Calculate attack rates.

CHAPTER 1, FOOD SAFETY

(b) Calculate the interval for each case, and determine the

range of incubation periods by identifying the shortest and longest

incubation period. Calculate the median incubation period. (Make a list of

the individual incubation periods from shortest to longest. The middle

value on the list, or the average of the two middle values if there is an

even number of cases, is the median incubation period.)

period for a common source outbreak of staphylococcal food poisoning.

Table 1-12 shows the incubation periods grouped by two-hour intervals.

Both the median incubation period (3.5 hours) and the large number of cases

with illness onset between 2 and 4 hours after eating the suspect food are

consistent with staphylococcal food poisoning.

(4) Calculate attack rates.

(a) Attack rates, the percentage of ill persons, may be food or

meal-specific. For either type of attack rate to be meaningful, the

investigator must have food and/or meal histories on both ill and well

persons who were at risk of eating the suspect food or meal.

(b) Food-specific attack rates help pinpoint a suspect food

within a meal, and can support observations and conclusions on food

handling that contributed to the outbreak. Meal-specific attack rates are

appropriate when an investigation has not pinpointed a particular meal; the

results may help focus further investigative efforts.

become ill after they ate a particular food or meal by the total number of

persons (both cases and controls) who ate that food or meal, and multiply

the results by 100. Do the same for the persons who did not eat that

particular food or meal. A highly suspect food or meal will have the

highest attack rate for those who ate that food or meal, and the lowest

attack rate for those who did not eat that food or meal. The difference

between the two rates provides an easy method of comparing different meals

or different foods.

(d) When investigating a disease with a long incubation period

(e.g., hepatitis A), attack rates based on food preference rather than

actual consumption may be necessary. A person's food preferences may be

determined by asking if, when given a choice, they always or usually eat

certain foods (e.g., raw oysters), purchase particular brand items, or dine

at a particular restaurant.

(e) Table 1-14 is an example of a food specific

attack rate analysis. Persons who reported that they ate potato salad have

a high rate of illness. The difference in attack rates is greatest for

potato salad, which implicates this food item as the vehicle in the

outbreak. Not all people who reported eating potato salad became ill.

Some people may not accurately remember what they ate or did not eat. The

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