- The starting point for the occurrence of a communicable disease is the existence of a reservoir or source of infection.
- The source of infection is defined as “the person, animal, object or substance from which an infectious agent passes or is disseminated to the host (immediate source).”
- The reservoir is defined as any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host.
- Simply, it is the natural habitat of the infectious agent in which it metabolizes and replicates.
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Source of Infection
- We refer to the source of infection as the origin from which a host acquires the infection, either endogenous (i.e. originating from a person’s own commensal microbial flora) or exogenous (i.e. an individual, animal or object that in the external environment of the host).
- Usually, the source can be identified as an individual, animal or object in a specific place, and at a specific time.
- Thus, a person can be a source of infection; either for him/her self (endogenous) or to other people (directly through personal contact, or indirectly, e.g. by contaminating food or beverages).
- In addition to people, also animals can be sources of infection.
- Objects may be sources of infection; food, water, air-conditioning systems, showers, medical instruments, recreational waters, door knobs, cotton handkerchiefs, etc.
- Most man-made products that may be sources of infection are required to be produced while limiting the risk of contamination.
- In most outbreak investigations, the principal objective is to identify the source of the infection.
- Interestingly enough this sometimes leads to semantic problems: an identified ‘source’ (e.g. a chocolate cake) is usually contaminated by some other source (e.g. the baker of the cake or the eggs used).
- Tracing back such a ‘chain of transmission’ usually leads back to the reservoir.
- In a number of articles, the concept of ‘source’ and ‘reservoir’ are used as synonyms, though strictly speaking, they are not.
Source vs Reservoir of Infection
- The terms reservoir and source are not always synonymous.
- For example, in hookworm infection, the reservoir is man, but the source of infection is the soil contaminated with infective larvae.
- In tetanus; the reservoir and source are the same that is soil.
- In typhoid fever, the reservoir of infection may be a case or carrier, but the source of infection may be feces or urine of patients or contaminated food, milk or water.
- Thus the term “source” refers to the immediate source of infection and may or may not be a part of the reservoir.
Types of Reservoirs
The reservoir of infection may be of three types:
- Human reservoir
- Animal reservoir, and
- Reservoir in non-living things.
- By far the most important source or reservoir of infection for humans is the man himself.
- He may be a case or carrier.
- A case is defined as “a person in the population or study group identified as having the particular disease, health disorder or condition under investigation”.
- A variety of criteria (e.g., clinical, biochemical, laboratory) may be used to identify cases.
- Broadly, the presence of infection in a host may be clinical, subclinical or latent.
- The clinical illness may be mild or moderate, typical or atypical, severe or fatal depending upon the gradient of involvement.
- In epidemiological terminology, the term primary case refers to the first case of a communicable disease introduced into the population unit being studied.
- The term index case refers to the first case to come to the attention of the investigator; it is not always the primary case.
- Secondary cases are those developing from contact with primary case.
- A suspect case is an individual (or a group of individuals) who has all of the signs and symptoms of a disease or condition, yet has not been diagnosed as having the disease or had the cause of the symptoms connected to the suspected pathogen.
- Whatever may be the “gradient of infection”, all infected persons, whether clinical or subclinical, are potential sources of infection, because the disease agent is leaving the body through frequent stools, vomiting, and coughing, sneezing or other means and is potentially available for transfer to a new host.
- In some diseases, either due to inadequate treatment or immune response, the disease agent is not completely eliminated, leading to a carrier state.
- A carrier is defined as “an infected person or animal that harbors a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for others”.
- As rule carriers are less infectious than cases, but epidemiologically, they are more dangerous than cases because they escape recognition, and continuing as they do to live a normal life among the population or community, they readily infect the susceptible individuals over a wider area and longer period of time, under favorable conditions.
- The “Typhoid Mary” is a classic example of a carrier.
Carriers may be classified as:
(a) lncubatory: Incubatory carriers are those who shed the infectious agent during the incubation period of disease. That is, they are capable of infecting others before the onset of illness. This usually occurs during the last few days of the incubation period, e.g., measles, mumps, polio, pertussis, influenza, diphtheria and hepatitis B.
(b) Convalescent: That is, those who continue to shed the disease agent during the period of convalescence, e.g., typhoid fever, dysentery (bacillary and amoebic), cholera, diphtheria and whooping cough.
(c) Healthy: Healthy carriers emerge from subclinical cases. They are victims of subclinical infection who have developed carrier state without suffering from overt disease, but are nevertheless shedding the disease agent, e.g., poliomyelitis, cholera, meningococcal meningitis, salmonellosis, and diphtheria.
(a) Temporary: Temporary carriers are those who shed the infectious agent for short periods of time. In this category may be included the incubatory, convalescent and healthy carriers.
(b) Chronic: A chronic carrier is one who excretes the infectious agent for indefinite periods. Chronic carrier state occurs in a number of diseases, e.g., typhoid fever, hepatitis B, dysentery, cerebra-spinal meningitis, malaria, gonorrhoea, etc. Chronic carriers are far more important sources of infection than cases.
III. Portal of Exit
- The source of infection may sometimes be animals and birds.
- These, like the human sources of infection, maybe cases or carriers.
- The diseases and infections which are transmissible to man from vertebrates are called zoonoses. There are over 100 zoonotic diseases which may be conveyed to man from animals and birds.
- The best known examples are rabies, yellow fever and influenza. The role of pigs and ducks in the spread of epidemic and pandemic influenza both as reservoirs, carriers and “amplifying hosts” is now well established.
- The migrations and movements of animals and birds may carry serious epizootiological and epidemiological risks.
- There is evidence that genetic recombination between animal and human viruses might produce “new” strains of viruses (e.g., influenza viruses).
Reservoir in non-living things
- Soil and inanimate matter can also act as reservoirs of infection.
- For example, the soil may harbour agents that cause tetanus, anthrax, coccidioidomycosis, and mycetoma.
- Park, K. (n.d.). Park’s textbook of preventive and social medicine.
- Beaglehole, Robert, Bonita, Ruth, Kjellström, Tord & World Health Organization. (1993). Basic epidemiology, Updated reprint. World Health Organization.
- Hennekens CH, Buring JE. Epidemiology in Medicine, Lippincott Williams & Wilkins, 1987.