INVESTIGATION OF AN OUTBREAK

Dr. Khaled Dhaifullah Al GhaidanyConsultant family physician

Pattern of spread of infectious diseases

Epidemic: the occurrence of an unusually increased number of cases within limited time and the cases can be traced to a common source

Outbreak: localized small epidemic in a closed or confirmed community in camps, schools, ..etc

Pandemic: large expanding epidemic involving different adjacent countries e.g. Influenza

Endemic: Constant presence of a disease in the locality e.g. Schistosomiasis in Egypt

Sporadic: Occurrence of scattered unrelated cases irregularly distributed in time, place, person e.g. poliomyelitis

Zoonosis: Infectious disease transmitted under natural condition from intermediate animal to man e.g. Brucellosis

Epizootic: Epidemic or outbreak in animals e.g. anthrax

Enzootic: Endemic disease among animals e.g. brucellosis

Exzootic: imported disease of animals

Steps of investigation

1. Verify the diagnosis2. Establish the existence of an outbreak3. Identify and count cases4. Perform descriptive epidemiology5. Develop hypothesis6. Evaluate the hypothesis7. Implement control and preventive

measures8. Communicate the findings

Verify the diagnosis

To ensure that the problem has been properly diagnosed and to rule out laboratory errors.

Revision of clinical and lab. results A case definition must be based on clinical

criteria with restriction by time, place, and person

A doctor called to an “outbreak of food poisoning” is unwise to begin by collecting information about recent meals before confirming that the clinical history is compatible with the diagnosis

Establish the existence of an outbreak

Avoid pseudo-epidemics: 1. Sudden increase in a doctor’s

awareness of a disease 2. Improvement of reporting system3. Increased notification rate A single case of botulism is

considered as an outbreak needing investigation

Identify and count cases

Once a case definition is developed, start case finding from all possible sources by active or passive methods

Take a history from each confirmed or suspected cases

Clinical examination may be conducted to detect minor or modified forms of the disease

Laboratory investigation may be needed

Perform descriptive epidemiology

Time: Draw epidemic curve (histogram: No. of cases

by their date of onset) to identify the date of exposure to the source of infection, and the epidemic pattern (e.g. common source or propagated)

Place:Draw the spot map ( where cases live, work, or

may be exposed). It may provide important etiologic clues.

Person: collect personal data (age, sex, marital status,

occupation, leisure activity, use of medications, tobacco and drugs

Develop hypothesis

From the revision of descriptive epidemiology a hypothesis can be generated.

It should address the following:1- source of the agent2- mode of transmission3- the causative exposure

Evaluate the hypothesis

It can be evaluated by comparing it with the established facts when clinical, laboratory, environmental, and/or epidemiological evidence obviously support it

If this is not the case, then analytic epidemiology is used to test the hypothesis through case control or cohort study.

Laboratory and environmental studies may be used to confirm the epidemiological findings

Implement control and preventive measures

Must be applied as soon as possible even if the source is not known yet.

Control measures are applied at the weak link in the chain of infection ( might be the agent, the host, or the reservoir)

Control measures: NotificationIsolationQuarantine