Question from a student 20170503c

Q:

From the outbreak investigation questions in the past papers, part (a) asks about the suspected food and part (b) asks about the implicated food.

I would like to ask:

     1) for answering the suspected food, do we ONLY use the attack rate difference to deduce the answer?

     2) is it necessary that the suspected food MUST BE THE SAME as the implicated food?

For 2012-2013 Q8, are these answers correct?

       (a) Pizza is the suspected food as it has the highest attack rate difference among the others. One point needs to be aware that there is no bacteria found in pizza.

       (b) B. cereus is the implicated microorganism as it is consistent to the shortest incubation period (5 hours) while C. perfringens is not. Therefore, the implicated food is bread

A: 

I do not have the 2012-13 question at hand, so I just answer according to my memory of the questions.

1. You can use whatever evidence you think appropriate to deduce the suspected food. The attack rate difference is an important factor in the deduction or elimination. Especially when the difference is very small or negative.
2. Suspected food and implicated food can be different. 

a. No bacteria does not mean the food is clean. There can be toxins.

b. The bread, if having a very small attack rate difference, or negative attack rate difference, would probably not the implicated food. It is possible that the pizza had Staphylococcus aureus grown on and toxin produced.