Describe the investigation of an epidemic.

Investigation of an Epidemic

• The related terms — endemic, the constant presence of a disease in an area; hyperendemic, persistently at a high level; sporadic, scattered and irregular cases; pandemic, an epidemic crossing international boundaries and affecting a large number of people; and epizootic and zoonosis, in animals and transmissible to man
• The objectives of investigating an outbreak are two — to control the present outbreak, and to prevent future ones by understanding what happened
• The steps follow a standard sequence, and reciting it in order is the answer:
• 1. Verify the diagnosis — by clinical examination and laboratory confirmation of a sample of cases. Everything that follows depends on this, and acting on a wrong diagnosis wastes the whole effort
• 2. Confirm the existence of an epidemic — by comparing the current number with the expected number for that time, place and season, using past records
• 3. Define the population AT risk — obtain a map, the population figures and the denominator
• 4. Rapid search for all cases — using a case definition applied consistently, which should be sensitive and broad at first (to find all cases) and may be tightened later; house-to-house search where necessary
• 5. Data analysis by time, place and person — construct the epidemic curve, a spot map, and attack rates by age, sex and other attributes
• The epidemic curve indicates whether the source is a point source, a continuous source or propagated, and allows the probable time of exposure to be estimated by counting back one incubation period from the peak
• 6. Formulate a hypothesis about the source, the mode of transmission and the environmental factors
• 7. Test the hypothesis — typically by comparing attack rates among those exposed and not exposed to each suspected vehicle. In a food-borne outbreak, the food-specific attack rate table identifies the vehicle: the culprit has a high attack rate among those who ate it and a low rate among those who did not, and the difference between them should be the largest
• 8. Evaluate the ecological factors — water supply, sanitation, food handling, vectors, housing, and any recent breakdown or change
• 9. Further investigation of the population at risk — serological surveys, environmental sampling and testing of food handlers or contacts
• 10. Write the report — a full record with recommendations, without which nothing is learned for the future
• Control measures are not the last step — they begin immediately and run IN parallel with the investigation. This is the point most often got wrong in answers: one does not wait for the analysis to be complete before acting
• The control measures are directed at the three links of the chain:
• Control of the reservoir or source — early diagnosis, notification, isolation, treatment, disinfection, and removal of the contaminated food or water source
• Interruption of transmission — chlorination of water, safe disposal of excreta, food hygiene, vector control, hand hygiene and personal protection
• Protection of the susceptible host — immunisation (active or passive), chemoprophylaxis, and improved nutrition
• Also essential — notification to health authorities; health education of the community, without which measures are not adopted; arrangements for treatment of cases; and continued surveillance until the outbreak is over, defined as two incubation periods passing with no new case
• The commonest outbreaks in the Indian setting — cholera and other diarrhoeal disease, typhoid, hepatitis A and E, food poisoning, measles, dengue and chikungunya, and viral encephalitis
• Control measures begin on day one, in parallel with the investigation rather than after it
• Count back one incubation period from the peak to estimate when exposure probably occurred
• Verify the diagnosis before anything else, since every subsequent step depends on it being right
• An epidemic is defined relative to the expected, so one case of polio is an epidemic and a thousand colds may not be
• Start with a broad sensitive case definition to find all cases, and tighten it later for analysis
• The food-specific attack rate table finds the vehicle, which shows a high rate among eaters and a low rate among non-eaters
• The largest difference between the two rates identifies the culprit food more reliably than the highest rate alone
• A spot map may reveal the source, as it did for Snow and the Broad Street pump
• An outbreak is over after two incubation periods without a new case, and surveillance continues until then
• Health education is part of control, since measures that are not understood are not adopted
• Notify the health authority promptly, which is both a legal obligation and the trigger for wider response
• Write the report, without which nothing is learned and the next outbreak repeats the same course
• Attack the reservoir, the transmission and the host together, since measures against one link alone are rarely enough