Salmonella stpaul outbreak -- part 1

You probably have seen in the news that there was a salmonella stpaul outbreak. You probably know that it started thinking it was tomatoes. However, no contaminated tomatoes were discovered, so the authorities started looking into foods that are associated with tomatoes, such as peppers and cilantro.

The seemingly slow speed of tracing the outbreak, and the reluctance of the FDA to rule out some foods, led to a firestorm of criticism. In this post, Swampy is going to try and explain the basics of how these investigations work.

When a doctor sees a patient that may have a food borne illness, the doctor may (but is not necessarily required to) report the illness to the local public health department, who will test for the usual suspects. When the cause of the illness is discovered, they'll run a genetic test and place the organisms genetic profile on a reporting system. In the case of the salmonella outbreak, it was on a system called PULSENET.

The software then links the profile to other reported profiles across the country. When it notices that the same genetic profile is popping up in multiple states, the system flags a CDC investigator.

Some health departments, but not all, automatically launch an investigation when they see certain events. For instance, Minnesota has a policy that every case of e coli 0157:H7 is followed up. If you are the person who got sick, someone from the health department will ring your doorbell. They will want to know what you ate, and if it is your food, where you bought it. They'll take samples of your food.

Okay, so now the CDC investigators know of an outbreak. At this point, they will interview the cases, and try to mathematically solve for which food was the culprit. Each outbreak works a little differently. In some cases, the mathematical analysis points to a strong correlation. For the outbreak under discussion, people who ate raw tomatoes were 7 times more likely to get salmonella. That was the point where you saw on the news to avoid fresh tomatoes, but nobody could tell you if a particular tomato was safe.

CDC then forwards the results to FDA. It is the FDA that starts the arduous task of trying to figure out where the tomatoes came from. Tracing food takes a long time as most of the records are paper.

Where do fresh tomatoes come from? Well, at the time of the outbreak, all the tomatoes came from one of two places -- South Florida and Mexico.

Vegetables that are shipped into the US are inspected by USDA. The goal of border inspection is protection of US industry (making sure that insects are not given a free ride where they would infect US crops), and compliance with US regulations (making sure that the items in the truck are allowed into the US). The inspectors take a small sample of the good and test it according to regulations. In the case of vegetables, it is usually a visual inspection for pests and general condition. There is no routine test for contamination.

The consequence of this is that the process of tracing back to the source is identical whether the farm is in the US or in Mexico.

(end of part 1)