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.
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.
It is now thought that there are five varieties of this species that exists world-wide:
Trichinella spiralis spiralis – temperate zone – high infectivity for pigs, rats and man.
Trichinella spiralis nelsoni – tropics – low infectivity for pigs and rats and high infectivity for lions, hyenas.
Trichinella spiralis nativa – Arctic – low infectivity for pigs, found in polar bears, resistant to freezing.
Trichinella spiralis pseudospiralis- New Zealand – low infectivity for pigs, rats and mice.
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).
The Blood Nematodes
These nematodes are known as filariae and consist of a group of nematodes, which have successfully invaded the blood stream, connective tissue, or serous cavities of vertebrates. They are long thread –like nematodes.
Many of them are of medical and veterinary importance attacking man and various domestic animals being transported by various vectors, including mosquitoes. The nematodes from this order do require intermediate hosts for the completion of their life cycle.
The morphology of these nematodes consists of a cylindroid pharynx with an anterior muscular portion and a posterior glandular portion; the males have spirally coiled tails.
Sexually mature female worms release microfilaria, which are pre-larval stages. These are released into the bloodstream. Most species are known to be ovoviviparous and some have “sheathed” microfilaria.
The filarial nematodes which parasitise in man consist of Wuchereria bancrofti, Loa loa, Onchocerca volvulus.
They inhabit a range of locations within the body: lymph glands, deep connective tissue, subcutaneous tissues or mesenteries. Invasions of these tissues usually result in inflammatory reactions, which is a typical symptom of a human filarial infection. In some cases these result in fleshy deformities known as elephantiasis.
It has been estimated that approximately 1 billion people in tropical and subtropical countries are exposed to the risk of filarial infections and at least 200 million are infected with filariasis. The species which are primarily responsible for these human filarial infections are: Wuchereria bancrofti, Brugia malayi and Onchocerca volvulus.
Wuchereria bancrofti
Wuchereria bancrofti is a nematode causing lymphatic filariasis throughout the tropics and subtropics and is transmitted by mosquitoes. There are two strains of
W. bancrofti:
1. The nocturnal periodic strain which is widely distributed in endemic regions i.e. Africa, India and the Far East and also parts of China, Korea and Japan, the microfilariae are in their highest concentrations between the hours of 10pm and 2am.
2. The sub-periodic strain which is found in the Pacific region, and has a microfilaraemia all the time with the highest numbers being detected between noon and 8pm.
Humans are the only known reservoir host of W. bancrofti. Infection rates in some communities in East Africa exceed 30% of adults causing revolting swellings of the legs or genital system, known as elephantiasis in man. The adult worm occurs in tightly coiled nodular masses in the major lymphatic ducts.
The main vector is Culex quinquefasciatus, a mosquito that is particularly common in towns and cities, breeding in organically polluted water, resting in houses and feeding by night on their human occupants. Typical breeding sites include: storm drains blocked with domestic refuse, accumulations of domestic waste water, inadequately covered septic tanks and pit latrines.
In rural areas throughout Africa Anopheles gambiae and Anopheles funestus are involved in transmission. Elsewhere other anopheline mosquito species may transmit bancroftian filariasis in rural areas, while in Papua New Guinea Mansonia may act as a vector.
Life cycle
Microfilariae enter the host during a blood meal when the vector, a mosquito, punctures the skin. The infective larvae enter through the wound and migrate to the peripheral lymphatics where they grow to mature male and female worms. They can live there for several years. After mating, the gravid females release sheathed microfilariae into the peripheral blood where they can be detected 8 – 12 months after the initial infected bite.
The mosquito acquires the infection by ingestion of the microfilaria in the blood meal. The microfilariae lose their sheath on arrival in the stomach of the mosquito due to gastric juices. The larvae migrate to the thoracic muscles and develop into infective larvae over a period of 6 – 14 days. The larvae then migrate to the mouthparts of the mosquito, which infects the host during a blood meal.
The blood stages of filariae, mifcrofilariae, vary in the times when they are present in the peripheral blood, corresponding with the peak biting time of the vector. Thus, iocturnally periodic forms the microfilaria are present in the peripheral blood circulation at night; during the day they reside in the deep tissues, particularly the lungs.
Morphology
The adult worms are white and threadlike. The male measures between 2.5 – 4cm whereas the female is larger, measuring between 8 – 10cm.
The microfilariae are 230 – 275mm in length. The tail of the microfilariae of
W. bancrofti tapers to a delicate point and exhibits no terminal nuclei. The sheath the microfilariae of W. bancrofti stains with haematoxylin stain.
Clinical signs of disease
Many patients are asymptomatic. Patients may present with fever, lymphangitis and lymphadenitis. Lymphangitis commonly affects the lower extremities and there may also be genital and breast involvement. An inflammatory reaction occurs in the lymphatic vessels that harbour the adult worms. Oedema develops which may resolve after the first few of attacks. A late complication resulting in thickening and verrucous changes in the skin known as elephantiasis may occur after recurring lymphangitis. Secondary bacterial and fungal infections may occur in patients with long-standing elephantiasis.
Obstruction of the genital organs may result in hydrocoele formation and scrotal lymphoedema. Obstruction of the retroperitoneal lymphatics may cause the renal lymphatics to rupture into the urinary tract producing chyluria.
Some patients with filariasis do not exhibit microfilaraemia but develop tropical pulmonary eosinophilia, which is characterised by peripheral eosinophilia, wheeze and cough. High eosinophilia, high IgE level and high anti-filarial antibody titres are features of this syndrome.
Laboratory diagnosis
Sheath may or may not stain with Giemsa; is stained with haematoxylin stains. Discrete nuclei. Empty space between the nuclei and the body wall. No nuclei in tip of tail. Innerbody is rarely visible in Giemsa. Is not stained with haematoxylin. Cephalic space as long as it is broad. Tip of tail may be bent underneath the body. Found in blood.
Loa loa
Loa loa, also known as the African eye worm, is a filarial nematode endemic in the rain forests of West and Central Africa. It is transmitted by Chrysops species, also known as mango flies or horse flies, and humans are the only known reservoir. It is estimated that 2-13 million humans are infected with the larvae.
Adults migrate in the subcutaneous tissues of man and monkeys, with them eventually migrating across the eyeball under the conjunctiva.
Life cycle
The adult worms live in the subcutaneous and deep connective tissues and the microfilariae are found in the peripheral blood, where they can be in ingested by the Crysops fly (day biting fly). The adults can live in the tissues for up to 17 years
Once the microfilariae have been taken up by the Chrysops during a blood meal they develop within the fat body. They develop through to L3 within 10 – 12 days. The microfilariae, L3 re-enter the host’s blood stream when the fly takes another blood meal. They reach adult worms within 4- 6 months living in the subcutaneous and deep connective tissues.
The microfilariae exhibit diurnal periodicity, the highest numbers being detected in blood between 10am and 2pm.
Morphology
Adult males of Loa loa are 2 – 3.5cm long and the females from 5 – 7cm. The microfilariae of Loa loa are 250 – 300mm. They possess a sheath, which stains blue-grey with Delafield’s haematoxylin. The sheath is not stained with Giemsa. The tail gradually tapers to a rounded end, the densely packed nuclei extending to the tip.
Clinical signs of disease
Many patients infected with Loa loa appear to be asymptomatic and the migration of the adult worm through the subcutaneous tissues often goes unnoticed, unless passing beneath the conjunctiva of the eye. They can be seen crossing the eye, but it is a rapid process taking approximately 15 – 20 minutes. Hypereosinophilia and increased antibody levels, especially IgE are also noted. Eyeworm episodes are as equally common in man as well as women with common re-occurrences. There is an increased incidence with age.
The most common pathology associated with Loa loa infections are Calabar swellings, which are inflammatory swellings resulting in a localized subcutaneous oedema. These swellings are due the host’s response to the worm or its metabolic products and can be found anywhere in the body but most commonly in the extremities. These swellings last from 1 – 3 days. They develop rapidly and last one to three days, usually accompanied by localized pain, urticaria and pruritis. There is a higher frequency of calabar swellings in women with common re-occurences.
Serious complications such as cardiomyopathy, encephalopathy, nephropathy and pleural effusion have been recorded.
Laboratory diagnosis
Kinked and sheathed microfilaria. Sheath is not stained with Giemsa stain; it is stained with haematoxylin. Nuclei crowded extending to tip of tail; tip of tail tapers. Cephalic space as long as it is broad. Innerbody is not usually stained. Found in blood.
Microfilaria worms found in tissue and skin
The main species of microfilariae found in the skin and tissue are Onchocerca volvulus and Mansonella streptocerca. Microfilariae of Onchocerca volvulus and less often, Mansonella streptocerca migrate through the skin causing itching and skin texture changes and occasionally arrive in the eye where they cause blindness. Detection of these microfilariae is from skin snips or nodule biopsies. When high numbers of microfilariae are present, they can occasionally be found in the blood and urine.
Onchocerca volvulus
Onchocerca volvulus is mainly found in West Africa and Central and South America. Onchocerciasis, also known as river blindness, is a major public health problem, especially in West Africa; there an eradication program has been established. It is one of the world’s most distressing diseases of helminth origin, often resulting in blindness. Onchocerca volvulus is transmitted by the species Simulium or black fly whose breeding habitat is fast flowing rivers or streams, therefore, there is a patchy distribution of the disease as it is specified to where water courses are. The adult worms are found iodules or onchodermata in superficial sites, but may invade other tissues.
It is estimated that there are 18 million cases worldwide with 17.5 million being found in Africa. Nigeria is the most infected region. The rate of morbidity is high in relation to those with an infection.
The life cycle is similar to W. bancrofti, except that the intermediate hosts are various species from the genus Simulium (Black flies), the most important species is Simulium damnosum.
The microfilariae are ingested by a Black fly during a blood meal, from where they are carried to the midgut where they penetrate the epithelium and migrate, via the haemocoele, to the indirect flight muscles. Here they undergo two moults, L1 – L3 and develop into infective L3 larvae which move to the mouth parts. Development is completed in 6 – 9 days.
When the infected fly takes another blood meal the infective larvae are once again transmitted into another host (definitive host). The microfilariae are released from the mouth parts and transmitted directly into the hosts bloodstream. Moulting takes place form L3 – L4 within 2 – 5 days and the larvae then migrate widely through the body under the skin and between muscles, ligaments and tendons. The final moult to L5 occurs at 1.5 – 2.5 months after transmission. Male worms are known to mature in about 4 months later. Female worms initiate the formation of the nodules and the males may join later. The sexually mature female worms release microfilariae, which migrate out from the nodules into the skin and other tissues, most significantly into the eye.
Morphology
The whitish adult worm lies coiled within capsules in the fibrous tissue. The female can measure up to 50cm while the males are shorter measuring up to 5 cm. The microfilariae of O. volvulus are unsheathed and are usually found in the skin. They measure between 221 – 287mm long.
Clinical signs of disease
Clinical manifestations are due to microfilarie in the epidermis.
Light infections may be asymptomatic or cause pruritis. This leads to scratching which can result in infection. Lyphadenopathy may also be a feature of early infection. After months or years, onchodermatitis results in secondary stage of thickening due to intradermal oedema and pachydermis. There is a loss of elastic fibres resulting in hanging groin, hernias and elephantiasis of the scrotum. There is finally atrophy of the skin resulting in loss of elasticity. There is mottled depigmentation of the skin.
Ocular lesions are related to the intensity of the microfilariae in the skin. Ocular lesions include sclerosing keratitis, secondary glaucoma and cataract, coroidoretinitis and fluffy corneal opacities. The major complication of onchocerciasis is the development of lesions in the eye, which may result in blindness or other distressing ocular diseases.
Laboratory diagnosis
1. Analysis of Skin Snips
Small amounts of skin are collected by using a needle to raise the skin and then to slice about 1 mg of skin to a depth of 0.5mm. Snips are collected from several sites, usually the shoulders or the buttocks and sometimes the chest and calves. The snips are placed immediately in 0.5ml normal saline in a microtitre plate and left for 4 hours to allow the microfilariae to migrate out of the tissues. After 4 hours, the samples are examined using an inversion microscope. The microfilariae should still be moving and can be identified from the table below. The microfilariae can also be collected by filtration or centrifugation and the deposit containing microfilariae can be stained with Giemsa at pH 6.8.
2. Analysis of Biopsies
Biopsies of tissue nodules can be dabbed on to a slide to produce impression smears and then stained with Giemsa stain at pH 6.8 for the presence of microfilariae.
Dracunculus medinensis
Dracunculus medinensis is a non-filarial parasite as it only has one uterus whereas filaria have two. It is usually associated with places where there is a lack of clean drinking water e.g. step wells in India, covered cisterns in Iran, and ponds in Ghana. The life cycle usually involves copepod intermediate host. They are parasitic in the connective tissue or coelom of vertebrates. The disease associated with this parasite is known as Dracunculiasis.
Mature female worms, which are gravid with microfilariae, migrate to the superficial layers of skin of humans, especially those regions, which are most likely to come in contact with water, such as the ankle, foot, arms and shoulders. Here the worms secrete a substance (substance is unknown), which causes a blister to rise over its anterior end where it has pierced the lower layers. The blister eventually forms into an ulcer which on contact with water, the uterus is projected out of the ulcer cavity, and a cloud of milky white secretion, containing hundred of active larvae, is released. Once out of the water again the uterus dries and shrivels preventing the release of further larvae.
If the microfilariae is ingested by an appropriate species of Cyclops, they break though the soft mid-intestine wall and come to lie in the body cavity. The larvae undergo two moults and become infective in approximately 3 weeks. Humans become infected by accidentally ingesting through drinking water the infective Cyclops. Upon ingestion the larvae are activated to penetrate through the gut wall, and migrate through the tissues, moulting twice and finally becoming lodges in the viscera or subcutaneous tissues. Maturation of the worms is slow taking about 1 year to reach sexual maturity before the females are ready to migrate to the skin to release their larvae.
Morphology
The adult female worm measures up to 1 metre in length whereas the male measures about 2cm.
Clinical signs of disease
After ingestion of the Cyclops, there is no specific pathology associate with the mucosal penetration and larval maturation in the deep connective tissues. Erythema and tenderness can be associated with blister formation. The patient can also exhibit vomiting, diarrhoea, asthmatic attacks. Symptoms usually subside when the lesion erupts. If the worm is removed, healing usually occurs without any problems. If the worm is damaged or broken during removal, there may be intense inflammatory reaction with possible cellulitis along the worms migratory tract. This can result in arthritis and synovitis.
Laboratory diagnosis
The best remedy for removing the adult worm is a slow process of daily gently rolling the worm around a small stick and slowly pulling it out of the skin. With this method you must be careful not to pull apart the worm as it will recoil back into the skin and cause secondary infections.
At this moment in time this parasite is being effectively controlled due to a strict control program. The program includes stopping people from drinking infected water, putting muslin over the water jars, which they use to collect the water in, thus preventing the cyclops from being collected in water. Educating the communities about the parasite and adding temphos to the water, thus, killing off any microfilariae in water.
