look for the characteristic protoscoleces, which can be either invaginated or evaginated. The cyst fluid will also reveal free hooklets and tissue debris. 1% eosin may be added to the fluid to determine the viability of the protoscoleces. Viable protoscoleces exclude eosin whereas nonviable protoscoleces take up the eosin.
3. Histological examination of the cyst wall after surgical removal.
Western Blots
One serological test, which is proved to be of value to diagnosing Hydatid disease, is the Western Blot. The test presents a definitive means for detection of human antibodies to the cestode E. granulosus.
Diagnosis can be made using the Western Blot assay for the detection of IgG antibodies in serum reactive with E. granulosus antigens present on a membrane. Field studies support a sensitivity of 80% and specificity of 100% in patients with hepatic cysts.
This assay is known as the Qualicode Hydatid Disease Kit, the principle behind the test is that it is a qualitative membrane-based immunoassay manufactured from E. granulosus proteins. The E. granulosus proteins are fractionated according to molecular weight by electrophoresis on a ployacrylamide slab gel (PAGE) in the presence of sodium dodecyl sulfate (SDS). The separated E. granulosus proteins are then transferred via electrophoretic blotting from the gel into strips for testing of individual samples.
During the procedure, the strips containing the E. granulosus proteins are incubated with serum specimens and washed to remove unbound antibodies.
Visualisation of human immunoglobulins specifically bound to E. granulosus proteins is performed by sequential reaction with goat anti-human immunoglobulin-alkaline phosphatase conjugate and BCIP/NBT substrate. Bands corresponding to the positions of the resoled E. granulosus proteins will be visualized on the strip, indicating the presence in the serum sample of IgG antibodies directed against
E. granulosus antigens. Band positions are compared to those on a reference strip developed using Hydatid disease positive control.
Prevention
1. Safe disposal of dog feces.
2. Education to prevent feeding uncooked offal to dogs.
Echinococcus multilocularis
The larvae of Echinococcus multilocularis is a particularly dangerous species causing multilocular (alveolar) hydatid disease in man and animals and is common in the highlands of Europe i.e. Switzerland and Germany, in Canada, Alaska and Northern Russia. The most common definitive hosts are foxes and wolves in addition to domestic cats and dogs when they have access to infected rodents.
Life cycle
Foxes are the primary definitive hosts, although, under domestic circumstances dogs can act as the definitive host. Rodents are the intermediate hosts. Man is an accidental host by the ingestion of eggs where multilocular cysts are formed. In these cysts, the limiting membrane is thin, and the germinal epithelium may bud off externally resulting in proliferation in any direction.
Metastases may occur. Unlike E. granulosus, there is little fluid in the cysts of E. multilocularis.
Morphology
The morphology is in general very similar to that of E. granulosus, but the adults are much smaller.
Unlike E. granulosus, cysts of E. multilocularis in man do not contain daughter cysts with scolices. Instead, the larval cyst, or as it is referred to as an alveolar or multilocular hydatid cyst, forms a multicystic structure made up of proliferating vesicles embedded in a dense fibrous stroma, which is often mistaken for a hepatic sarcoma. In older cysts the hydatid fluid is replaced by a jelly-like mass.
Clinical Signs of Disease
Cysts are formed primarily in the liver and their growth in the vena cava or portal vein results in metastases in the lung or brain. Clinical signs of disease are similar to that of E. granulosus.
Diagnosis
1. Laboratory diagnosis can be made by ELISA.
2. Clinical diagnosis is made by ultrasound.
Differences between the hydatid cysts of E. granulosus and E. multilocularis
|
Echinococcus granulosus |
Echinococcus multilocularis |
|
Slow development of cyst |
Rapid development of cyst |
|
Cysts have thick-walled chambers |
Cyst has thin-walled chambers |
|
Separated by connective tissue |
Not separated by connective tissue |
|
Cyst is fluid filled |
Cyst is gelatinous filled |
|
Cyst is free of host material |
Cyst is contaminated by host material |
The Cestodes
The cestodes (or tapeworms) form a group of worms, exhibiting two unmistakable morphological features; they all possess flat, ribbon-like bodies and lack an alimentary canal.
Adult tapeworms usually inhabit the alimentary canal of their hosts (though they occasionally are found in the bile or pancreatic ducts) and attach themselves to the mucosa by means of a scolex. Despite the lack of the digestive system they do absorb food from the hosts intestine; thereby providing the tapeworms a habitat that is associated with high nutritional levels causing the tapeworms high growth rate.
Larvae on the other hand show a wide range of habitat preferences, being found in almost any organ of both vertebrate and invertebrate hosts. Though, most larval species show a preference for a particular site.
This lack of the alimentary canal markedly separates tapeworms from nematodes and trematodes. The outer tegument of the body must serve not only as a protective coating but also as a metabolically active layer through which nutritive material can be absorbed, along with secretions and waste material to be transported out of the body.
The body consists of a chain of segments or proglottids, which can be immature, mature or gravid; the latter contains a fully developed uterus packed with eggs. Therefore, each tapeworm is made up of a ‘string of individuals’ having a complete set of reproductive organs in progressive degrees of sexual maturity and budding off from a body attached to the host tissue by a head or scolex.
Except for Hymenolepis nana, which can develop directly in the same host, the lifecycle of tapeworms involves both an intermediate and definitive host.
Taenia species
Taenia species are the most common cestode parasites of humans. More than 60 million people are infected with T. saginata (“beef” tapeworm) world-wide and about 4 million are infected with T. solium (“pork” tapeworm). T. saginata has a comsmopolitan distribution, but is more common in developing countries where hygiene is poor and the inhabitants have a tendency of eating raw or insufficiently cooked meat. T saginata is the most common adult tapeworm found in man. T solium is virtually extinct in Europe and the USA.
The adults of both species live in the small intestine of man, the definitive host. The gravid segments are very active and escape through the anus, releasing large numbers of eggs in the perianal region or on the ground where they can survive for long periods. When ingested by pigs or cattle, the eggs hatch, each releasing an oncosphere, which migrates through the intestinal wall and blood vessels to reach striated muscle within which it encysts, forming cysticerci. When inadequately cooked meat containing the cysts is eaten by man, the oncospheres excyst, settle in the small intestine and develop there into adult cestodes over the next 3 months or so. The segments of T. solium are somewhat less active than those of the beef tapeworm but its eggs, if released in the upper intestine, can invade the host (auto-infection), setting up the potentially dangerous larval infection known as cysticercosis in muscle of any other site. (Peters & Gilles, 1995)
Both humans and cattle or pigs are necessary to the complete life cycle of Taenia species. (In Europe and the USA cattle are the normal intermediate hosts, but in the tropics several other ruminants, e.g. goat, sheep, llama and giraffe, may serve as the intermediate hosts.) Eggs ingested by the intermediate hosts usually contain oncospheres. The oncospheres then hatch out in the duodenum, pass into the intestine where they penetrate the intestinal wall and are then carried by the circulation to be deposited in tissues (usually muscle). There they develop into cysticerci larva, which are white and ovoid, measuring approximately 8 x 5mm.
Humans become infected by ingesting inadequately cooked beef or pork with cysticerci, containing an invaginated protoscolex. The protoscolexes evaginate and pass into the small intestine where they attach themselves to the mucosa and develop into adult worms.
Eggs and proglottids are passed out in feces, and are then eaten by the intermediate host, thus, perpetuating the life cycle.
Morphology
Ova of Taenia species are spherical, yellowish brown and measure 31 – 34mm in diameter. The shell is thick and radially striated. Within the shell, the onchosphere has 3 pairs of hooklets. However, the microscopical appearance of the ova of T. saginata and T. solium are identical.
The length of the adult T. saginata is 4 – 8 meters long and that of T. solium is 3 – 5metres long. The proglottids of Taenia species can be identified by the number of uterine branches; 7 – 13 for T. solium and 15 – 20 for T. saginata. If the scolex is recovered, the 4 suckers and rostellum of hooklets of T. solium will distinguish it from T. saginata, which has 4 suckers but no hooklets.
Clinical Signs of Disease
The presence of the adult worm rarely causes symptoms apart from slight abdominal irritation with diarrhoea, constipation or indigestion. The accidental ingestion of the embryonated ova of T. solium may result in cysticercosis in man. An infection due to an adult Taenia, in man or animals, is referred to as taeniasis.
T. saginata (the “beef” tapeworm) does not cause human cysticercosis.
When the embryonated eggs are ingested, the embryos hatch out, migrate through the intestinal wall and are carried around the body in the circulation and deposited in various tissues. Muscle and subcutaneous tissues are usually infected, but cysticerci can infect most organs and tissues. Human cysticercosis is usually asymptomatic unless the infection is particularly heavy or cysticerci are formed in some vital area e.g. the brain, resulting in neurological sequelae.
Comparison of T.saginata and T.solium
|
Characteristic |
Taenia saginata |
Taenia solium |
Intermediate host |
Cattle, reindeer |
Pig, wild boar |
|
Site of development |
Muscle, viscera |
Brain, skin, muscle |
|
Scolex: adult worm |
No hooks |
Hooks |
|
Scolex: cysticercus |
No rostellum |
Rostellum & hooks |
|
Proglottis: uterine branches |
23 (14 – 32) * |
8 (7 –11) * |
|
Passing of proglottids |
Single, spontaneous |
In groups, passively |
|
Ovary |
2 lobes |
3 lobes |
|
Vagina: sphincter muscle |
Present |
Absent |
* No universal agreement to the number of uterine branches in these 2 species. As a rough guide, specimens with more than 16 branches are likely to be those of T. saginata and those with less than 10 branches are ikely to be of T. solium.
Laboratory diagnosis
Diagnosis of intestinal taeniasis can be made by recovery of the characteristic ova in stool. However, the ova of T. solium and T. saginata are identical and diagnosis is made by the recovery of the segments or scolex.
The diagnosis of cysticercosis depends upon serology. MRI scans may reveal the presence of lesions in the brain. Calcified cysticerci are less often seen in the brain: in about one-third of cases, 10 years or more after infection. Occasionally, the diagnosis is made histologically on surgical specimens. Calcification in muscles usually appears 3 – 5 years after initial infection, and are most typically seen as spindle-shaped calcifications, most numerous in the thighs.
Hymenolepis nana
Hymenolepis nana, the dwarf tapeworm, is the smallest tapeworm to infect humans. This cestode belongs to a large family known as Hymenolepididae.
The diagnostic features of this family are: scolex armed with one circlet of five hooks; 1 – 3 large testes and sacciform uterus. In addition to the H.nana, three other species, H. diminuta, H. microstoma and H. citelli have been used extensively for studies on cestodes.
Hymenolepis nana has a cosmopolitan distribution and is thought to be the most common tapeworm throughout the world. The infection is more frequently seen in children although adults are also infected, causing hymenolepiasis.
Life cycle
The life cycle of H. nana does not require an intermediate host; complete development occurs within the villi of a single host, resulting in a ‘direct’ life cycle. Though it can also utilise an insect as an intermediate host.
The eggs that are released from mature proglottids in the upper ileum are usually passed out in feces. If swallowed by another human they develop into hexacanth oncospheres and burrow into the villi of the small intestine.
This is where they develop into tailless cysticercoids and then migrate towards the ileum and attach thear for proglottids formation. The eggs, which are ingested by insects, such as fleas, beetles or cockroaches hatch to form tailed cysticercoids, which remain unmodified as long as they are inside the insect. If they are accidentally swallowed by a human they pass down to the ileum and establish themselves. (Peters & Gilles, 1995)
Morphology
The egg containing the oncosphere bears three pairs of hooklets and is surrounded by a membrane. This membrane has 2 polar thickenings from which arise threadlike filaments extending into the space between the membrane and the colourless hyaline shell, unlike those of H. diminuta, which do not possess any filaments.
The adult tapeworm is normally 2.5 – 4cm long. The scolex is knob like in shape, has a rostellum with hooklets and 4 suckers. The segments are wider than they are long. Ova are spherical or ovoid measuring 30 – 47mm in diameter. This is what distinguishes it morphologically from H. diminuta.
Clinical Signs of Disease
Infections due to H. nana may cause no symptoms even with heavy worm burdens. However, symptoms of restlessness, irritability, anorexia, abdominal pain and diarrhoea have been reported. Heavy worm burdens may be caused by autoinfection, which can be a problem in the immunocompromised.
Laboratory Diagnosis
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Diagnosis is based on recovery and identification of the characteristic ova in a formol-ether concentrate of feces. Adult worms and proglottids are rarely seen in stool samples.
Diphyllobothrium latum
Members of this order, commonly known as pseudophyllids, are chiefly parasites of fish-eating mammals, birds and fish. They typically are found with a scolex which is characterised by two shallow elongated bothria situated with one dorsally and one ventrally. The proglottids are flattened dorsoventrally.
Diphyllobothrium latum is an intestinal tapeworm, known as the human ‘broad’ tapeworm. It is the largest tapeworm found in man. The term ‘broad’ relates to the fact that the proglottids are generally wider than longer. It is an important human parasite. The adult worms of two other species of the genus, D. dendriticum and D. ditremum are chiefly parasite of fish-eating birds and mammals.
The tapeworm, D. latum has a wide distribution, occurring especially in countries bordering on the Baltic Sea (Finland, Sweden etc.): and also in Russia, Switzerland and North America. It is in these countries where the populations are known to eat uncooked or partly cooked (i.e. smoked) fish.
Apart from man they are found in many other hosts, especially the dog, cat and pig. This is due to the host countries allowing the domestic animals access to the offal from the infected fish.
The life cycle of this tapeworm requires two intermediate hosts.
The eggs are passed out in human faeces, once in water they hatch out into small ciliates coracidium larvae, which swim until ingested by Copepods. It is in these intermediate hosts that growth and development of the 1st larval stage are completed (they are now known as procercoids). When these crustaceans (fresh water) are eaten by fish, the procercoid larvae continue to develop in the flesh of fish and become known as plerocercoid larvae. It is this stage of the larvae, which develops in man when they eat undercooked fish and they grow into adult worms in the small intestine.
Morphology
The egg is usually ovoid and has a small knob at the opercular end and is yellowish-brown in colour with a smooth shell, of moderate thickness. They measure 58 – 75mm by 40 – 50mm in size.
Adult worms can reach up to a length of 10 metres or more and may contain up to 3,000 proglottids. The scolex is spatulate with no rostellum or hooklets. It has 2 shallow grooves or bothria, which are unlike the typical 4 suckers seen on the Taenia species. The proglottids measure 3mm long and 11mm wide and have a rosette shaped central uterus.
Clinical Signs of Disease
The infection caused by D. latum is due to the ingestion of raw, poorly cooked or pickled fresh water fish. The symptoms associated with D. latum infection may be absent or minimal with eosinophilia. There may be occasional intestinal obstruction, diarrhoea, and abdominal pain. The most serious symptom is the onset of pernicious anaemia. This is due to vitamin B12 deficiency, caused by excessive absorption of the vitamin by the adult worm and the absorption of cobalamins from the host intestine (occurring only in a small percentage of people).
Laboratory diagnosis
Laboratory diagnosis depends on the recovery of characteristic eggs from a formol ether concentrate of feces. Proglottids may also be seen in fecal samples usually in a chain of segments from a few centimeters to about 0.5 meters in length.
Taenia solium
The adult form of T. solium causes taeniasis solium. T. solium larvae cause cysticercosis. Teniasis and cysticercosis occur worldwide but is endemic in areas of Asia, South America, and Eastern Europe.
Morphology. T. solium can be identified by its scolex with 4 suckers and circle of hooks and by its gravid proglottids, which have 7-12 primary uterine branches. Larva of T.solium called cysticercus. A cysticercus consists of a pea-sized fluid-filled bladder with an invaginated scolex.
T. solium cysticercus.
T. solium scolex
T. solium scolex
Life cycle. Humans are infected by eating raw or undercooked pork containing the larvae (fecal-oral way of transmittion).
In the small intestine, the larvae attach to the gut wall and take about 3 months to grow into adult worms measuring up to 5 m. The gravid terminal proglottids detach daily, are passed in the feces, and are accidentally eaten by pigs. A 6-hooked embryo (oncosphere) emerges from each egg in the pig’s intestine. The embryos burrow into a blood vessel and are carried to skeletal muscle. They develop into cysticerci in the muscle, where they remain until eaten by a human. Humans are the definitive hosts, and pigs are the intermediate hosts.
A different and far more dangerous sequence occurs when a person ingests the eggs in food or water that has been fecally contaminated or ingestion of T. solium eggs after vomiting (in patient with taeniasis). The cysticerci can become large in eye, subcutaneous tissue, brain, lung, heart, and muscle. In the brain, they manifest as a space-occupying lesion. Living cysticerci do not cause inflammation, but when they die they can release substances that provoke an inflammatory response.
Clinical manifestation of teaniasis solium: abdominal pain, nausea, diarrhea, weight loss, infection may by asymptomatic.
Clinical manifestation of cysticercosis in the brain: headache, meningoencephalitis, vomiting, and seizures; in the eyes: uveitis, retinitis.
Diagnosis of teaniasis solium: gravid proglottids (with 7 – 12 uterine branches) may be found in the stools.
Diagnosis of cysticercosis depends on demonstrating the presence of the larvae of T.solium in tissue by x-ray, or computed tomography scan.
Prevention of taeniasis involves cooking pork adequately and preventing pigs from ingesting human feces by disposing of waste properly.
Prevention of cysticercosis consists of treatment of patients to prevent autoinfection plus observation of proper hygiene, including hand washing, to prevent contamination of food with the eggs.
Taenia saginata causes taeniasis saginata.
Taeniasis saginata occur worldwide but is endemic in areas of Asia, South America, and Eastern Europe.
Morphology. T. saginata can be indentified by its scolex with 4 suckers but in contrast to T.solium, no hooklets.
Its gravid proglottids have 17-35 primary uterine branches. Larva of T.saginata called cysticercus.
Life cycle. Humans are infected by eating raw or undercooked beef containing larvae (fecal-oral way of transmission). In the small intestine, the larvae attach to the gut wall and take about 3 months to grow into adult worms measuring up to 10 m. The gravid terminal proglottids detach, are passed in the feces, and are eaten by cattle.
Embryos (oncospheres) emerge from the eggs in the cow’s intestine and burrow into a blood vessel, where they are carried to skeletal muscle. They develop into cysticerci in the muscle. The cycle is completed when the cysticerci are ingested. Humans are the definitive hosts, and cattle the intermediate hosts.
Clinical manifestation of teniasis saginata: abdominal pain, nausea, diarrhea, weight loss, infection may by asymptomatic. In some, proglottids appear in the stools and may even protrude from the anus.
Laboratory diagnosis: gravid proglottids (with 17-35 uterine branches) may be found in the stools.
Prevention. Prevention of taeniasis saginata involves cooking beef adequately and preventing cattle from ingesting human feces by disposing of waste properly.
Diphyllobothrium latum, the fish tapeworm, causes diphyllobothriasis.
The disease is found worldwide but is endemic in areas where eating raw fish is the custon, such as Scandinavia, northern Russia, Japan, Canada, USA (northern Michigan, Minnesota).
Morphology. Diphyllobothrium latum can be indentified by its scolex with 2 elongated sucking grooves by which the worm attaches to the intestinal wall.
The proglottids are wider than they are long, and the gravid uterus is in the form of a rosette.
Scolex of D. latum
Adult D.latum is the longest of the tapeworms, measuring up to 13 m. Larva of D.latum called plerocercoid.
Life cycle.
Humans are infected by eating raw or undercooked fish containing larvae (fecal-oral way of transmittion). In the small intestine, the larvae attach to the gut wall and develop into adult worms. Gravid proglottids release fertilized eggs, and the eggs are then passed in the stools. The immature eggs must be deposited in fresh water for the life cycle to continue. The embryos emerge from the eggs and are eaten by tiny copepod crustacea (first intermediate hosts). There, the embryos differentiate and form procercoid larvae in the body cavity. When the copepod is eaten by freshwater fish, eg, pike, trout, and perch, the larvae differentiate into plerocercoids in the muscle of the fish (second intermediate host). The cycle is compelled when raw or undercooked fish is eaten by humans (definitive hosts).
Clinical disease: infection by D.latum causes little damage in the small intestine. In some individuals, megaloblastic anemia occurs as a result of vitamin B12 deficiency caused by preferential uptake of the vitamin by the worm. Most patients are asymptomatic, but abdominal discomfort and diarrhea can occur.
Diagnosis depends on finding the typical eggs, oval, yellow-brown eggs with an operculum (lidlike opening) at one end, in the stools.
Prevention involves adequate cooking of fish and proper disposal of human.
MEDICAL HELMINTHOLOGY: ROUNDWORMS – HUMAN PARASITES
Nematodes (also known as phylum Nemathelmintes) are roundworms with a cylindrical body and a complete digestive tract including mouth and anus. They are pseudocoelomate worms. The body is covered with a noncellular, highly resistant coating called a cuticle, which is molted as they grow.
Nematodes have separate sexes; the female is usually larger than the male. The male typically has a coiled tail.
The medically important nematodes can be divided into 2 categories according to their primary location in the human body, namely intestinal and tissue nematodes.
Nematodes (or “round worms”) are non-segmented helminths known as make up a large assemblage of relatively simple structured organisms. They possess bilateral symmetry and a complete digestive tract with oral and anal openings; they taper to a relative point at both ends. They are also found to have separate sexes, with the male being smaller than the female, ranging in size from a few millimeters to over a meter in length. Their cylindrical non-segmented bodies allow them to be easily distinguishable from other helminths.
Once hatched in the intestine they undergo an incredible migration. The larvae initially burrow into the mucosa, penetrate blood vessels and appear as second stage larvae in the liver within six hours post-infection. Here they remain for several days and develop into third stage larvae, L3. These larvae then migrate to the heart and are carried to the lungs via the pulmonary arteries, arriving within 4 to 7 days. From there they break out of the capillaries into the alveoli and finally work their way up the trachea to the pharynx and reach the small intestine on the 8th or 10th day post-infection.
Within the intestine, the larvae begin their third moult and become fourth stage larvae by the tenth day. The pre-patent period of Ascaris suum in pigs (40 – 53 days) is less than that of Ascaris lumbricoides (54 – 61 days) in humans. Two to three months after ingestion of the eggs, the females lay eggs in the intestine.
The fertilised female can lay about 200,000 eggs per day. Eggs require oxygen and moisture to embryonate and the worm is often found associated with Trichuris trichiura.
Ascaris lumbricoides
Morphology
Ascaris lumbricoides is the largest of the intestinal nematodes found in man. The male measures 15cm with a diameter of 3 – 4mm and has a curled tail with protruding spicules. The female is 20 –35cm long with a diameter of 5mm with a straight pointed posterior end. The mouth has one dorsal and 2 ventral lips. Both are creamy white and the cuticle has fine circular striations.
The ova can be unfertilised, fertilised or decorticated and can show considerable variation in shape and size. They measure 85 – 95mm by 43 – 47mm. The fertilised ova are easily recognised, oval in shape with a thick wall showing an irregular bumpy surface. They measure 45 – 75mm by 35 – 50mm. The outer covering has an albuminoid coat, stained golden brown by bile. The outer wall lies directly on top of a thick smooth shell, which is not easily distinguishable. Some have lost their albuminoid wall. The unfertilised ova are longer and narrower than the fertile ova, measuring 75 – 85mm by 35 – 50mm.
The shell layers of the egg provide a very resistant structure, which can withstand many chemicals, which make them ideal parasites of the intestine.
Clinical signs of disease
Small burdens of worms in the intestine may cause no symptoms. The patient may have symptoms of pneumonitis with cough and low-grade fever during the migration of the larvae through the liver and lungs. This can be accompanied by wheezing, coughing and eosinophilia. In heavy worm burdens the adult worms actively migrate in the intestine resulting in intestinal blockage, vomiting and abdominal pain but infections may also be asymptomatic.
The worms can penetrate through the wall of the intestine, or into the appendix, travel up the common bile duct, which may become blocked or they may then enter the gallbladder or liver. A heavy worm burden in children may lead to severe nutritional impairment and retardation in growth.
Life cycle of A. lumbricoides
Laboratory diagnosis
The adults of A. lumbricoides may be expelled through the anus, mouth or nose. It is important to distinguish the adult worms from earthworms, which are segmented and are often collected as a contaminant from toilets.
The microscopic examination of stool deposits after concentration reveals the characteristic bile stained ova. Eggs may be difficult to identify if an excess of iodine is added to the wet preparation as they retain the stain thus resembling debris. Ova may also become decorticated. In most symptomatic cases identification is easy due to the vast number of eggs, which can be found within a few seconds of starting to scan the slide.
Hookworm species
Hookworms infective to man comprise of 2 species, Necator americanus and Ancylostoma duodenale. They are classified as one of the most destructive of human parasitic helminths. There is no intermediate host, with man being the only definitive host.
It is estimated that there are some 900 million cases of infection world-wide. The infection is serious where the worms insidiously undermine the health of their hosts.
They occur in areas where sanitary and environmental conditions favour the development of the eggs and larval infections (warm, damp soil).
The geographic distributions of the two species are remarkably divided into:
Necator americanus, which predominately is a New World hookworm, where it was introduced from Africa to the Western Hemisphere. It can also be found in the Far East, Asia, Africa, South America and Oceania.
Ancylostoma duodenale is an Old World hookworm; it is the only species of Europe and areas bordering on the Mediterranean. It can also be found in the Middle East, North China, Africa, Asia and South America.
Life cycle
The adult worms live in the small intestine, attached firmly to the mucous membrane of the gut lining, and feed on blood and tissue. The adult females deposit their eggs in the gut (they can produce up to 20,000 eggs per day); the eggs are then passed out in feces. The rhabditiform larvae hatch in warm, damp soil (light sandy loam), feeding on bacteria. After about one week during which they have gone through 2 moults, they become infective and climb into a suitable position waiting for a suitable host to pass by. The larvae enter the host by penetrating unbroken skin (it is now recognised that A. duodenale can successfully enter man by oral ingestion, this may be more important for this species than skin penetration). The larvae then enter blood vessels and are carried to the heart, lungs and trachea. They are then swallowed and develop into adult worms in the small intestine. Larvae that are initially swallowed may not show this migration.
Larvae live for an average of 3 – 6 weeks in the tropics (A. Duodenale can live at lower temperatures than N. americanus can, and so is found in more temperate climates).
Morphology
Both species have similar general morphology and measure approximately, females 10 – 13mm and males 8 – 11mm. The general morphology of the two species resembles those of Nippostrongylus brasiliensis, the rat hookworm, but they are approximately twice the size of the rat hookworm (species not discussed here).
The male species has a posterior copulate bursa, which is absent from the female. The females though possess a vulva opening, which is found almost one third of the body length from the posterior end, they also have two ovaries. Most of the female body is occupied with eggs.
The mouth (or buccal cavity) of the two species show a conspicuous pair of chitinous plates on the dorsal surface. Ancylostoma duodenale buccal cavity bears 2 hook like teeth on the top and 2 triangular cutting plates on the bottom. While the mouth of N. americanus has 4 cutting plates, 2 on the ventral and 2 on the dorsal surfaces. The head is curved in both species but Necator adults it is finer but more pronounced forming a definite “hook” at the anterior end. The buccal cavity is used to attach the worms securely to the mucosa of the small intestine. With the teeth and cutting plates used to pierce the mucosa.
The bursa (the characteristic external feature which forms an umbrella-like extension surrounding the cloaca) of both male species is well developed. Necator adults are distinguished from Ancylostoma by the split dorsal rays and the close arrangement of the lateral rays.
The ova are oval and transparent with a smooth thin shell and measure 56 – 75mm by 36 – 40mm. They are usually passed in the 4 – 8 cell stage in faeces and may be embryonated. The ova of both species of hookworm are similar.
Clinical signs of disease
Larval penetration of the skin may lead to pruritis, often termed as “ground itch” at the site of penetration. Respiratory symptoms may arise during the larval migration.
The adult worm in the intestine may cause intestinal necrosis and blood loss as a result of the attachment of the adult to the intestinal mucosa. Patients with acute infections may experience nausea, vomiting, abdominal pain, diarrhoea and eosinophilia.
Chronic infections may lead to iron deficiency and anaemia resulting from the excessive loss of iron. Heavy worm burden in children may have serious consequences including death.
Cutaneous larva migrans
If man comes in contact with hookworm larva of the dog (or cat), A. braziliense or A. caninum, penetration of the skin may take place. The larvae are unable to complete the migration to the small intestine and become trapped. Trapped larvae may survive for weeks or even months, migrating through the subcutaneous tissues.
Trapped larvae have been known to produce severe reaction, forming tunnels through the tissues, causing intense itchy skin eruption, producing a red track under the skin, which demonstrates accurately the wanderings of the larvae.
Often intense pruritis and scratching may lead to secondary bacterial invasion, known as “creeping eruption” or “cutaneous larval migrans”.
First-stage rhabditoid larvae that hatch from eggs are 250-300 μm long by 17 μm. They have a long buccal canal and their genital primordium is small and difficult to see. Infective, third-stage, filariform larvae are 500-600 μm long. These have a pointed tail and a ratio of esophageal to intestinal length of 1:4. The sheath about the larvae is conspicuously striated.
Laboratory Diagnosis
Adults of hookworm species may be passed out spontaneously in feces. The microscopic examination of stool deposits after concentration reveals the characteristic ova.
Eggs of this species are indistinguishable from those of Ancylostoma duodenale. If these eggs hatch in feces because of a delay in fecal examination, the first-stage larvae must be differentiated from those of Strongyloides stercoralis, which normally are passed in feces. Whereas hookworm first-stage larvae have a long buccal canal and an inconspicuous genital primordium, the larvae of Strongyloides have a short buccal canal and a prominent genital primordium. Stool specimens must not be refrigerated before attempting to culture larval stages, as Necator is especially sensitive to cold.
Because hookworm species cannot be differentiated on the basis of their eggs, it is necessary to culture larvae or to recover adult worms for morphologic study to make a specific diagnosis.
Trichuris trichiura
Trichuris trichiura, more commonly known as the “whip worm”, due to the whip-like form of the body. They have a cosmopolitan distribution, though, it is more commonly seen in tropical climates and in areas where sanitation is poor. They seem to occur in areas particularly where Ascaris and hookworm are found due to the eggs requiring the same conditions to allow for embryonation both species can be found in human together.
There are several species within this genus each infecting specific hosts, but only T. trichiura infects man, causing human trichuriasis. It is a parasite that infects much more people than is generally appreciated, up to 800 million people throughout the tropics and temperate regions.
Life cycle
Eggs require a warm, moist environment with plenty of oxygen to ensure embryonation, but once they have embryonated they are extremely resistant to environmental conditions.
Adult worms are found in the caecum and upper part of the colon of man. In heavy infection they can be found in the colon and the terminal ileum. They attach to the mucosa by the anterior end or by embedding the anterior portion of the body in the superficial tissues, obtaining nutrition from the host tissues.
Once fertilised the female worms lay several thousands of eggs, which are unsegmented at the oviposition and are passed out in feces. Once they have been passed out they require an embryonation period in the soil, which may last from 2 weeks to several months, after which they become infective.
When embryonated eggs are swallowed by human hosts, larvae are released into the upper duodenum. They then attach themselves to the villi lower down the small intestine or invade the intestinal walls. After a few days the juveniles migrate slowly down towards the caecum attaching themselves to the mucosa, reaching their final attachment site simultaneously.
The larvae reach maturity within 3 weeks to a month after infection, during which they undergo 4 moults. There is no lung migration and the time from ingestion of infective eggs to the development of adult worms is about 3 months.
Infection is achieved by swallowing soil that contains embryonated eggs. Therefore, children are most commonly seen to possess the infections, as they are more likely to swallow soil.
Morphology
The adult worms of T. trichuria are characterised by the enormously elongated capillary-like oesophagus (anterior end) with the anus situated in the extreme tip.
The thin anterior portion of the worm is found embedded in the mucosa. There are no lips, and the vulva is at the junction of the thread-like and thickened regions of the body. The posterior end is much thicker and lies free in the lumen of the large intestine.
The female measures 35 – 50mm long and the male 30 – 45 mm long.
The ova are characteristically barrel shaped, bile stained with bipolar plugs. They measure 50 – 54mm by 20 – 23mm.
Clinical signs of disease
Most infections due to this nematode are light to moderate with minimal or no symptoms. However, a heavy worm burden may result in mechanical damage to the intestinal mucosa due to the adult worm being threaded into the epithelium of the caecum. Abdominal cramps, tenesmus, dysentery and prolapsed rectum may occur in these cases.
If a prolapsed rectum is observed, many worms may be seen adhering to the mucosa of the rectum.
Symptomatic infections are usually only seen in children. The majority of infections are chronic and mild, with nonspecific symptoms like diarrhoea, anaemia, growth retardation, and eosinophilia.
Laboratory Diagnosis
The adult worms of T. trichiura are rarely seen in feces. The microscopic examination of stool deposits after concentration reveals the characteristic barrel shaped ova. In symptomatic infections numerous numbers of eggs can be seen due to the prolific nature of the female worms, even in mild infections many eggs can be seen in the smear.
Strongyloides stercoralis
Strongyloides stercoralis is an intestinal nematode commonly found in warm areas, although it is known to survive in the sub-tropics (hot and humid conditions). The geographic range of Strongyloides infections tend to overlap with that of hookworm due to the eggs requiring the same environmental conditions to induce embryonation.
This parasite is interesting in that it contains a free-living stage (exogenous) and a parasitic stage (endogenous) where their larvae undergo development in both stages.
Life cycle
The life cycle has three phases:
The parasitic adult females lay eggs while they are in the duodenum where they hatchproducing rhabditiform (non-infective) larvae.
1. The larvae can have two fates in life, one where they are passed out in feces to continue down the free-living pathor they develop into infective filariform larvae whilst travelling down thesmall intestine.
2. The larvae which, develop in the environment can also undergo different development. Some larvae undergo directdevelopment (homogonic) or indirect development (heterogonic).
The non-infective first stage (rhabditiform) larvae develop into free living adults in the soil within 2 – 5 days andproduce infective third stage or filariform larvae which can penetrateexposed skin (heterogonic development). This phase is common in moist, warm tropical countries.
In the non-infective rhabditiform larvae, which are excreted in feces, develop into infective filariform larvae in the soil (homogonic development). These infective larvae penetrate exposed skin. There is no development of free living adult worms and this phase is common in temperate zones. The larvae never undergo sexual maturity.
Both types of larvae can become established in the host by penetrating the skin or by oral ingestion.
The larvae, which infect the host by penetrating the skin, undergo a migration through the dermal tissues and into the circulation to the heart and lungs, then up the bronchi and trachea, where they are eventually swallowed and pass down into the intestine. On reaching the mucosa of the duodenum the females develop and produce eggs. Adult males are unable to attach themselves to the mucosa, therefore, for any copulation to take place they must mate in the lumen of the intestine.
3. The non-infective rhabditiform larvae develop into infective filariform larvae while passing down the small intestine. Autoinfection occurs when the larvae reinfect the host by penetrating the intestinal mucosa or the perianal or perineal skin. The larvae migrate to the lungs via the circulatory system and then return to the intestine.
From initial infection to maturity usually less than 4 weeks pass.
Morphology
The first stage rhabditiform larvae measure approximately 250mm long and 20mm wide. They have a bulbed oesophagus and a short buccal cavity. In an old specimen, rhabditiform larvae of S.stercoralis must be differentiated from those of hookworm, which have a longer buccal cavity. The third stage or filariform larva is approximately 500mm long and has a notched tail compared with that of hookworm, which is sheathed and has a long slender tail.
Adults are slender and possess and extremely long oesophagus, which in the female extends 1/3 to1/2 of the body. The anal opening is ventral and the tail is pointed.
Eggs are rarely found in the stool as they hatch in the intestine. They are oval and thin shelled, resembling those of hookworm but are smaller measuring 50 – 58mm by 30 – 34mm.
Clinical Signs of Disease
Disease associated with infections due to S.stercoralisis varied, ranging from some patients being totally asymptomatic to the hyperinfection syndrome. There are 3 areas of involvement in Strongyloides infections: skin, lungs and intestine.
1. Initial skin penetration of the filariform larvae usually causes very little reaction, however with repeated infections the patient may mount a hypersensitive reaction, thus, preventing the larvae from completing its life cycle. The term larva currens is used when there is a rapidly progressing urticarial track.
2. The migration of larvae through the lungs may stimulate an immune response, which can result in a cough, wheezing and fever.
3. Symptoms associated with intestinal strongyloidiasis may mimic a peptic ulcer due to ulceration of the intestinal mucosa. In heavy infections the intestinal mucosa may be severely damaged resulting in malabsorption. There may also be lower gastrointestinal bleeding. Eosinophilia may be high.
Hyperinfection syndrome
The autoinfective capability of larvae may be responsible for long-term infections, which persist for many years. The parasite and host reach an equilibrium state where neither host nor parasite suffers any adverse reactions. If this equilibrium is disturbed e.g.immunosuppression, the infection proliferates with immense numbers of larvae migrating to every tissue in the body, especially the lungs. This condition is referred to as disseminated strongyloidiasis. This results in tissue damage, pneumonitis, brain damage or respiratory failure.
Laboratory diagnosis. Microscopy
Laboratory diagnosis depends on finding larvae in stool, sputum or duodenal aspirates.
Strongyloides larvae may be present in stool in very small numbers and culture methods maybe needed to encourage the rhabditiform larvae to develop into filariform larvae and migrate from the sample.
The Enterotest or string test can be used to recover larvae from duodenal aspirates.
Larvae must be distinguished from hookworm larvae especially if it is an older sample. Rhabditiform larvae are most commonly seen.
A good concentration technique is essential to increase the chances of seeing larvae, though they are easily killed making diagnosis more difficult.
Serological tests are of value in the diagnosis of strongyloidiasis when larvae cannot be found. An enzyme-linked immunosorbent assay (ELISA) using larva antigen, is usually employed.
Enterobius vermicularis
Disease: Enterobiasis (pinworm infection, oxyuriasis).
Geographic Distribution: Worldwide.
Location in Host: Cecum, appendix, colon, and rectum.
Morphology.
Adult Worms. Males are 2.5 mm long by 0.1-0.2 mm wide, and have a blunt posterior end and a single spicule, 100-140 μm long. Females are 8-13 mm long by 0.3—0.5 mm wide, and have a long pointed tail. In both sexes, there are cephalic inflations, and the esophagus is divided into three parts — corpus, isthmus, and bulb.
Eggs. Elongate, flattened on one side, with a thick, colorless shell, 50-60 μm by 20-30 μm. Eggs are partially embryonated when laid.
Life Cycle
Females usually emerge from the anus at night and discharge their partially embryonated eggs on the perianal surface.
Eggs embryonate to the infective first stage within 4 to 6 hours. Infection usually is by direct transmission of eggs to mouth by hands or through fomites. Parasites develop in the lower intestinal tract, and the prepatent period is 3 to 4 weeks. Adults normally live for only a few months.
Diagnosis
Eggs usually are detected in cellulose tape preparations applied to the patients perianal region in the early morning prior to the patient’s bathing or using the toilet. Eggs are sometimes found in fecal preparations; however, routine diagnosis by fecal examination is unreliable because eggs are not introduced into the fecal stream. Instead these eggs are discharged onto the surface of fecal material as it passes through the rectum. Not infrequently, adult females are seen around the anus or on the surface of stool specimens. Rarely, immature larval stages of pinworms, especially female worms, are found in fecal specimens. These developing larvae lack cephalic inflations for their first 2 to 3 weeks of development, but the characteristic morphology of the esophagus seen in adult worms is present in these larvae and aids in the correct diagnosis.
Because eggs are not usually found in routine fecal examinations, cellulose tape preparations are the most reliable means for detecting this infection.
Enterobiasis is a familial and group infection that is more prevalent in children. It is very common in daycare nurseries and institutional settings.
Trichinella spiralis
Trichinella spiralis was first seen by James Paget but was named and described by his Pofessor, Richard Owen. The family Trichinellidae contains only one single genus Trichinella and was originally thought only to contain the one species, Trichinella spiralis, which causes the serious and often fatal disease in man known as trichinosis (trichinelosis). It is a parasite of carnivorous animals and is especially common in rats and in swine fed on uncooked garbage and slaughter house scraps, humans become infected by eating raw pork, with sausages being the most common cause of infection. It is a cosmopolitan parasite and prevalent in many European countries with the highest interest being in China.
Trichinella spiralis is a “domestic” parasitic nematode long recognized to cause a zoonosis transmitted to man by the ingestion of infected pork.
Life cycle
Infection in the definitive hosts is acquired by the hosts eating raw or undercooked flesh e.g. pork, containing encapsulated larvae. Rats are probably the most highly infected “natural” hosts and pigs become infected by eating infected pork scraps or occasionally rats, which inhabit their stalls. For man sausages are a dangerous source of the parasite as a small fragment of infected pork, (after mincing), may become widely distributed among a number of sausages.
Humans become infected by eating raw meat containing encysted larvae. The cyst becomes digested and releases the larvae, which invade the intestinal mucosa. They develop and mate in the second day. By the 6th day of infection, the female adults deposit motile larvae, which are carried by the intestinal lymphatics or mesenteric venules to other tissues in the body. The very active muscles, such as the diaphragm, jaws, tongue, larynx and eyes, are invaded and the larvae become encapsulated by the 21st day following infection. Calcification of the cysts occurs as early as five months, but usually begins after 6 –18 months. The cyst wall is derived from the host’s muscle fiber and the larvae remain viable for many years with no further development occurring. When muscles are eaten by the definitive host, sexual maturation in the intestinal phase, as explained earlier, occurs rapidly.
Morphology
The adult female worm is about 2-3mm long and 90mm in diameter. The male is smaller measuring 1.2mm long by 60mm in diameter.
The female adult worms are ovoviparous and up to 1500 larvae may be released by a single worm.
Clinical signs of disease
Symptoms during the intestinal phase may go unnoticed or may be severe. Epidemics can result in outbreaks of gastro-enteritis, 2 to 7 days after the ingestion of raw pork. Diarrhoea with or without abdominal pain may last for several weeks. Eosinophilia and fever occur in most cases. Leucocytosis is common and hyperglobulinaemia is characteristic. Myocytosis and circum orbital oedema are classical signs. There can also be central nervous system involvement.
Pathogenicity
The primary pathogenic effect of Trichinella comes from the destruction of the striated muscle fibres in which it encysts. There can be neurological manifestations of trichinosis and death may be ascribed to myocarditis, encephalitis or pneumonitis.
Laboratory diagnosis
Diagnosis of trichinosis depends on the clinical signs, such as myalgia, periorbital oedema, fever and eosinophilia in a patient with a history of eating pork or sausages.
Serological tests are available but may be negative if carried out within 3 – 4 weeks after infection. Circulating antibodies to T. spiralis appear on 2 – 4 weeks after infection. Redefined diagnostic antigens for their detection are currently being developed. A simple IFAT employing fragments of larvae, as antigen is a useful diagnostic tool. Latex tests with extracted larval antigens are also proved to be valuable in the acute stage, during which high antibody titers develop.
Muscle biopsy is available with the muscle being digested in pepsin, which frees the encapsulated larvae or by a simple device whereby the muscle sample is compressed between 2 glass plates to make it semi-transparent, allowing you to see any encapsulated larvae using a “trichinoscope” (a simple magnifying system).
