General Epidemiology. Epidemic process. Ways Infectious disease control.
The Subject Matter of Epidemiology
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The word “epidemiology” has been used since the time when most of the infectious diseases were epidemic. Today, when infectious morbidity has considerably decreased, the concept of epidemiology includes the study of objective laws of aetiology, distribution and control of infectious diseases in a human community, and also elaboration of methods to prevent and control these diseases.
The following definition of the term “epidemiology” was formulated at the International Symposium of Epidemiologists in Prague (I960):
Epidemiology is an independent branch of medicine studying aetiology and spreading of infectious diseases in a human community and is aimed at prevention, control, and final eradication of these diseases.
General and special epidemiologies are distinguished. General epidemiology studies the laws of distribution of infectious diseases among people (characteristics of sources of infection, the mechanism of transmission, susceptibility to infection, and the like) and the general principles of prevention and control of these diseases. Special epidemiology studies epidemiologic characteristics of each particular infectious disease and the methods to prevent and control it.
History of epidemiology. Ancient people had their own concept of contagiosity of some diseases and took first prophylactic measures; they rejected people with infectious diseases from their community, used variolation (deliberate inoculation with smallpox virus), disinfection, etc. All these measures were empirical and their efficacy was low. At those times it was impossible to prove instrumentally that infectious diseases might be evoked by living microorganisms, but numerous epidemics of black plague, smallpox and typhus, especially in the 14-15th centuries, aroused such suspicions in physicians. Fracastorius, an Italian physician (1483-1553), produced a theory that proved contagiosity of these diseases.
The Concept of Infection
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An infectious process is the interaction of a pathogenic microorganism with a macroorganism under certain environmental and social conditions. The concept “infectious disease” means the condition manifested by a disease state of a patient and the so-called carrier state.
The specific properties of infective agents, various pathogenicity and virulence of these agents, as well as the quantity of microorganisms that enter the macroorganism, resistance of the macroorganism and duration of specific immunity account for the multitude of clinical manifestations of infection.
Infection can be clinically pronounced or it may be asymptomatic, which is known as the carrier state (parasite, bacterium, virus carrier state). A clinically manifest infection can run a typical or atypical course. Patients with a typical form of infection demonstrate all symptoms specific for a given disease. One or several symptoms of a given disease are absent from the clinical picture of an atypical form, or the symptoms can be modified. A disease can be acute or run a protracted or even a chronic course.
A clinically manifest disease is usually classed as mild, moderate, and severe; according to the duration, the disease can be acute or chronic.
An acute infection (smallpox, measles, plague) is characterized by a short stay of the causative agent in the body and development of specific immunity in the patient toward the given infection.
A person with a subclinical infection (acute and chronic) looks in full health, and the disease can only be diagnosed by detecting the causative agents, specific antibodies, and functional and morphological changes in the organs and tissues that are specific for a given disease. Such patients (or carriers) are a special danger for the surrounding people since they are the source of infection. At the same time, a repeated subclinical infection in poliomyelitis, diphtheria, influenza, and some other acute infections promotes formation of an immune group of people (herd immunity). Acute and chronic subclinical forms (carrier state) are more common in typhoid fever, paratyphoid B, salmonellosis, viral hepatitis B, etc.
The Concept of Epidemic Process
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Microorganisms causing infectious diseases parasitize on host and persist due to continuous reproduction of new generations which change their properties in accordance with evolution of the environment conditions. Living inside its host, the microorganism persists for a definite period of time. Then the pathogenic microorganism can survive by changing its residence, i.e., by moving to another host via a corresponding transmission mechanism. This continuous chain of successive transmission of infection (patient-carrier), manifested by symptomatic or asymptomatic forms of the disease, is called an epidemic process.
According to Gromashevsky, the source of infectious microorganisms is an object which is the site of natural habitation and multiplication of the pathogenic microorganisms, and in which the microorganisms are accumulared. Since pathogenic nucroorgansms are parasites, only living macroorganism can be such an object, i.e., a human or an animal.
An epidemic focus is the residence of infection source including the surrounding territory within the boundaries of which, the source can, under given conditions, transmit a given disease through the agency of the pathogenic microorganisms. The focus of infection remains active until the pathogenic microorganisms are completely eradicated, plus the maximal incubation period in persons that were in contact with the source of infection. The following three obligatory factors are necessary for the onset and continuous course of an epidemic process: the source of pathogenic microorganism, the mechanism of their transmission, and macroorganisms susceptible to infection.
Severity of the disease is of great epidemiologic importance for determining “the source of infection”. If the disease is severe, the patient remains in bed and can only infect his relatives. But it is difficult to diagnose the disease if it runs a mild course; besides, the patient often does not attend for medical aid and continues performing his routine duties (at the office, school, and the like) thus actively promoting the spread of infection.
Carrier of infection is another source of morbidity. According to modern views, carrier state is an infectious process that runs an asymptomatic course. But those who sustained an infectious disease, convalescents, and also healthy persons (transition) can also be carriers of infectious microorganisms. True, carriers release pathogenic agents into the environment in a smaller quantity than patients with clinically manifest diseases, but they are danger to community too since they actively associate with healthy people and spread the infection.
Recovery from some infectious diseases, e.g. dysentery, typhoid fever, paratyphoid, diphtheria, meningococcal infection, viral hepatitis B, is not always attended by complete destruction of the microbes in the patient. Carrier state can persist in persons who sustained diphtheria or meningococcal infection after their clinical recovery: acute carrier state can last from several days to several weeks. Persons who sustained typhoid fever or paratyphoid B can be the source of spread of the pathogenic microorganisms for months. Carrier state can persist for years or even for the rest of life (chronic carrier state) in 3-5 per cent of cases, which can be explained by defective immune system.
Humans can be infected by wild animals when hunting, during stay in wild environment contaminated with excrements, when drinking water or eating food that may be contaminated with excrements of wild animals. Birds can also be transmitters of infection (omitosis, salmonellosis, etc.).
Mechanism of transmission. For the epidemic to break out it is not sufficient to have a source of infection alone. The causative agent can survive only if it is transmitted from one host to another, because any given macroorganism destroys the pathogenic microorganisms by specific antibodies that are formed in it in response to the ingress of these microorganisms. Death of an individual host terminates the life of the parasitizing microorganisms. The only exception are spore-forming microbes (causative agents of anthrax, tetanus, botulism). The combination of routes by which the pathogenic microorganisms are transmitted from an infected macroorganism to a healthy one is called the mechanism of infection transmission.
The method by which microbes are excreted from an infected macroorganism (the first phase) depends on the locus of infection in the infected individual or a carrier. If pathogenic microorganisms reside on respiratory mucosa (influenza, measles, pertussis) they can be released from the patient only with expired air or with droplets of nasopharyngeal mucus. If the infection is localized in the intestine, the pathogenic microorganisms can be excreted with faeces (dysentery). The pathogenic organisms in the blood infect blood-sucking arthropods.The presence of the causative agents outside a macroorganism (the second phase) is connected with various environmental objects. Pathogenic microorganisms excreted from the intestine get on soil, linen, household objects and water, while those liberated from the airways are borne in air. The environmental elements that transmit the pathogenic agent from one person to another are called transmission factors. The pathogenic agent can sometimes be transmitted by direct contact with an infected individual or a carrier (venereal diseases, rabies).
The following main factors are involved in transmission of infection: air, water, foods, soil, utensils, arthropods (living agents).
Water is another very important medium by which infection can be transmitted. Pathogenic microorganisms can get into water by various routes: with effluents, sewage, with runoff water, due to improper maintenance of wells, laundry, animal watering, getting of dead rodents into water, etc. Spontaneous purification of water depends on ambient temperature, chemical composition, aeration degree, exposure to sun rays, the properties of the microorganisms, and other factors. Infection is transmitted by drinking contaminated water, using this water for domestic purposes, bathing, etc. Water can be the medium for transmission of cholera, typhoid fever, leptospirosis, dysentery, viral hepatitis A, tularaemia, and other diseases. If potable water gets contaminated with faecal sewage, water-borne infection can become epidemic with rapid spreading.
Transmission of infection with food is especially important since pathogenic microorganisms can multiply in foodstaffs. Food can be infected by contact with an infected person or a carrier, by insects or rodents. Food can be infected during improper transportation, storage, and cooking. The form in which a given food is taken is also epidemically important (uncooked natural foods, thermally processed foods, hot or cold foods). Consistency of foodstaff and its popularity are also important factors. Milk and meat are common transmission media. Dairy products (curds, sour cream), vegetables, fruits, berries, bread and other foods that are not cooked before use are important transmission factors as well. Milk, dairy products can transmit dysentery, typhoid fever, brucellosis, tuberculosis, etc. Meat and fish can be an important factor in development of salmonellosis. Intestinal diseases are often transmitted through vegetables, fruits and baked products.
Soil is contaminated by excrements of humans and animals, various wastes, dead humans and animals. Contamination of soil is an important epidemiologic factor because soil is the habitat and site of multiplication of flies, rodents, etc. Eggs of some helminths (ascarides, Trichuris trichiura, hookworms) are incubated in soil. The pathogenic microorganisms of soil can pass into water, vegetables, berries that are eaten by man uncooked.
It is especially dangerous to use faecal sewage to fertilize soil where cucumbers, tomatoes and other vegetables are grown. Tetanus, gangrene, and anthrax are transmitted through soil.
The role played by various environmental objects in transmission of diseases depends on contact with the source of infection, probability of transfer of a contaminated object to a healthy person, and also on the character of chemical and physical effect that a given object can produce on the pathogenic microorganism.
The objects at patient’s room can be the transmitting factor for influenza, tuberculosis, children’s infections, dysentery, typhoid fever, and other diseases. Domestic animals can be the source of infection, while arthropods can transmit infection.
Utensils and household objects such as dishes, cups, plates (in hospitals, canteens, etc.) can become a transmissing factor for tuberculosis, scarlet fever, typhoid fever, diphtheria. Soiled linen and underwear can promote the spread of infection such as scabies, intestinal or droplet infections.
Toys, pencils, and other objects in children’s use are important transmitting factors.
Living objects that transmit infection can be divided into two groups: specific and non-specific (mechanical). Specific carriers are lice, fleas, mosquitoes, ticks, etc. They transmit infection by sucking blood (inoculation) or contaminating human skin with their excrements. Inside specific transmitters of infection, the pathogenic microorganisms multiply, accumulate, and with time become dangerous to the surrounding. A louse, for example, sucks blood of a typhoid fever patient and excretes the pathogenic microorganisms with faeces only in 4-5 days. Non-specific carriers transmit the pathogenic microorganisms by purely mechanical method. Flies, for example, carry microbes of dysentery, typhoid fever, viral hepatitis and some other diseases that are found on their bodily surfaces, on the limbs, in the proboscis and the intestine. Gadflies transmit microbes causing anthrax and tularaemia by their stinging apparatus.
Transmitting factors determine also the third phase of transmission mechanism-inoculation of the successive biological object (host). The pathogenic factor is inhaled with air, ingested with food and water, or is transmitted into the blood by arthropods.
The forms of realization of the transmission mechanism, including the combination of factors involved in spreading of a corresponding disease, are known as the transmission routes of the infective agents.
The following transmission routes are distinguished: contact, air-bome (or dust-bome in some diseases), food- and water-borne, transmission by arthropods and soil, through the placenta, by medical parenteral and other manipulations.
Susceptibility and immunity. Susceptibility of people to a given infection is a very important factor in infection spreading. Susceptibility of an individual or of a community are distinguished. Susceptibility to a disease is a biological property of tissues of a human or an animal, characterized by optimum conditions for multiplication of pathogenic microorganisms. Susceptibility is a species property, that is transmitted by hereditary trait. Many infectious diseases can affect only a certain species of animals. Some anthroponoses, e.g. typhoid fever, scarlet fever, gonorrhoea do not affect animals even after artificial inoculation, because the animals are protected by hereditary (species) immunity.
But hereditary immunity is not an absolute property. Under some unfavourable conditions, immunity of a macroorganism can be altered. For example, overheating or cooling, avitaminosis, or some other unfavourable factors can promote the onset of a disease that would not, under normal conditions, affect man or animal. Pasteur, for example, exposed hens to cold to artificially provoke anthrax in them (the disease that does not affect hens under normal conditions).
The following kinds of immunity are distinguished: hereditary (species), acquired (natural: active, passive; artificial: active, passive).
Some features of epidemic process. An epidemic develops and is maintained only by the interaction between the source of infection, specific mechanism of its transmission, and susceptible population under giveatural and social conditions. The role of these motive forces during subsequent infection is different. The most active is the source of infection, the carrier of the infective factor, the pathogenic microorganisms multiply in it with subsequent release into the environment. The mechanism of infection transmission is decisive. It can be active ingress of the pathogenic factor into a healthy macroorganism through the agency of living carriers, inhalation with air, ingestion with food and water, or persistence of viable pathogenic microorganisms on various non-living objects before they enter another living organism. Susceptibility plays a passive role. In the presence of susceptibility, a person gets infected, while in the absence of such susceptibility a person is not afflicted.
The intensity of an epidemic process can also be different. Three stages of quantitative changes are usually distinguished in the epidemic course: sporadic incidence, epidemic, and pandemic.
Sporadic incidence is a normal (minimal) morbidity characteristic of a given infection for a given country or region. Many infectious diseases occur as single cases.
Group incidence of infectious diseases in a community is assessed in everyday medical practice as an epidemic outburst.
An epidemic is characterized by morbidity that 3-10 times exceeds the sporadic occurrence of a given disease in a given locality; it is also characterized by development of multiple epidemic foci.
Pandemic is characterized by widespread epidemic throughout large territories.
Endemic* characterizes an epidemic qualitatively. An endemic disease constantly occurs among population of a given area. Long existence of any infectious disease in a given country or area can be due to the presence of some natural factors.
Exotic disease is an opposite notion. It is used to designate an infectious disease that does not normally occur in a given country or area and can only be brought from a foreign country.
In veterinary the terms epidemic, pandemic, and endemic are replaced by epizootic, panzootic, and enzootic, respectively.
A focus of infection is a site or area where cases of an infectious disease can occur or has already occurred.
The quantitative and qualitative changes in the epidemic process depend on the natural and social conditions that can activate the source of infection, the transmission factor, or susceptibility of population, thus increasing their epidemiologic activity, or on the contrary, decreasing it.
The effect of natural conditions on the transmission mechanism of infection is especially marked when the pathogenic microorganisms are transmitted by living carriers. Absence of living transmitters (ticks, mosquitoes) during a certain season or reduction of their population reduces the human infection rate, and hence is important for the course of the epidemic process.
Pavlovsky has worked out a theory of natural nidality of transmissible diseases. He showed that many infectious diseases exist iature independently of man, in a certain combination of natural conditions in a given locality, in the presence of warm-blooded animals and arthropods that are depots of the pathogenic microorganisms. For example, ticks transmit encephalitis from diseased animals to healthy ones. Besides, ticks transmit the virus to their posterity.
According to Pavlovsky, natural nidality of transmissible diseases is characterized by indefinitely long existence of the pathogenic microorganisms, their specific transmitters and animals (reservoirs of the pathogenic microorganisms) during renewal of their generations independently of man in various biocenoses, both during the course of their evolution and at a given period of time.
Natural nidi of non-transmissible diseases can exist as well. For example, carriers of leptospirosis are not involved in circulation of the pathogenic microorganisms. Spread of this disease is confined within a certain geographic area where a particular rodent lives. Diseases with natural nidality are characterized by seasonal morbidity which is associated with biology of the carriers.
Many animals give posterity in spring; hence vernal rises in brucellosis morbidity. Plague exists in its latent form during hibernation of gophers and marmots. As rodents return to active life in spring, the infection activates and rapidly spreads among the young generation.
Natural processes have their effect oon-living transmission factors as well. Open water bodies get contaminated more easily with effluents and serve as the source of water-bome epidemic of typhoid fever during the cold season when spontaneous purification of water is slowed down and the microorganisms causing intestinal infections survive for longer periods of time.
Presence of people in enclosures promotes transmission of air-home infections, while wearing warm clothes without proper hygiene of individuals promotes multiplication of lice, carriers of louse-borne and recurrent fever. The effect of the natural factor on susceptibility is insignificant. It only increases or decreases nonspecific body resistance (barrier function of the skin, mucosa, blood, bile, etc.).
The social factor is more important epidemiologically. It includes the concept of living conditions of population: the quality of dwelling, density of population in residential buildings and areas, conveniences (water supply and sewage system), well-being of population, nutrition, cultural standards, sanitation, health-care system, social structure of a community, etc.
The course of an epidemic depends strongly on the living conditions, i.e., on population density, intensity of association between the source of infection and the surrounding people, the character of occupation, traffic, time of detection of carrier state or developing disease, and time of hospitalization or isolation in home conditions. Poor ventilation, overcrowded residence, inadequate insolation and ventilation of rooms and suboptimal sanitation promote spread of tuberculosis and other infectious diseases.
Domestic animals, poultry, and wild animals can be the source of infection. Man can be infected by a domestic animal due to inadequate veterinary control, untimely detection of diseased animals and their isolation, slaughter or treatment. Rodents and wild animals are regularly reduced in their number which decreases considerably their epidemiologic danger.
The condition of water supply and sewage systems, rational and timely cleaning of settlements are important for the spread of intestinal infections such as typhoid fever, paratyphoid, dysentery, cholera, poliomyelitis, viral hepatitis, etc.
Inadequate control and poor organization of food catering is responsible for spread of infectious diseases. Food can be infected by carriers among those who work in food catering, food shops, children’s and medical institutions. People can be infected by meat of diseased cattle and milk and dairy products manufactured from the milk of infected animals.
Labour conditions are often important for the development and spread of infectious diseases. Animal breeders, veterinary workers, those engaged in handling and processing animal materials (leather, wool, etc.), get infected by diseased animals (anthrax, brucellosis, etc.). These diseases can thus be occupational. Besides, factors decreasing resistance of people (hard labour, overcrowded dwellings, cooling and other debilitating factors) can also promote spread of infection.
Migration of population during social conflicts (famine, war), disasters, such as earthquake, flood, or fires, that are associated with destruction of dwellings and worsening of the living conditions and cause mass-scale migration of the victims, intensify the epidemic spread of infectious diseases, that previously occurred as single cases.
Classification of Infectious Diseases
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In the 19th century, infectious diseases were classed as contagious (transmissible from person to person), miasmatic (transmitted through air), and contagious-miasmatic. Late in the 19th century, in view of advances made in bacteriology, the diseases were classified according to their aetiology. These classifications could not satisfy clinicians or epidemiologists since diseases with different pathogenesis, clinical course and epidemiologic characteristics were united in one group. Classifications based on clinical and epidemiologic signs proved ineffective too.
The classification proposed by Gromashevsky seems to be more reasonable than many others. It is based on the location of infection in the macroorganism. In accordance with the main sign, that determines the transmission mechanism, all infectious diseases are divided by the author into four groups: (1) intestinal infections; (2) respiratory infections; (3) blood infections; (4) skin infections. According to Gromashevsky, each group is subdivided into anthroponoses and zoonoses; their epidemiology and prevention differ substantially.
Intestinal infections. Intestinal infections are characterized by location of the causative agents in the intestine and their distribution in the environment with excrements. If the causative agent circulates in the blood (typhoid fever, paratyphoid A and B, leptospirosis, viral hepatitis, brucellosis, etc.), it can also be withdrawn through various organs of the body, e. g. the kidneys, lungs, the mammary glands.
As a microbe is released into the environment with faeces, urine, vomitus (cholera), it can cause disease in a healthy person only after ingestion with food or water. In other words, intestinal infections are characterized by the faecal-oral mechanism of transmission.
Maximum incidence of intestinal infections occurs usually during the warm seasons.
The anthroponoses include typhoid fever, paratyphoid, bacterial and amoebic dysentery, cholera, viral hepatitis A, poliomyelitis, helminthiasis (without the second host). The zoonoses include brucellosis, leptospirosis, salmonellosis, botulism, etc.
The main means of control of intestinal infection are sanitary measures that prevent possible transmission of the pathogenic microorganisms with food, water, insects, soiled hands, etc. Timely detection of the diseased and carriers, their removal from food catering and the like establishments is also very important.
Specific immunization is only of secondary importance in intestinal infections.
Respiratory infections. This group includes diseases whose causative agents parasitize on the respiratory mucosa and are liberated into the environment with droplets of sputum during sneezing, cough, loud talks, or noisy respiration.
People get infected when the microbes contained in sputum get on the mucosa of the upper airways. If the causative agent is unstable in the environment, a person can only be infected by lose contact with the sick or carrier (pertussis).
Pathogenic microorganisms causing some diseases can persist for a period of time in an enclosure where the sick is present. Infected particles of sputum or mucus can dry and be suspended in the air. Some diseases of this group can spread through contaminated linen, underwear, utensils, toys, etc.
Since susceptibility of people, and especially of children to respiratory infection is very high, and since the infection is easily transmitted from the diseased (or carriers) to healthy people, almost entire population of a given area usually gets infected, and some people can be infected several times. Some diseases of this group form a special subgroup of children’s infections (diphtheria, scarlet fever, measles, pertussis, epidemic parotitis, chickenpox, rubella). A durable immunity is usually induced in children who sustained these diseases. The main measure to control respiratory infections is to increase non-susceptibility of population, especially of children, by specific immunization.
It is important to timely reveal the sick and carriers, and also to break the mechanism of infection transmission: control of overcrowding, proper ventilation and isolation of enclosures, using UV-lamps, wearing masks, respirators, disinfection, and the like.
Blood infections. The diseases of this group are transmitted by blood-sucking insects, such as fleas, mosquitoes, ticks, etc., which bite people and introduce the pathogenic agent into the blood.
Tick-bome encephalitis, Japanese В encephalitis and some other infections are characterized by natural nidality which is due to specific geographic, climatic, soil and other conditions of infection transmission. The morbidity is the highest during the warm season which coincides with the maximum activity of the transmitters-ticks, mosquitoes, etc.
Control of blood infections includes altering natural conditions, improvement of soils, draining swamps, destroying sites where the insects multiply, disinsection measures against mosquitoes, ticks, etc., detoxication of sources of infection by their isolation and treatment, carrying out preventive measures.
If the source of infection are rodents, measures to control them are taken.
Active immunization is also effective.
Skin infections. The diseases of this group occur as a result of contamination of the skin or mucosa with the pathogenic microorganisms. They can remain at the portal of infection (tetanus, dermatomycoses), or affect the skin, enter the body and be carried to various organs and tissues with the circulating blood (erysipelas, anthrax). The transmitting factors can include bed linen, clothes, plates and dishes and other utensils, that can be contaminated with mucus, pus or scales. Pathogenic microorganisms causing venereal diseases, rabies, AIDS, and some other diseases are transmitted without the agency of the environmental objects. Wound infections are characterized by damage to the skin as a result of injury (tetanus, erysipelas).
The main measures to control skin infections include isolation and treatment of the source of infection, killing diseased animals, homeless dogs and cats, improving sanitation and living conditions of population, personal hygiene, control of traumatism, and specific prophylaxis.
