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Point Source Epidemic - Park's PSM (Exam Notes)

Classification of Epidemics (Park)

• (a) Single exposure / "Point-source" epidemics
• (b) Continuous or multiple exposure epidemics

• (a) Person-to-person
• (b) Arthropod vector
• (c) Animal reservoir

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Point-Source Epidemic - Definition

The exposure to the disease agent is brief and essentially simultaneous, and the resultant cases all develop within one incubation period of the disease.

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Epidemic Curve - Point Source

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Key Features (3 Points - High Yield)

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Median Incubation Period

• Defined as the time required for 50% of cases to occur following exposure.
• This is a key point of interest in point-source epidemics - it helps identify the causative agent.

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Important Note - Etiology

• Contamination of air, water, food, or soil by industrial chemicals or pollutants
• Examples: Bhopal gas tragedy (India), Minamata disease (Japan - methyl mercury in fish)

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How Incubation Period Helps Identify Type (Exam Favourite)

• Point source epidemic - all cases occur within one incubation period
• Propagated epidemic - cases occur later than the known incubation period (multiple waves/peaks)

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What Happens if Epidemic Continues Beyond One Incubation Period?

1. Continuous or multiple exposure to a common source, OR
2. A propagated spread from person to person

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Comparison Table - Point Source vs Propagated (High Yield)

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Continuous/Repeated Exposure (Common-Source Subtype)

• No explosive rise - more extended or irregular pattern
• Example: Contaminated water supply, nationally distributed food/vaccine
• Classic example: Legionnaire's disease outbreak, Philadelphia, 1976

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One-Liner Definitions to Remember

• Point source epidemic = brief simultaneous exposure → all cases in ONE incubation period → single-peak explosive curve, no secondary waves
• Median incubation period = time for 50% of cases to occur after exposure
• Serial interval = time between onset of primary and secondary case (used to estimate incubation period)

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Important Short Questions - Epidemiology (Park's PSM)

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SECTION 1: DEFINITIONS & CONCEPTS

Q1. Define Epidemiology

• Unit of study = defined population (vs. clinical medicine = individual case)

Q2. Aims of Epidemiology (IEA - 3 aims)

1. Describe distribution and magnitude of health/disease in populations
2. Identify aetiological factors (risk factors) in pathogenesis
3. Provide data for planning, implementation and evaluation of health services

Q3. Epidemiological Triad

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SECTION 2: MEASUREMENTS IN EPIDEMIOLOGY

Q4. Rate vs Ratio vs Proportion

Q5. Attack Rate

• Incidence rate expressed as %, used only during epidemics
• Formula: (New cases / Total population at risk) × 100
• Reflects the extent of the epidemic

Q6. Secondary Attack Rate (SAR)

• Number of exposed persons developing disease within the incubation period following exposure to a primary case
• Used to measure the transmissibility of a disease within households/closed groups

Q7. Incidence vs Prevalence

Q8. Relationship Between Prevalence and Incidence

• Long duration disease → high prevalence relative to incidence (e.g., TB, diabetes)
• Short/rapidly fatal disease → low prevalence (e.g., food poisoning, homicide)

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SECTION 3: CONCEPT OF CAUSATION

Q9. Germ Theory of Disease

• Louis Pasteur era: one-to-one relationship (single agent → disease)
• Model: Disease agent → Man → Disease
• Limitation: doesn't explain why not everyone exposed develops disease

Q10. Web of Causation

• Concept by Brian MacMahon
• For multifactorial diseases (e.g., CHD), multiple causes form a complex web
• No single cause is necessary or sufficient
• Useful for chronic non-communicable diseases

Q11. Bradford Hill's Criteria for Causation (6 key criteria)

1. Temporal association - Cause must precede effect (most essential)
2. Strength of association - Higher RR = stronger causal link
3. Specificity - One cause → one effect
4. Consistency - Same finding in different studies/populations
5. Biological plausibility - Makes biological sense
6. Coherence - Does not conflict with known biology/natural history

From "Smoking and Health" - US Surgeon General's Report 1964

Q12. Types of Association

• Spurious association - Arises from bias (selection bias, information bias)
• Indirect association - Due to a confounding variable (e.g., altitude → iodine deficiency → goitre; altitude and goitre are indirectly associated)
• Direct (causal) association - True cause-effect relationship

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SECTION 4: NATURAL HISTORY & ICEBERG

Q13. Natural History of Disease

• Stages: Susceptibility → Sub-clinical disease → Clinical disease → Disability/Death
• Basis for Levels of Prevention (Leavell & Clark)

Q14. Iceberg Phenomenon

• Visible tip = clinical cases seen by physician
• Submerged portion = latent, inapparent, presymptomatic, undiagnosed cases + carriers
• Waterline = border between apparent and inapparent disease
• Examples with large submerged portion: hypertension, diabetes, anaemia, mental illness
• Importance: Hidden cases = undiagnosed reservoir of infection

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SECTION 5: EPIDEMIOLOGICAL STUDIES

Q15. Types of Epidemiological Studies

Q16. Descriptive Epidemiology

• Studies distribution of disease by Person, Place, Time
• Generates hypotheses (does NOT test them)
• Answers: Who? Where? When?

Q17. Case-Control Study (Retrospective)

• Starts with cases (disease present) and controls (disease absent)
• Looks BACKWARDS for exposure
• Direction: Effect → Cause
• Measure of association: Odds Ratio (OR)
• Best for: rare diseases, chronic diseases (e.g., cancer)
• Advantage: Quick, cheap, small sample size
• Disadvantage: Recall bias, cannot calculate incidence/RR directly

Q18. Cohort Study (Prospective)

• Starts with exposed and non-exposed groups (disease-free)
• Follows forward and looks for disease development
• Direction: Cause → Effect
• Measure of association: Relative Risk (RR)
• Best for: common diseases, studying multiple outcomes of one exposure
• Advantage: True RR, temporal sequence established
• Disadvantage: Time-consuming, expensive, loss to follow-up

Q19. Relative Risk (RR)

• RR = Incidence in exposed / Incidence in non-exposed
• RR = 1: No association; RR >1: Positive association; RR <1: Protective
• Used in cohort studies

Q20. Attributable Risk (AR)

• AR = Incidence (exposed) - Incidence (non-exposed)
• Expressed as %: AR% = [(IE - IN) / IE] × 100
• Indicates how much disease is attributable to the exposure and could be prevented by eliminating it

Q21. Odds Ratio (OR)

• Used in case-control studies (incidence not known, so RR cannot be calculated directly)
• OR = (a × d) / (b × c) from the 2×2 table
• When disease is rare, OR ≈ RR

Q22. Randomized Controlled Trial (RCT)

• Gold standard of experimental epidemiology
• Features: Randomization, Control group, Blinding
• Eliminates confounding by randomization
• Measures: Efficacy (can it work?) and Effectiveness (does it work in practice?)

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SECTION 6: TYPES OF EPIDEMICS

Q23. Types of Epidemics

• Point source (single exposure) - explosive, one peak, all cases within ONE incubation period
• Continuous/repeated exposure - more prolonged, irregular curve

Q24. Point Source vs Propagated Epidemic

Q25. Epidemic Curve

• Graphical display of cases over time
• Suggests: (1) Time relation to exposure, (2) Seasonal pattern, (3) Common-source vs propagated spread
• Median incubation period = time for 50% of cases to occur after exposure

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SECTION 7: SURVEILLANCE & SCREENING

Q26. Surveillance (Epidemiological)

• Types: Passive, Active, Sentinel surveillance
• Sentinel surveillance = monitoring a sample of carefully selected reporting sites/populations to get early warning signals

Q27. Uses of Incubation Period (4 uses - Park)

1. Tracing source of infection (short IP → easier to trace)
2. Period of surveillance/quarantine = maximum incubation period
3. Immunization - passive immunoglobulins within IP can prevent clinical illness
4. Identifying point source vs propagated epidemic
5. Prognosis - shorter IP → worse prognosis (tetanus, rabies)

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ONE-LINE ANSWERS (Quick Recall)

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Epidemic vs Endemic vs Outbreak (+ Related Terms)

From Park's Textbook of Preventive and Social Medicine

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Definitions (Word Origins Included - Asked in Exams)

1. EPIDEMIC

"The occurrence in a community or region of cases of an illness, specific health-related behaviour, or other health-related events clearly in excess of normal expectancy."

• The community/region and time period must be specified precisely
• Epidemicity is relative - compared to the usual frequency of the disease in the same area, same season, same population
• Even 2 cases of a disease long absent from a population may constitute an epidemic
• Epidemic threshold for common diseases (e.g., influenza) = 2 standard errors above endemic level

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2. ENDEMIC

"The constant presence of a disease or infectious agent within a given geographic area or population group, without importation from outside; may also refer to the 'usual' or expected frequency of the disease within such area."

• Example: Common cold is endemic - somebody always has one
• An endemic disease when conditions are favourable may burst into an epidemic (e.g., hepatitis A, typhoid fever)

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3. OUTBREAK

"A sudden increase in occurrence of a disease in a particular time and place."

• A single case of a communicable disease long absent from a population may also constitute an outbreak
• Or emergence of a previously unknown disease may also constitute an outbreak
• Term is often used interchangeably with epidemic but implies a more localised event

• Clustering of cases/deaths in time and/or space
• Unusual increase in number of cases or deaths
• Even a single case of measles, AFP, cholera, plague, dengue or JE
• Acute febrile illness of unknown aetiology
• Shifting in age distribution of cases
• Sudden increase/high vector density
• Natural disasters

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4. PANDEMIC

"An epidemic occurring over a very wide area, crossing international boundaries, and usually affecting a large number of people."

• Requirements for a pandemic:
  1. Agent must be able to infect humans
  2. Agent must be able to cause disease in humans
  3. Agent must spread easily from human to human
• Examples: Influenza pandemics, cholera pandemics, COVID-19

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5. SPORADIC

"Cases occur irregularly, haphazardly from time to time, and generally infrequently."

• Cases are widely separated in space and time
• Little or no connection with each other
• No recognizable common source
• Examples: Tetanus, Herpes zoster, Meningococcal meningitis
• May be the starting point of an epidemic when conditions become favourable

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6. EXOTIC

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7. ENZOOTIC

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Comparison Table (Exam Summary)

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Key One-Liners for MCQs

• Epidemic = excess over normal expectancy in a defined area and time
• Endemic = constant presence without importation
• Pandemic = epidemic + international boundaries
• Outbreak = epidemic but more localised in time and place
• Sporadic = irregular, no connection between cases
• Hyperendemic = high prevalence, all age groups equally
• Holoendemic = children most affected, adults relatively spared - e.g., malaria
• Epidemic threshold = 2 SD above endemic level (influenza)

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