Q.2 - Outbreak Investigation in a Slum (Waterborne Disease)

i) Setup for Investigation of an Outbreak in a Slum (3 marks)

When the government contacts you to investigate the outbreak, you would set up the following:

Immediate actions on arrival:

Constitute an Rapid Response Team (RRT) - including an epidemiologist, microbiologist, clinician, sanitary inspector, and health worker
Secure and seal the suspected well (preliminary report already implicates it) - prevent further use for drinking/cooking
Collect water samples from the well for microbiological and chemical analysis
Collect clinical samples (stool, blood, vomit) from affected cases for laboratory analysis
Establish a case definition based on fever and neurological symptoms described in the preliminary report
Set up a temporary field hospital/treatment facility in the slum area
Initiate line listing of all cases - record name, age, sex, address, onset date, symptoms, exposure history

Operational setup:

Get a map of the slum - identify the well, households, drains, sewage disposal
Identify and train local health workers to assist in house-to-house survey
Coordinate with local authorities (municipal corporation, district health officer)
Establish surveillance system for daily reporting of new cases

ii) Type of Epidemiological Study (1 mark)

The most appropriate study for this outbreak investigation is a Retrospective Cohort Study (also called a Cross-sectional/cohort survey or Epidemiological field investigation).

Since the source is suspected to be a common source (well water), a cohort study comparing attack rates among:

Those who used the well vs. those who did not
Those who drank/cooked with well water vs. those who used other sources

This calculates attack rates and Relative Risk (RR) to confirm or refute the well as the source.

In resource-limited situations or when the population is defined, a case-control study may be used if the number of cases is small, comparing cases (diseased) with controls (healthy neighbours) in terms of well water usage.

iii) Steps for Conducting the Epidemiological Study (3 marks)

As per Park's Textbook of Preventive & Social Medicine, the steps of epidemic/outbreak investigation are:

Step 1 - Verification of Diagnosis Clinically examine a sample of cases. Collect laboratory specimens (stool, blood, CSF given neurological involvement) to confirm the causative agent. Do not delay field investigation for lab results.

Step 2 - Confirmation of Existence of an Epidemic Compare current case numbers with expected baseline (previous years' data for the same period). An epidemic exists when observed frequency exceeds expected frequency by more than 2 standard deviations.

Step 3 - Defining the Population at Risk

Obtain/prepare a detailed map of the slum
Conduct a house-to-house census to count the total population (gives the denominator for attack rates)
Identify sub-groups: those using the well vs. those not using it

Step 4 - Rapid Search for All Cases and Their Characteristics

Conduct a medical survey using house-to-house visits
Use a standardized epidemiological case sheet (case interview form) collecting: name, age, sex, occupation, date of onset, symptoms, exposure to well water, foods eaten, contacts
Search for additional unreported cases through hospital records and community informants
Continue case search daily until area is declared epidemic-free (2 incubation periods after last case)

Step 5 - Data Analysis (Time, Place, Person)

Time: Draw an epidemic curve (x-axis = date of onset, y-axis = number of cases) - a sharp peak suggests common-source outbreak
Place: Prepare a spot map of cases; check if they cluster around the implicated well
Person: Calculate attack rates by age, sex, water-use habits; compute food/water-specific attack rates

Step 6 - Formulation of Hypotheses Based on time-place-person data, hypothesize: (a) source = well water, (b) causative agent (likely waterborne pathogen - Vibrio cholerae, E. coli, etc. given the fever + neurological symptoms), (c) mode of spread = ingestion of contaminated water

Step 7 - Testing of Hypotheses Compare attack rates in those who used the well vs. those who did not. Perform chi-square test and calculate Relative Risk. Correlate with lab findings from water samples. If contamination confirmed, hypothesis is accepted.

Step 8 - Evaluation of Ecological Factors Inspect the well - check for: proximity to drains/latrines, depth, structural integrity, contamination sources (seepage, flooding). Assess sanitation and hygiene practices in the slum.

Step 9 - Institution of Control Measures

Close/disinfect the contaminated well (chlorination or temporary closure)
Provide safe alternative water supply (tankers)
Treat cases (ORS, specific antibiotics based on causative agent)
Health education on boiling water, hand hygiene

Step 10 - Preparation of Report Write a detailed outbreak investigation report with findings, conclusions, and recommendations for preventing recurrence.

iv) WHO Criteria for Declaring a PHEIC (3 marks)

Under the International Health Regulations (IHR) 2005, a Public Health Emergency of International Concern (PHEIC) is declared by the WHO Director-General when an event meets at least 2 of the following 4 criteria (assessed using the decision instrument in Annex 2 of IHR):

Criteria (any 2 of 4 must be satisfied):

Serious public health impact - The event has serious public health impact (based on the number of cases, case fatality rate, or unusual clinical presentation)
Unusual or unexpected - The event is unusual (unexpected in terms of nature of agent, source, or mode of transmission) or unexpected (occurring in an area, season, or population that is not typical)
Significant risk of international spread - There is a significant risk that the disease may spread across international borders (based on movement of people, animals, vectors, or goods)
Significant risk of international trade/travel restrictions - There is a significant risk of international travel or trade restrictions being applied

Additional criteria considered:

Information from State Parties has been received
The event is not fully explained by normal activity

PHEICs declared by WHO (named examples):

H1N1 Influenza Pandemic - 2009
Polio resurgence - 2014
Ebola Virus Disease (West Africa) - 2014
Zika Virus - 2016
Ebola (DRC) - 2019
COVID-19 - 2020
Mpox (Monkeypox) - 2022

Reference: Park's Textbook of Preventive and Social Medicine; Harrison's Principles of Internal Medicine 22E

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