Question

The following is a short description of the 1854 cholera epidemic of London investigated by Dr. John Snow. Read it and answer the questions that follow.

A total of 862 out of 186,787 people died due to cholera in a sub-district of London during a 48 day period from July 8 to August 26,1854. 844 cholera deaths occurred in a sub-population of 167,654 people using water supplied by Southwark Company, while 18 cholera deaths occurred in 19,133 people using water supplied by Lambeth Company. Dr. John Snow conducted a study of the area and found that each company supplied both rich and poor, both large and small houses, and there were no differences in either the health condition or occupation of the persons receiving the water of the two companies. On the basis of these findings, John Snow argued that drinking water supplied by Southwark Company, whose source was the polluted water of the Thames River, caused the cholera epidemic. Cholera deaths decreased after the water source was changed to less polluted water.

1. Calculate the risk ratio of cholera deaths among people using water supplied by Southwark Company as compared to Lambeth Company.

2. Do you think that this risk ratio is strong enough to support a cause-effect relationship between cholera deaths and water supply? Why or why not?

3. John Snow's study was done long before the identification of the causative organism of cholera-- Vibrio cholera. Thus, Sir Bradford Hill's guideline of biological plausibility was lacking when John Snow did his study. What does this tell you about the nature of the biological plausibility criteria?

4. As you know, temporality is one of the key causal guidelines proposed by Hill. What kind of evidence would need to be provided about the 1854 cholera epidemic to support Hill's guideline about temporality?

5. A marked reduction in cholera deaths occurred after the improvement in water supply. Which one of Hill's guidelines is supported by this fact?

Answer 1

This is a question related to solving the risk ratio parameters and then to analyse its siginificance to the assumption made.

first of all let us determine the risk ratio .

The risk ratio is the relative measure of the risk of the group under treatment group to the cotrolled/free group.It is used to estimate the strength of the association between treatments or risk factors, and outcome

To measure the risk ratio, we will first of all build the 2*2 contigency table of the control group and treatment/experiement group with respect to affected and unaffected cases.

From the given question the 2*2 contigency table and the corresponding risk ratio will be as follows

2). We know that while we assume the causal effect between the exposure and the outcome, values of RR can be interpreted as follows:

If Risk Ratio(RR)

• RR = 1 means that exposure does not affect the outcome;
• RR < 1 means that the risk of the outcome is decreased by the exposure;
• RR > 1 means that the risk of the outcome is increased by the exposure.

Now compare the RR obtained from the question.

It is 5.35 >1 ,indicating that the cause has a very strong impact on the effect/outcome.

Hence from RR data only , we should agree with Dr. Snows assertion that bad water supplied from south park co was the cause of epidemic.

3). This is more of an interpretation question . During Dr. Snow's study, the biological plausiblity was lacking as it was not explored by then.

I can only infer that the presence of biological plausibility is dependent on the current state of knowledge.

4). In at all we need to make Hill's guidelines relevant during Dr. Snow's experiment and assertions, we will simply have to show that the exposure to Vibro Cholera which passed through contaminated water to the exposed subjects was present before diagonsis of disease and its occurance in general.

5). When water supply was improved, the cholera cases reduced. Now assuming Hill's guidelines and conditions prevelant (cholera present) ,we can safely infer that presence of Vibro Cholera was a significent contirbutor to cholera cases .