If the first link in the chain of causation (the disease agent) is deemed to be the weakest link, the most desirable control measure would be to eliminate the reservoir or source. Elimination of the reservoir may be easy with the animal reservoir (e.g., bovine tuberculosis, brucellosis) but not in humans; because the general measures of reservoir control comprise – early diagnosis, notification, isolation, treatment, quarantine, surveillance and disinfection - all directed to reduce the quantity of the agent available for dissemination.
Early diagnosis or identification
Rapid identification is the first step in the control of a communicable disease. It is the cornerstone on which the disease control is built. Prompt detection of cases (and carriers) and their treatment is like stamping out the "spark" rather than calling the fire brigade to put out the fire caused by the spark. Frequently, laboratory procedures may be required to confirm the diagnosis.
Early diagnosis is required for
treatment of patients
to study the time, place and person distribution (descriptive epidemiology)
Once an infectious disease has been detected or suspected, it should be notified to the local health authority who will in-turn initiate control measures – provision of medical care to patients in a hospital.
Certain diseases are statutorily notifiable. The list varies from country to country and even within the same country. The diseases considered to be serious menaces to public health and non-communicable diseases or conditions such as cancer, congenital defects, accidents etc. are included in the list of notifiable diseases.
Notification enables early detection of disease outbreaks, which in turn permits immediate action to be taken by the health authorities to control their spread.
Notification of infectious diseases is often made by the attending physician or head of the family but anyone, including the lay people (religious, political and administrative leaders, teachers and others) can report, even on suspicion. In all cases the diagnosis is verified by the local health authority.
Under the International Health Regulations (IHR), certain prescribed diseases are notified by the national health authority to WHO. These can be divided into:
Health administrations are required to notify to WHO, Geneva for any notification of communicable diseases under international surveillance and International Health Regulations.
An epidemiological investigation is called for whenever there is disease outbreak. Broadly the investigation covers the identification of the source of infection and of the factors influencing its spread in the community. These include geographical situation, climatic condition, social, cultural and behavioural patterns, the character of the agent, reservoir, the vectors and vehicles, and the susceptible host populations.
Definition: "separation for the period of communicability of infected persons or animals from others in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible, or who may spread the agent to others
In general, infections from human/animal sources can be controlled by physical isolation of the case or carrier and if necessary, treatment until free from infection, provided cases and carriers can be easily identified and carrier rates are low.
Purpose of Isolation: To protect the community by preventing transfer of infection from the reservoir to the possible susceptible hosts.
Type of isolation: varies with the mode of spread and severity of the disease.
high security isolation.
Type of isolation should be determined after assessing the relative risks to the patient and to others. Hospital isolation is better than home isolation. Isolation is difficult in rural areas. In some situations (e.g., cholera outbreaks) the entire village or rural community may have to be isolated. Isolation may also be achieved in some diseases by ‘ring immunization’.
Ring Immunization: ‘Encircling the infected persons with a barrier of immune persons through whom the infection is unable to spread’.
Duration of isolation: Determined by the duration communicability of the disease and the effect of chemotherapy on infectivity.
Isolation can help to control infectious diseases like diphtheria, cholera, streptococcal respiratory disease, pneumonic plague etc. However, in certain conditions isolation does not help like;
Subclinical infection and carrier state (e.g. polio, hepatitis A and typhoid fever)
Disease is infectious before it is diagnosed, (e.g. mumps)
To control diseases like leprosy, tuberculosis and STD physical isolation is replaced by chemical isolation
Reporting of cases after the disease has spread widely.
These limitations indicate that isolation which is a "barrier approach” to the prevention and control of infectious disease is not successful to a great extent and may give rise to a false sense of security. In modern day disease control, isolation is replaced by surveillance because of improvements in epidemiological and disease control technologies. Today, isolation is recommended only when the risk of transmission of the infection is exceptionally serious.
Many communicable diseases have been tamed by effective drugs.
Purpose of treatment: to kill the infectious agent when it is still in the reservoir i.e. before it is disseminated.
Action of Treatment:
Reduces communicability of the disease
Decreases duration of illness
Prevents development of secondary cases.
In some diseases (e.g. syphilis, tuberculosis, and Ieprosy), early diagnosis and treatment is of primary importance in interrupting transmission. Treatment is also extended to carriers. Types of treatment:
Mass treatment: Every individual in the community receives drugs irrespective of the presence of disease (e.g. trachoma). If the treatment is inadequate or inappropriate, it may induce drug resistance in the infectious agent and may frustrate attempts to control disease by chemotherapy.
Definition: "the limitation of freedom of movement of such persons or domestic animals exposed to communicable disease for a period of time not longer than the longest usual incubation period of the disease, in such manner as to prevent effective contact with those not so exposed". Quarantine measures are also "applied by a health authority to a ship, an aircraft, a train, road vehicle, and other means of transport or container, to prevent the spread of disease, reservoirs of disease or vectors of disease".
modified quarantine: e.g. a selective partial limitation of freedom of movement, such as exclusion of children from school
segregation: separation for special consideration, control of observation of some part of a group of persons (or domestic animals) from the others to facilitate control of a communicable disease. E.g. removal of susceptible children to homes of immune persons".
In contrast to isolation, quarantine applies to restrictions on the healthy contacts of infectious disease. Quarantine, is no longer used as a method of disease control, owing to better techniques of early diagnosis and treatment. It has been replaced by active surveillance.