Study Notes on Snow's Cholera Hypotheses and Epidemic Analysis

Part I: Snow's Hypotheses on Cholera

1.1 Snow’s basic hypotheses regarding cholera:

  • Communicable Disease:
    • Cholera is spread from person to person.
  • Transmission:
    • Direct: person-to-person.
    • Indirect: via contaminated food, water, or fomites (inanimate objects).
  • Entry and Exit:
    • Morbid matter enters the body via the mouth and exits via feces.
  • Agent Properties:
    • Cell-like structure
    • Very rapid reproduction (short incubation period)
    • High viability for indirect transmission
    • High infectivity (minute quantities sufficient for infection)
  • Pathogenesis:
    • The agent multiplies in the intestine and affects mucosal surfaces, causing significant fluid loss from blood capillaries.
  • Hygiene Link:
    • Cholera outbreaks are associated with poor hygiene.

1.2 Evidence for Hypotheses:

  • Circumstantial Evidence:
    • While evidence is not definitive, it supports the hypotheses:
    • a,b) Communicability and transmission are likely.
    • c,d) Probable entry and exit points.
    • e) Agent properties remain conjectural.
    • f) Pathogenesis based on solid pathological studies and therapeutic outcomes.

1.3 Modern Acceptance:

  • All Snow's hypotheses are supported by contemporary knowledge.

Part II: Recognizing the Epidemic

2.1 Identifying the Epidemic:

  • Snow recognized the epidemic by comparing past incidence and prevalence rates.
  • Defined the onset date as August 31 - September 1 due to a sharp increase in deaths.

2.2 Suspicion of the Pump:

  • Suspected the Broad Street pump as a water source linked to the outbreak.
  • Investigated water samples and found impurities after initial examinations.
  • Analyzed 83 deaths within three days of the outbreak and found proximity to the pump linked to most cases.

2.3 Pump Handle Removal:

  • The pump handle was removed on September 8, with the epidemic nearly resolved.

2.4 Evidence and Limitations:

  • Evidence supporting the pump's role was circumstantial:
    • Showed possible contamination from a nearby sewer.
    • Water contained possible organic impurities.
    • Most cases related to pump use indicated a strong association.

Part III: Experimental Design and Limitations

3.1 Experimental Setup:

  • The setup provided controlled conditions, isolating the water supply as a variable.
  • Large sample size with random allocation of subjects increases reliability.

3.2 Infection Dynamics:

  • Increased infections reduce the pool of susceptibles, leading to a decrease in new cases.
  • Other transmission methods increase as disease spreads, complicating assessments of water supply efficacy.

Part IV: Analyzing Host Factors

4.1 Infectious Period:

  • Understanding communicability periods is vital; organisms in contaminated materials may become non-viable over time.

4.2 Host Resistance:

  • Neglect of host factors weakens conclusions regarding disease transmission.

4.3 Population Size Impact:

  • In smaller populations, the susceptible pool is depleted faster than in larger populations, affecting disease dynamics.

4.4 Behavioral Factors:

  • The importance of human habits in disease spread is emphasized, offering a rationale for outbreak management.

Part V: Suggestions for Improvement

  • Chlorine use in water supplies.
  • Ensure effective washing of food.
  • Implement fly control mechanisms.
  • Utilize vaccination as disease control adjunct.
  • Update sanitary practices in mines and sewage treatment.

Part VI: Epidemiological Concepts Usage

  • Snow employs various fundamental epidemiological concepts which include:
    • Case Series: Illustrates initial cholera cases, suggesting transmissibility.
    • Mapping: Used to identify epidemic sources effectively.
    • Attack Rates: Documented daily increases in cholera attacks, indicating an epidemic.
    • Mortality Rates: Comparison of cholera death rates between populations utilizing different water supplies (notably 71 deaths per 10,000 homes vs. 5 per 10,000 homes).
    • Relative Risk: Established fatality differences demonstrating the higher risk associated with drinking contaminated water.