Concept of Infectious Disease Epidemiology, Chain of Infection and Sources and Reservoir
Communicable diseases are transmitted from the reservoir, source of infection to susceptible host. Due to rapid travel and transport, reintroduction of infectious diseases occurs not only locally but also humans, insects, livestock and food products carry infectious agents from country to country and from continent to continent. iInfectious agents carried by humans are often asymptomatic or in the incubation period whereas those carried by vectors, livestock and food are often silent. Knowledge of the portals of exit and entry, and modes of transmission provide a basis for determining appropriate control measures. Control measures are directed against the segment in the infection chain that is most susceptible to intervention. In the hospital, patients are treated or isolated to prevent transmission where as in the community soil may be decontaminated or covered to prevent the escape of the agent. Source of infection is defined as the person, animal, objects or substances from which an infectious agent passes or is disseminated to the host. A reservoir is defined as any person, animal, arthropod, plant, soil or substance in which an infectious agent lives, grows and multiplies on which it depends primarily for survival and where it reproduces itself in such manner that it can be transmitted to a susceptible host. Reservoir is the natural habitat in which the organism metabolizes and replicates
Summary
Communicable diseases are transmitted from the reservoir, source of infection to susceptible host. Due to rapid travel and transport, reintroduction of infectious diseases occurs not only locally but also humans, insects, livestock and food products carry infectious agents from country to country and from continent to continent. iInfectious agents carried by humans are often asymptomatic or in the incubation period whereas those carried by vectors, livestock and food are often silent. Knowledge of the portals of exit and entry, and modes of transmission provide a basis for determining appropriate control measures. Control measures are directed against the segment in the infection chain that is most susceptible to intervention. In the hospital, patients are treated or isolated to prevent transmission where as in the community soil may be decontaminated or covered to prevent the escape of the agent. Source of infection is defined as the person, animal, objects or substances from which an infectious agent passes or is disseminated to the host. A reservoir is defined as any person, animal, arthropod, plant, soil or substance in which an infectious agent lives, grows and multiplies on which it depends primarily for survival and where it reproduces itself in such manner that it can be transmitted to a susceptible host. Reservoir is the natural habitat in which the organism metabolizes and replicates
Things to Remember
ses of infectious disease epidemiology
1. Identification of causes of new emerging infections e.g. HIV, SARS
2. Surveillance of infectious disease
3. Identification of source of outbreaks
4. Studies of routes of transmission and natural history of infections
5. Identification of new interventions
Based on the transmission of disease, the cases are classified as follows:
Index case : the first case identified
Primary case: the case that brings the infection into the population
Secondary case: a case infected by a primary case
Tertiary case: a case infected by a secondary case
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Concept of Infectious Disease Epidemiology, Chain of Infection and Sources and Reservoir
Concept of Infectious Disease Epidemiology
Communicable diseases are transmitted from the reservoir, source of infection to susceptible host. Due to rapid travel and transport, reintroduction of infectious diseases occurs not only locally but also humans, insects, livestock and food products carry infectious agents from country to country and from continent to continent. iInfectious agents carried by humans are often asymptomatic or in the incubation period whereas those carried by vectors, livestock and food are often silent. Knowledge of the portals of exit and entry, and modes of transmission provide a basis for determining appropriate control measures. Control measures are directed against the segment in the infection chain that is most susceptible to intervention. In the hospital, patients are treated or isolated to prevent transmission where as in the community soil may be decontaminated or covered to prevent theescape of the agent.
Uses of infectious disease epidemiology
1. Identification of causes of new emerging infections e.g. HIV, SARS
2. Surveillance of infectious disease
3. Identification of source of outbreaks
4. Studies of routes of transmission and natural history of infections
5. Identification of new interventions
| Term | Definitions |
| Acute disease | Disease in which symptoms develop rapidly and that runs its course quickly |
| Chronic disease | Disease with usually mild symptoms that develop slowly and last a long time |
| Sub-acute disease | Disease with time course and symptoms between acute and chronic |
| Asymptomaticdisease | Disease without symptoms |
| Latent disease | Disease that appears after along time of infection |
| Communicable disease | Disease transmitted from one host to another |
| Contagious disease | Communicable disease that easily spreads |
| Non-communicabledisease | Disease arising from out site of host or from opportunistic pathogen |
| Local infection | Infections confine to a small region of the body |
| Systemic infection | Widespread infections in many systems of the body often travel in the blood and lymph |
| Focal infection | Infection that serves as source of pathogen for infection at other sites in the body |
| Primary infection | Initial infection within a given patient |
| Secondary infection | Infection that follow a primary infection often by opportunisticpathogens |
Table. Terms used to classify infectious diseases
Chain of Infection
In this processof transmission of communicable diseases, etiological agent, transmission factors or agents and host are linked in a chain.
Fig. Visceral leishmaniasis transmission cycle
Etiological agent → Transmission → Host
(Reservoir, source, (contact, common (Individual host factors,
pathogenicity, vehicle, airborne, nonspecificresistance,
infection dose) vector-borne) specific immunity)
Sources and Reservoir
Source of infection is defined as the person, animal, objects or substances from which an infectious agent passes or is disseminated to the host.
A reservoir is defined as any person, animal, arthropod, plant, soil or substance in which an infectious agentlives, grows and multiplies on which it depends primarily for survival and where it reproduces itself in such manner that it can be transmitted to a susceptible host. Reservoir is the natural habitat in which the organism metabolizes and replicates. For example, in typhoid fever, the reservoir of infection is human but the source of infection may be feces or urine of patients or contaminated food or water. Similarly, the reservoir ofClostridium botulinumspores in soil, but the source of most botulism intoxications is improperly canned food containingC. botulinumspores and a neurotoxin released by the bacteria.
'Sources' is the immediate source of infection and may or may not be a part of areservoir. The reservoir may be homologous (man to man- cholera) or heterologous (other than men, animals, and birds).
There are three types of reservoirs:
1. Human reservoir
2. Animal reservoir
3. Reservoir in non-living things
1. Human reservoir
Man himself is the important source or reservoir of infection. He may be a case or carrier. Diseases that are transmitted from person to person without intermediates include the sexually transmitted diseases, measles, mumps, streptococcal infection, and many respiratory pathogens.
Cases
A case is defined as a person in the population or study group identified as having the particular disease, health disorder or conditionn under investigation. The cases are identified by clinical, biochemical, and other laboratory tests. the cases may be clinical, sub-clinical or latent on the basis of clinical signs and symptoms.
Clinical cases (illness)
It may be mild or moderate, typical or atypical, severe or fatal. Mild case may become source of infection because they are ambulant and spread the infection wherever they go but severe cases are confined to bed.
Sub-clinical cases
It may be inapparent, covert, missed or abortive cases. All are important as asource of infection because they don't manifest signs and symptoms. Sub-clinical infection can be detected by laboratory tests, recovery of anorganism, antibody response, and biochemical and skin sensitivity tests. In sub clinical infection, the case discharges infectious agent but in latent infection, the host does not shed infectious agent.
Based on the transmission of disease, the cases are classified as follows:
Index case :the first case identified
Primary case:the case that brings the infection into the population
Secondary case: a case infected by a primary case
Tertiary case:a case infected by a secondary case
Carriers
A carrier is defined as an infected person or animal that harbours a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for others. Epidemiologically, carriers are more infectious because they are not recognized and lives as normal in community. To be a carrier:
- There must be the presence of the disease agent in the body.
- Absence of recognizable signs and symptoms of disease.
- Shedding of the disease agent in the excretions.
Carrier may be classified as below:
A. By type
a. Incubatory
b. Convalescent
c. Healthy
B. By duration
a. Temporary
b. Chronic
C. By portal of exit
a. Urinary
b. Intestinal
c. Respiratory
d. Others
A. By type
a. Incubatory carriers
Incubatory carriers shed the infectious agent during the incubation period of diseases. This occurs during the last few days of the incubation period. e.g. measles
b. Convalescent carriers
These carriers continue to shed infectious agent during recovery (convalescence) period. Convalescence carrier can pose threat to the unprotected household. For example, in typhoid fever, patient may excrete bacilli for six to eight weeks.
c. Healthy carriers
These are sub-clinical cases who have developed carrier state without suffering from overt disease and shed the disease agent. eg.g. cholera, diphtheria, etc.
B. By duration
a. Temporary carriers
Temporary carriers are those who shed the infectious agent for ashort period of time incubatory, convalescent and healthy carriers are temporary carriers.
b. Chronic carriers
Chronic carriers excrete the infectious agent for anindefinite period. The longer the carrier state, the higher the risk to the community. Some chronic carriers excrete the infectious agent intermittently and some continuously . The duration of carrier state varies with disease e.g. typhoid fever.
C. By portal of exit
Carriers are also classified according to the portal of exit of the infectious agent. Therefore, there are the urinary carriers, intestinal carrier, nasal carrier etc.
2. Animal reservoir
The animal reservoir may also be case or carrier. The diseases, which are transmitted from animal to man, are called zoonotic diseases. e.g. rabies, yellow fever etc. Histoplasmosis is carried all over the world by birds. The genetic recombination between animal and human viruses might produce new strain of viruses (e.g. influenza virus).
3. Reservoir in non-living things
Soil and other inanimate things also act as reservoir of infection. For example, the causative agent of anthrax, tetanus, and coccidioidomycosis etc.
References
Gordis, L. Epidemiology. third edition. 2004.
Joshi, Banjara. Fundamentals of Epidemiology. Kathmandu: Quality Printing Press, 2007
Park, K. Park's Text Book of social and prevention Medicine. 18th edition. 2008.
Lesson
Methods of transmission of diseases
Subject
Microbiology
Grade
Bachelor of Science
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