Investigation of an epidemic

The emergence of an epidemic consistently indicates a significant shift in the prevailing equilibrium among the agent, host, and environment. It necessitates a prompt and comprehensive investigation of cases to identify the factor(s) responsible and guide the implementation of control measures to prevent further spread. Emergencies triggered by epidemics pose a significant challenge to national health administrations. Epidemiology plays a crucial role in the investigation of epidemics.

Objectives of an epidemic investigation

1. To define the magnitude of the epidemic outbreak or involvement in terms of time, place and person.
2. To determine the particular conditions and factors responsible for the occurrence of the epidemic.
3.  To identify the cause, source(s) of infection, and modes of transmission to determine measures necessary to control the epidemic
4. To make recommendations to prevent recurrence.

Steps of an epidemic investigation

1. Verification of diagnosis

Verification of diagnosis constitutes the initial crucial step in an epidemic investigation. Occasionally, reports may be spurious, stemming from misinterpretation of signs and symptoms by the general public. Therefore, swift on-site verification of the diagnosis is imperative. It is not mandatory to examine every case exhaustively for diagnosis; a clinical examination of a representative sample can often suffice. While laboratory investigations are highly valuable for confirming diagnoses, epidemiological investigations should not be postponed until laboratory results are available.

2. Confirmation of the existence of an epidemic

The subsequent step involves confirming the presence of an epidemic, typically achieved by comparing disease frequencies with those of previous years during the same period. An epidemic is considered to be present when the number of cases (observed frequency) surpasses the expected frequency for that population, based on historical data. A commonly employed criterion is the identification of cases exceeding two standard deviations from the endemic occurrence, particularly for diseases like influenza. However, certain epidemics, such as common-source outbreaks of cholera, food poisoning, and hepatitis A, are readily identifiable without such comparisons. These types of epidemics exhibit obvious patterns. In contrast the existence of modern epidemics (e.g., cancer, cardiovascular diseases) is not easily recognized unless comparison is made with previous experience.

3. Defining the population at-risk

• Obtaining a map of the area: Before commencing the investigation, it is imperative to obtain a detailed and up-to-date map of the region. In the absence of such a map, it may be necessary to create one, incorporating information on natural landmarks, roads, and the precise locations of all residential units along each road or in remote areas. The area can be subdivided into segments using natural landmarks as boundaries, and these segments may be further divided into smaller sections. Each section can then be assigned numerical designations for individual dwelling units (houses).
• Population enumeration: The denominator in epidemiological investigations may pertain to the entire population or specific subgroups. It could also be associated with total events. For instance, if the denominator represents the entire population, a comprehensive census of the population, categorized by age and sex, must be conducted in the defined area through house-to-house visits. To facilitate this task, a sufficient number of lay health workers may be employed. This approach allows for the determination of the population size. The population census is crucial for later computation of essential attack rates within various groups and subgroups. Without an appropriate denominator representing the “population at risk,” it is not possible to calculate accurate attack rates.

4. Rapid search for all cases and their characteristics

• Medical survey: Simultaneously, conduct a comprehensive medical survey within the defined area to identify all cases, including those who have not sought medical care and individuals potentially exposed to the risk. Ideally, a complete survey, involving the screening of each member of the population for symptoms or signs of the specific disease, should be undertaken to capture all affected individuals. Lay health workers can be trained to administer an “epidemiological case sheet” or questionnaire for data collection.
• Epidemiological case sheet: Equip the epidemiologist with an “epidemiological case sheet” to gather data from confirmed cases and individuals apparently exposed but unaffected. The case sheet, also known as a “case interview form,” should be meticulously designed based on the initial findings of a rapid preliminary inquiry. It should include essential information such as name, age, sex, occupation, social class, travel history, previous exposure, time of disease onset, signs and symptoms, personal contacts at home, work, school, and other locations, as well as details about special events, such as attended parties, consumed foods, and exposure to common vehicles like water, food, and milk. The collected information should be relevant to the studied disease. For instance, in the case of a food-borne illness, detailed food histories are crucial. A case review form ensures the completeness and consistency of data collection. In situations where the outbreak is extensive, and interviewing all cases is not feasible (e.g., influenza), a random sample should be examined, and data should be collected.
• Search for additional cases: Inquire with patients about other cases they may be aware of in their home, family, neighborhood, school, or workplace, particularly those with an onset within the incubation period of the index case. Consider cases admitted to local hospitals, as this may unveil not only additional cases but also evidence of person-to-person spread. The search for new cases (secondary cases) should be conducted daily until the area is declared free of the epidemic. This period is typically twice the incubation period of the disease since the occurrence of the last case.

5. Data analysis

The collected data should be continuously analyzed utilizing classical epidemiological parameters, focusing on time, place, and person. If the disease agent is known, these characteristics can be organized into the Agent-Host-Environment model.

• Time: Examine a chronological distribution of onset dates and construct an “epidemic curve.” Identify any temporal clustering of cases. An epidemic curve may indicate: (a) a temporal relationship with exposure to a suspected source, (b) whether it is a common-source or propagated epidemic, and (c) whether a seasonal or cyclic pattern suggests a specific infection.
• Place: Develop a “spot map” illustrating the geographic distribution of cases, and if possible, their correlation with potential sources of infection (e.g., water supply, air pollution, foods eaten, occupation). Clustering of cases may indicate a common source of infection. Analyzing geographic distribution can offer insights into the disease source and its mode of spread.
• Person: Analyze the data by age, sex, occupation, and other potential risk factors. Determine attack rates/case fatality rates for both exposed and unexposed individuals and according to host factors. For instance, in most food-borne outbreaks, calculate food-specific attack rates for each consumed item to pinpoint the source of infection.

The objective of data analysis is to identify common events or experiences and delineate the groups involved in these shared experiences.

6. Formulation of hypotheses

Based on the distribution of time, place, and person or the Agent-Host-Environment model, formulate hypotheses to explain the epidemic in terms of (a) a possible source, (b) causative agent, (c) possible modes of spread, and (d) environmental factors contributing to its occurrence. Arrange these hypotheses in order of relative likelihood. The development of tentative hypotheses should guide further investigation.

7. Testing of hypotheses

Evaluate all reasonable hypotheses by comparing attack rates in various groups exposed and unexposed to each suspected factor. This analysis helps determine which hypothesis aligns with all known facts. When multiple theories are presented, it may be challenging to immediately distinguish between sound and merely plausible ones. Turning back to hypotheses tested against the subsequent course of events can provide valuable insights.

8. Evaluation of ecological factors

Conduct an investigation into the circumstances involved to implement measures preventing further disease transmission. Examine ecological factors contributing to the epidemic, such as the sanitary status of eating establishments, water and milk supply, disruptions in the water supply system, human population movements, atmospheric changes (temperature, humidity, air pollution), and population dynamics of insects and animal reservoirs. Utilize a case-control approach to relate the disease to environmental factors, identifying sources of infection, reservoirs, and modes of transmission.

9. Further investigation of population at risk

Study the population at risk or a sample to gather additional information through medical examinations, screening tests, examination of suspected food, feces or blood samples, biochemical studies, and assessment of immunity status. The approach may be retrospective or prospective. For example, a serological study can reveal clinically inapparent cases and provide insights into the pathogenesis of the condition. Healthy individuals from the same universe can be studied in a case-control fashion, classifying them based on exposure to specific potential vehicles and whether they are ill or not.

10. Writing the report

Compose a comprehensive and persuasive report. Include the following information in the final report on the epidemic:

Information to be included in the final report on an epidemic

At the onset of an epidemic, it may be essential to institute temporary control measures based on the established facts of the disease. These measures can be adjusted or replaced as new insights are gained through the ongoing epidemic investigation. As noted by Frost (156), an epidemiological investigation transcends the mere collection of established facts; it involves the systematic organization of these facts into chains of inference, extending more or less beyond the bounds of direct observation.

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Sources: Park, K. (2021). Park’s Textbook of Preventive and Social Medicine (26th ed.). Bhanot Publishers.