Definitions & Epidemic Investigation - Park's Textbook of Preventive and Social Medicine

PART I: DEFINITIONS

1. EPIDEMIC

• The number of cases indicating an epidemic varies with the agent, size and type of population exposed, previous exposure history, and time and place of occurrence.
• Epidemicity is relative - it is judged against the usual frequency of the same disease in the same area, among the same population, at the same season.
• Even a single case of a disease long absent from a population, or a first invasion by a previously unrecognized disease, requires immediate reporting and full field investigation.
• Two cases of such a disease associated in time and place may be sufficient to constitute an epidemic.

2. ENDEMIC

• Example: Common cold is endemic because somebody always has one.
• Hyperendemic: Disease constantly present at a high incidence/prevalence and affects all age groups equally.
• Holoendemic: High level of infection beginning early in life, affecting most of the child population, leading to equilibrium where adults show less disease than children (e.g., malaria).
• An endemic disease, when conditions are favourable, may burst into an epidemic (e.g., hepatitis A, typhoid fever).

3. SPORADIC

• The cases are so few and separated widely in space and time that they show little or no connection with each other, nor a recognizable common source of infection.
• Examples: Tetanus, herpes zoster, meningococcal meningitis.
• A sporadic disease may become the starting point of an epidemic when conditions become favourable for spread.
• Many zoonotic diseases are characterized by sporadic transmission to humans.

4. PANDEMIC

• Only some pandemics cause severe disease in individuals or at a population level.
• Characteristics of an infectious agent for pandemic causation:
  1. The agent must be able to infect humans
  2. The agent must be able to cause disease in humans
  3. The agent must be able to spread easily from human to human
• Examples: Influenza pandemics, cholera pandemics, COVID-19 pandemic.

PART II: INVESTIGATION OF AN EPIDEMIC

• (a) Define the magnitude in terms of time, place, and person
• (b) Determine the conditions and factors responsible for occurrence
• (c) Identify the cause, source(s) of infection, and modes of transmission
• (d) Make recommendations to prevent recurrence

Note: Some steps can be done concurrently. The approach is applicable to almost any epidemic.

STEPS IN INVESTIGATION OF AN EPIDEMIC

Step 1: Verification of Diagnosis

• First and most important step.
• Reports may sometimes be spurious, arising from misinterpretation of signs and symptoms by the lay public.
• Verification must be done on the spot, as quickly as possible.
• Not necessary to examine ALL cases - a representative sample may suffice.
• Laboratory investigations are useful to confirm the diagnosis, but epidemiological investigations should not be delayed awaiting lab results.

Step 2: Confirmation of the Existence of an Epidemic

• Compare disease frequency during the same period of previous years (baseline data).
• An epidemic exists when observed frequency is in excess of expected frequency for that population based on past experience.
• An arbitrary limit of two standard errors from the endemic occurrence is used to define the epidemic threshold (e.g., influenza).
• Some epidemics are obvious (e.g., cholera, food poisoning, hepatitis A - common-source epidemics).
• Modern epidemics (cancer, cardiovascular disease) are less easily recognized without comparison.

Step 3: Defining the Population at Risk

• (a) Obtaining a map of the area: Prepare a detailed, current map with natural landmarks, roads, and location of all dwelling units; divide into segments and smaller sections with numbered dwelling units.
• (b) Counting the population: Conduct a complete census by age and sex via house-to-house visits, using lay health workers. This helps compute attack rates in various groups. Without an appropriate denominator ("population at risk"), attack rates cannot be calculated.

Step 4: Rapid Search for All Cases and Their Characteristics

• (a) Medical survey: Identify all cases including those who have not sought medical care, and those possibly exposed to risk. Complete screening picks up all affected individuals.
• (b) Epidemiological case sheet: Design a case interview form to collect: name, age, sex, occupation, social class, travel history, time of onset, signs and symptoms, contacts at home/work/school, special events (parties, foods eaten), exposure to common vehicles (water, food, milk), history of injections or blood products, etc.
• (c) Searching for more cases: Ask patients about other cases in home, family, neighbourhood, school, workplace. Search for secondary cases every day until the area is declared free of epidemic (usually taken as twice the incubation period since the last case).

Step 5: Data Analysis

• Time: Prepare a chronological distribution of onset dates and construct an "epidemic curve." Look for time clustering; the curve can indicate (a) time relationship with exposure to a suspected source, (b) whether common-source or propagated epidemic, (c) seasonal or cyclic pattern.
• Place: Prepare a "spot map" (geographic distribution) of cases and relate to possible sources (water supply, air pollution, occupation, etc.). Clustering indicates a common source.
• Person: Analyze by age, sex, occupation, and other risk factors. Determine attack rates/case fatality rates for those exposed vs. not exposed. In food-borne outbreaks, food-specific attack rates must be calculated for each food item eaten.

Step 6: Formulation of Hypotheses

• Based on time, place and person analysis, formulate hypotheses explaining the epidemic in terms of:
  • (a) Possible source
  • (b) Causative agent
  • (c) Possible modes of spread
  • (d) Environmental factors which enabled it to occur
• Place hypotheses in order of relative likelihood to guide further investigation.

Step 7: Testing of Hypotheses

• All reasonable hypotheses must be considered and weighed by comparing attack rates in various groups for those exposed vs. not exposed to each suspected factor.
• This determines which hypothesis is consistent with all known facts.
• The hypothesis is tested by the subsequent course of events.

Step 8: Evaluation of Ecological Factors

• Investigate circumstances to prevent further transmission.
• Examine ecological factors: sanitary status of eating establishments, water and milk supply, breakdown in water supply system, population movements, atmospheric changes (temperature, humidity, air pollution), population dynamics of insects and animal reservoirs.
• The outbreak can be studied in a case-control fashion.
• Primary concern: relating the disease to environmental factors to know source(s) of infection, reservoirs, and modes of transmission.

Step 9: Further Investigation of Population at Risk

• Additional information may be obtained by: medical examination, screening tests, examination of suspected food/faeces/blood samples, biochemical studies, assessment of immunity status.
• Approach may be retrospective or prospective.
• Serological studies may reveal clinically inapparent cases and illuminate pathogenesis.
• Classify all members as to (a) exposure to specific potential vehicles and (b) whether ill or not.

Step 10: Writing the Report

• The report should be complete and convincing.
• Contents of the final report include: background (geographic location, demographics, socioeconomic situation, health services), description of the epidemic, analytical data, conclusions, and recommendations to prevent recurrence.

Quick Summary Table

Real Epidemic Investigation Case Studies from Park's Textbook

CASE STUDY 1: John Snow and the Broad Street Pump - London Cholera Epidemic (1854)

Background

Method: The Spot Map

Key Epidemiological Findings

• Cases clustered tightly around the Broad Street pump
• The boundary lines of equal distance between the Broad Street pump and neighboring pumps coincided almost perfectly with the distribution of cases
• The Workhouse nearby (which had its own water supply) had very few cases despite being in the middle of the epidemic zone
• People who worked or lived near the Broad Street pump but obtained water from elsewhere were spared

Outcome

Park's comment: "It was by such a study (spot map of fatal cases), John Snow was able to focus attention on the common water pump in Broad Street as the source of infection... Snow was able to hypothesize that cholera was a water-borne disease, long before the birth of bacteriology."

CASE STUDY 2: Legionnaire's Disease - Philadelphia, USA (1976)

Background

Epidemiological Features

• Classified as a common-source, continuous or repeated exposure outbreak
• The outbreak continued beyond the range of one incubation period (unlike a point-source epidemic)
• There was no evidence of secondary cases among persons who had contact with ill persons - ruling out person-to-person (propagated) spread
• The source was ultimately traced to the air conditioning cooling tower of the hotel, from which contaminated aerosols were distributed

Significance

1. How to differentiate a continuous common-source epidemic from a point-source epidemic using the epidemic curve
2. The importance of ruling out person-to-person spread through careful contact tracing
3. The discovery of a previously unknown pathogen (Legionella) - a new disease entity

CASE STUDY 3: Maxcy's Investigation of Murine Typhus - USA (1920s)

Background

Method & Finding

• Maxcy plotted cases by their place of employment on a geographic map
• The analysis revealed that cases clustered in locations frequented by rodents (rats)
• This geographic pattern pointed strongly to a rodent reservoir and a rodent-associated vector (rat flea) as the transmission mechanism

Outcome

Park's comment: "It was by a spot map by 'place of employment' that Maxcy hypothesized a rodent reservoir for typhus fever in 1920s, which led to the discovery that typhus fever was not a single disease entity."

CASE STUDY 4: The AIDS Epidemic - Cluster Analysis (Early 1980s)

Background

Method & Finding

• Analysis of case clustering based on sexual contact revealed that cases were concentrated among men who had sex with men
• Clustering based on blood product use identified cases among haemophiliacs and blood transfusion recipients
• These two patterns of clustering provided the epidemiological clue that AIDS was an infectious disease transmitted through blood and sexual contact

Outcome

• An infectious (transmissible) agent
• Blood and sexual routes of transmission
• Target populations at highest risk

Park's comment: "Evidence of case clustering based on sexual contact or blood product use provided the clue that AIDS (Acquired Immune Deficiency Syndrome) was an infectious disease."

CASE STUDY 5: William Budd and Typhoid Fever - Rural England (1856)

Background

Method & Findings

• By careful observations of the pattern of cases in the community
• Budd concluded that the spread was by drinking water - not by miasma or sewer gas (which was the prevailing theory at the time)

Significance

• Water-borne transmission of typhoid
• Led to public demands for clean water supplies
• Contributed to the Public Health Act of 1875 in England

Summary: Epidemic Type vs. Classic Case Study

Key Epidemic Curve - Common-Source (Point-Source) Pattern

Outbreak Trigger Events (India - Park's Table)

Population Explosion, Demographic Trends in India & Control Measures

PART I: DEFINITION OF POPULATION EXPLOSION

"It required all of human history up to the year 1800 for the world population to reach one billion. The second billion came in 130 years (around 1930), the third billion in 30 years (around 1960), the fourth billion in 15 years (1974), the fifth billion in 12 years (1987), and the sixth billion in 12 years (1999)."

"Delay the first, postpone the second, and prevent the third" (referring to births)

PART II: DEMOGRAPHIC TRENDS IN INDIA

A. Population Growth

• India doubled from 238 million (1901) to 439 million in 60 years (1961)
• Doubled again to 846 million in only 30 years (1991)
• Crossed 1 billion on 11 May 2000
• At 1.4 billion (mid-2020), India supports 17.5% of the world's population on just 2.4% of the world's land area
• Projected to reach 1.53 billion by 2050 and surpass China as world's most populous country around 2027

B. Birth Rate and Death Rate Trends in India

• Southern states (Kerala, Tamil Nadu, Karnataka, Andhra Pradesh) have achieved TFR near or below replacement level (2.1)
• High-focus northern/central states (Bihar TFR 3.3, UP 3.1, Meghalaya 3.0) drive national population increase
• These high-fertility states are anticipated to contribute about 50% of the nation-wide increase in population

C. Age-Sex Composition (India, 2018)

• Population in age group 0-14 is showing decline - a positive demographic sign
• Proportion of elderly (60+) is increasing - will impose greater burden on health services
• India's young working-age population offers a potential "demographic dividend" if harnessed

D. Sex Ratio

• Haryana (843), Delhi (844), and Uttarakhand (840) have the most skewed sex ratios at birth
• Kerala (957) and Chhattisgarh (958) have the most balanced
• The low sex ratio at birth reflects sex-selective practices, posing a serious social and demographic problem

E. Density of Population

F. Urbanization

• Urban population (2019): 471.8 million (34.5%) - still predominantly rural
• Urbanization driven by: natural population growth + rural-to-urban migration (for employment, education, health)
• Major metros (2019): Mumbai (12.69 million), Delhi (10.93 million), Bengaluru (5.10 million)
• Continuous rural-urban migration constitutes a social crisis impairing quality of life

G. Literacy

• National literacy rate (2011): 74.04% (males 82.14%; females 65.46%)
• Highest literacy: Kerala 93.91%, Mizoram 91.58%
• Lowest: Bihar 63.82%, Arunachal Pradesh 66.95%
• Low female literacy in states like Rajasthan (52.66%) directly contributes to high fertility

H. Demographic Stage: Where is India?

PART III: MEASURES FOR CONTROL OF POPULATION EXPLOSION

A. DIRECT MEASURES (Family Planning / Contraception)

1. Spacing Methods (Temporary Methods)

• Condom - male barrier, also prevents STIs
• Diaphragm / Cervical cap - female barriers
• Spermicides

• Oral contraceptive pills (OCP) - combined estrogen-progestin; highly effective
• Progestin-only pills (Mini-pill)
• Injectable contraceptives (DMPA - Depo-Provera)
• Implants (sub-dermal progestin implants)

• Copper-T (Cu-T 380A) - highly effective, long-acting
• Hormonal IUDs (LNG-IUS)

• Rhythm method / Calendar method
• Lactational Amenorrhoea Method (LAM)
• Basal body temperature method
• Cervical mucus (Billings) method

2. Terminal / Permanent Methods (Sterilization)

• Vasectomy (conventional)
• No-scalpel vasectomy (NSV) - safer, more acceptable; currently promoted under National Family Welfare Programme

• Tubectomy/Tubal ligation (minilaparotomy - most common in India)
• Laparoscopic sterilization (using Falope rings or clips)
• Can be done as: interval procedure, post-partum, or concurrent with MTP

B. INDIRECT / SOCIOECONOMIC MEASURES

1. Education

• Universal primary and secondary education, especially for girls
• Population education at school and university level - making students aware of consequences of uncontrolled growth and benefits of a small family
• Low female literacy (e.g., Rajasthan 52.66%) is directly correlated with high TFR

2. Raising the Age of Marriage

• Prohibition of Child Marriage - legal minimum age: 18 for girls, 21 for boys
• Delayed marriage reduces reproductive span and total fertility

3. Improving Status of Women

• Employment opportunities for women
• Economic empowerment and financial independence
• Women with education and employment desire fewer children

4. Old Age Social Security

• One reason for large families: children as old age insurance
• State-supported old age security reduces dependence on children as economic assets

5. Economic Development

• Historically proven: countries that achieved economic development also achieved fertility decline
• "The best motivation is economic - a desire to improve standard of living"

6. Compulsory Education of Children

• Reduces child labor, raises cost of raising children, lowers desired family size

7. Mass Communication and IEC (Information, Education, Communication)

• Communication of small family norm through mass media (TV, radio, social media)
• Attitude surveys show >60% already have favorable attitudes to family planning
• The challenge is converting favorable attitudes into actual contraceptive use

8. Legislation

• Laws against sex determination (Pre-Conception and Pre-Natal Diagnostic Techniques Act - PC-PNDT)
• Child marriage prevention acts
• Incentives and disincentives for family size (controversial)

9. Voluntary Organizations

• Family Planning Association of India
• Family Planning Foundation of India
• Population Council of India
• Indian Medical Association, Indian Red Cross - all supplement government efforts

C. NATIONAL PROGRAMME (India's Family Welfare Programme)

• Renamed Family Welfare Programme (1977) to avoid coercive connotations
• Target-free approach adopted (1996) - reproductive health approach replacing numerical targets
• Current strategy: Reproductive and Child Health (RCH) approach
• Focus states: BIMARU states (Bihar, UP, MP, Rajasthan) + Jharkhand, Chhattisgarh, Uttarakhand, Assam, Odisha (highest TFR states)
• Two-child family norm as the campaign goal
• Demographic goal: achieve NRR = 1 (Net Reproduction Rate = 1, i.e., each woman replaces herself exactly)

Summary: Measures at a Glance