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Field Epidemiology: Reservoir Identification, House-to-House Survey, Epidemiological Case Sheet, Line Listing, Contact Tracing & Examination of Close Contacts

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1. Early Identification of the Reservoir of Disease

Definitions

• Source of infection: The person, animal, object or substance from which an infectious agent passes or is disseminated to the host.
• Reservoir of infection: Any person, animal, arthropod, plant, soil or substance (or combination) in which an infectious agent lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host. In short, it is the natural habitat where the organism metabolizes and replicates.

Note: Reservoir and source are not always synonymous. E.g., in typhoid, the reservoir is the case/carrier, but the source is contaminated food/water. In hookworm, the reservoir is man, but the source is contaminated soil.

Types of Reservoirs

Human Reservoir - Key Concepts

• Clinical cases: Mild/ambulant cases are epidemiologically more dangerous than severe ones because they spread infection while mobile.
• Subclinical (inapparent) cases: Do not appear in statistics but shed pathogen equally - dominant role in maintaining endemicity (e.g., polio, hepatitis A, rubella).
• Latent infection: Pathogen lies dormant without shedding (e.g., herpes simplex, Brill-Zinsser disease).
• Carriers: Shed pathogen without showing disease (healthy, incubatory, convalescent, or chronic carriers).

How to Identify the Reservoir Early

1. Early diagnosis - Rapid identification is the cornerstone of disease control. Laboratory confirmation (culture, serology, PCR) of index cases.
2. Notification - Statutory notification to local health authority triggers investigation.
3. Epidemiological investigation - Trace backward from the index case (first case to come to investigator's attention) to the primary case (first case introduced into the population) to identify the original source/reservoir.
4. Active case finding - House-to-house surveys to detect subclinical and unreported cases.

• Park's Textbook of Preventive and Social Medicine, p. 107-108

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2. House-to-House Survey

Purpose

• Detect clinical and subclinical cases missed by passive surveillance
• Estimate the true attack rate and disease burden
• Identify unreported/missed cases and carriers
• Map the geographic distribution of cases (spot map)
• Identify high-risk households and clusters
• Collect environmental samples (water, food)

Methodology

1. Define the area: Delineate the affected locality/ward/village.
2. Train field workers: Health workers, ANMs, ASHAs trained on case definitions and data collection tools.
3. Standardized questionnaire: Each household visited using a uniform pro-forma to record:
   • Name, age, sex, address of all members
   • Presence of illness, symptoms, date of onset
   • Vaccination status
   • Dietary/exposure history (for food/water-borne outbreaks)
   • Source of drinking water, sanitation status
4. Examination of contacts: All household members examined for signs/symptoms.
5. Sample collection: Specimens (stool, blood, throat swabs) from symptomatic AND suspected asymptomatic individuals.
6. Mapping: Cases plotted on a spot map to identify clusters and point vs. propagated source.

Outcome

• Generates a line list of all identified cases
• Fills epidemiological case sheets for each case
• Supports calculation of attack rates (age-specific, sex-specific, area-specific)

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3. Epidemiological Case Sheet (Investigation Pro-forma)

Standard Components

Importance

• Provides standardized data for descriptive epidemiology (time, place, person distribution)
• Used to generate line listing
• Helps establish the epidemic curve
• Identifies common exposures and risk factors
• Used for analytical epidemiology (case-control, cohort studies)

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4. Line Listing

Standard Columns in a Line List

Uses of Line Listing

• Spot trends: Identify clusters by time, place, or person
• Epidemic curve construction: Plot cases by date of onset
• Attack rate calculation: Cases per total exposed
• Hypothesis generation: Identify common exposure (same food, same water source, same event)
• Monitor outbreak progression: Updated daily during an active outbreak
• Contact tracing management: Track whether contacts have been followed up

The line list is a living document - updated continuously as new cases are found or lab results arrive.

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5. Examination of Close Contacts

Categories of Contacts

• Household contacts: Family members sharing the same dwelling
• School/workplace contacts: Classmates, colleagues
• Healthcare contacts: HCW exposed without PPE
• Social contacts: Persons who attended the same gathering/event

Objectives of Contact Examination

1. Early detection of secondary cases (including subclinical)
2. Interrupt transmission by treating or isolating infected contacts
3. Prophylaxis: Give post-exposure prophylaxis (e.g., rabies PEP, meningococcal chemoprophylaxis, TB preventive therapy)
4. Vaccination: Offer vaccine to susceptible contacts (ring vaccination)
5. Surveillance: Monitor contacts for development of symptoms during the maximum incubation period

Steps in Contact Examination

1. List all contacts from index case interview and house-to-house survey
2. Classify by exposure level: High-risk, medium-risk, low-risk
3. Clinical examination: Symptoms, signs of illness
4. Laboratory investigation: Targeted specimens based on disease (stool culture for cholera, throat swab for diphtheria, Mantoux for TB contacts, etc.)
5. Prophylaxis/vaccination as appropriate
6. Quarantine/home surveillance: High-risk contacts placed under observation for one maximum incubation period

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6. Contact Tracing

Principles

• Work backward from the case: Who exposed the patient? (Source tracing)
• Work forward from the case: Whom did the patient expose? (Secondary case finding)
• Prioritize based on period of communicability of the disease

Steps in Contact Tracing

• Obtain a detailed history of all contacts during the infectious period
• Identify location, duration, and nature of exposure for each contact

• Compile a complete contact list with name, address, relationship, date of last exposure

• Field workers visit or call each contact
• Inform them of potential exposure (without disclosing the source case's identity - confidentiality)

• Determine the nature and duration of exposure
• Classify as high/medium/low risk

• Examine contacts clinically
• Collect specimens for laboratory testing
• Record findings in the contact tracing form

• Treatment if found infected
• Chemoprophylaxis if indicated (e.g., rifampicin for meningococcal contacts, azithromycin for cholera household contacts)
• Vaccination (e.g., measles ring vaccination, rabies PEP)
• Quarantine/isolation for high-risk contacts during incubation period
• Health education: Teach contacts about symptoms to watch for and when to seek care

• Monitor all contacts until the maximum incubation period has passed
• If a contact develops symptoms, they become a new case and the cycle restarts

Contact Tracing by Disease Type

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Relationship Between All Steps - The Field Investigation Workflow

Notification of index case
        ↓
Early diagnosis + Case confirmation
        ↓
Epidemiological case sheet filled
        ↓
House-to-house survey → More cases found → Line listing updated
        ↓
Identify reservoir (human/animal/environment)
        ↓
Examine close contacts → Contact tracing
        ↓
Isolate cases | Treat cases | Prophylaxis for contacts
        ↓
Interrupt transmission → Control outbreak

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Key Points for Examinations (Community Medicine / PSM)

• The index case is the first to come to the investigator's attention; the primary case is the first actual case.
• Subclinical cases are epidemiologically more important than clinical cases in maintaining endemicity.
• Line listing is the foundation of descriptive epidemiology in an outbreak.
• Active case finding (house-to-house) detects far more cases than passive notification.
• Contact tracing works both backward (source) and forward (contacts at risk).
• The period for contact surveillance = one maximum incubation period from last exposure.
• Isolation applies to cases; quarantine applies to contacts - both aim to break the chain of transmission.