Question

Exposed persons Those who ate suspected foods Unexposed persons Those who did not eat suspected foods Suspected foods Ill Well Total Attack Rate % Ill Well Total Attack Rate % Ham 36 5 2 11 Potato Salad 40 4 9 6 Peas 16 15 10 13 An investigation of an outbreak of staphylococcal food poisoning occurred during the second week of January 2017 on campus. The case definition produced by the investigators was an acute onset of nausea, cramps, and vomiting. Complete the attack rate table above, including the attack rates for each suspected food, the relative risk (RR) for each suspected food and determine the most likely food responsible for the outbreak. Justify your response.

Answer 1

The calculations of the attack rate and relative risk are shown below:

Suspected foods	Ill (a)	Well (b)	Total (a+b)	Attack Rate %   (Ill/Total)	Ill	Well	Total	Attack Rate %    (Ill/Total)	Relative Risk (RR) = AR(Exposed)/AR(un-exposed)
Ham	36	5	41	36/41 = 0.88	2.00	11.00	13.00	2/13 =0.15	0.88/0.15 = 5.71
Potato Salad	40	4	44	40/44 = 0.91	9.00	6.00	15.00	9/15 = 0.60	0.91/0.60 = 1.52
Peas	16	15	31	16/31 = 0.52	10.00	13.00	23.00	10/23 = 0.43	0.52/0.43 = 1.19

Attack rate % = a/a+b

RR = AR(Exposed)/AR(Unexposed)

We can see from the above table that the relative risk for Ham is the most out of all the suspected foods with RR being 5.71

It means that risk of being ill for exposed persons who ate Ham is 5.71 times the risk of being hill those who did not eat Ham. Hence, Ham is the most likely food responsible for the outbreak