John%20Snow

1
John Snow

• Highlights from pp. 19 26 in Gerstman (2003)

2
Who was John Snow?

• Victorian surgeon / anesthesiologist
• Realized his obligation went beyond treating
  individual patients
• Quintessential hero of epidemiology
• Insightful theory of disease
• Impressive methods of studies

3
Snows Waterborne Theory

• Based his theory on two facts
• Early symptoms restricted to GI tract
• Epidemics followed commerce and armies
• Theorized cholera agent is
• swallowed
• passed from person-to-person
• multiplies within the host

4
Snows Epidemiologic Methods

• Ecological study compared rates by region
• Cohort study compared rates by water source
• Case-control study compared water source
  (exposure) in cases and non-cases

5
Ecological StudyKey data in Figure 1.13 (p. 24)

• Columns in Table
• Sub-district (neighborhood)
• Population (denominator)
• Cholera deaths 1853 (numerator)
• Mortality per 100,000 numerator / denominator
  100,000
• Water companies serving neigborhood
• Example of rate calculation
• Rate St. Saviour 45 / 19,709 100,000 227
• Rate Christchurch 7 / 16,022 100,000 43
• Water source
• St. Saviour Southwark and Vauxhall Water Only
• Christchurch Lambeth and Southwark and
  Vauxhall Water (both companies serving
  neighborhood)

6
Cohort Study Key data in Table 1.7 (p. 25)

• Data by household water source
• Rate per 10,000
• Rate Southwark Vauxhall 1263 / 40,046
  10,000 315
• Rate Lambeth 98 / 26107 10,000 37.5

7
Case-Control Study

• Collect data on all cases
• Determine source of water for cases and non-cases
  (controls)
• See text (pp. 23 26) for selected interviews
• Also see map Fig. 1.14 (next slide)
• Conclusion Cases more likely to use water from
  Broad Street Pump

8
Fig 1.14 (p. 25)
9
20th Century Epidemiology(p. 26)

• Shift from acute contagious disease to chronic
  degenerative (noninfectious) disease
• E.g.s.
• British Doctors Study (Long Term Effects of
  Smoking)
• Framingham Heart Study (Risk Factors for
  Cardiovascular Disease)