Investigating food and waterborne epidemics

Other epidemiological techniques are useful for investigating food- and water-borne epidemics, particularly case-control and cohort study methods.

Case-control studies An example of the use of a case-control study in a cholera investigation is given where fish were suspected to contain the aetiological agent. In this community, people preferred to eat fish marinated, but uncooked. Cases were interviewed as to whether they ate raw fish and compared with a similar group who had not had the disease. The results are set out in a two-by-two table: there was a significant finding w2 = 50.47; P<0.001.

Cases

Controls

Total

Ate raw fish

31

8

39

Did not eat raw

3

60

63

fish

Total

34

68

102

Suspected cause

Cases

Controls

Present

a

b

Absent

c

d

In the example above, the relative risk of contracting cholera after eating raw fish is

31 x 60

A reasonable estimate of the relative risk can be arrived at (as the incidence rates are not known) from the two-by-two table, using the odds ratio, ad/cb:

Cohort studies A cohort is a group of people all exposed to the same aetiological agent. By following this group over time, the risk of developing disease can be measured. A modification of the technique can be used in outbreak investigation, particularly food poisoning. This compares the attack rate in the persons exposed to the factor with the attack rate in those not exposed to the factor. In a food poisoning outbreak, where various foods are suspected, then the attack rates in those eating and not eating the range of foods can be compared. This is best illustrated by using an example as shown in Table 2.1. The relative risk for each food item is calculated as above and the results set down in a table. Most of the relative risk values are about 1, but there is over four times the risk of becoming ill if you ate fish, so the investigator would suspect fish as being the most likely cause.

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