Download John Snow

John Snow
Highlights from pp. 19 – 26 in
Gerstman (2003)
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
Snow’s 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
Snow’s 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 noncases
Ecological Study
Key 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)
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
Case-Control Study
• Collect data on all cases
• Determine source of water for cases and noncases (“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
Fig 1.14 (p. 25)
th
20
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)