Using the tag cloud widget for WordPress, I find that my most commonly used tag currently is “economics.” How can that be? It’s not even one of my categories. Perhaps it is my broad definition of economics? Perhaps it is my frequent discontent with the way that human behavior gets discussed in the economics literature? Maybe I’m really interested in economic questions. I think I definitely need to write more posts on diarrhea…
The latest CDC report on the ongoing Salmonella serotype Saintpaul suggests that we may indeed be nearing the end. Here is the epidemic curve as of 28 July:
While there is certainly still the possibility of more cases appearing that have not yet been reported or another introduction (see previous post), the fact that the epidemic curve has continued to trail off in two successive weeks is good news.
The epidemiological evidence continues to implicate fresh jalapeño chiles grown in México. As of 28 July, 1307 cases have been reported since 1 April 2008. Nearly every state in the union has reported at least one case from this outbreak, as can be seen on this map:
Again the most likely victims (at least to report illness) are 20-29 year-old adults while the least likely to report illness are 10-19 year-olds and those over 80. Once again, I suggest that this is consistent with a vehicle consumed along with alcoholic beverages. Salsa. Though contaminated jalapeños (or other produce) could, of course, make their way into other foods as well. The Saintpaul strain was relatively rare before this outbreak, leading CDC to suggest the following:
The previous rarity of this strain and the distribution of illnesses in all U.S. regions suggest that the implicated food is distributed throughout much of the country. Because many persons with Salmonella illness do not have a stool specimen tested, it is likely that many more illnesses have occurred than those reported.
Let’s hope that this outbreak is really coming to an end. It would be nice if we could identify the source of the infection. Foodborne disease is an important public health concern that probably does not receive the attention it should. A 1999 paper by Mead and colleagues (admittedly now a bit long in the tooth) estimated that foodborne diseases cause approximately 76 million illnesses, 325,000 hospitalizations, and 5,000 deaths in the United States each year. It is not hard to imagine that in the 10 years since this study the foodborne disease burden has increased in the U.S. Time for another study, I’d say.
The first produce to positively match the Salmonella serotype Saintpaul was announced today. A jalapeño chile, grown in México and processed at an unnamed plant in McAllen, Texas, tested positive for the strain of Salmonella that has sickened at least 1237 people since April. This is the first time that the bug has actually shown up on a sample of fresh produce. Perhaps it wasn’t tomatoes after all, but it is a nice demonstration of confounding in epidemiological inference (tomatoes and jalapeños are commonly served together in the form of salsa, making it difficult to determine which is the actual vehicle for infection if both are implicated). Now the question is where along the production chain the contamination took place and how we can make sure that we bring an end to this outbreak. All of the news stories and government reports on this topic can be found on the ProMedMail website.
The Centers for Disease Control and Prevention have just issued a new report on the ongoing outbreak of Salmonella serotype Saintpaul infection. Since April, 1237 people have been infected. The investigation has continued to focus on raw tomatoes but also jalepeño (and serrano) chiles and cilantro. This further supports my previous speculation, based on the age profile of the cases, that salsa consumed while drinking alcoholic beverages might be implicated in this outbreak. Epidemiological investigations are greatly complicated when multiple vehicles that are typically consumed together are implicated.
There is an interesting caution that accompanies the epidemic curve:
This has to do with the fact that it can be very misleading to read too much into an epidemic curve for an ongoing outbreak. Consider the epidemic curve for the 2003 SARS outbreak in Singapore:
Now imagine we were looking at the curve as it evolved on 12 March. We might have be tempted to say that the epidemic was coming to an end and, man, would we ever have been wrong! There are a couple of things at work here. First, epidemics often have multiple introductions and while theory tells us that an epidemic curve will be more or less bell shaped, this is based on the assumption of a single introduction. If you look at the SARS epidemic curve hard enough, you can see several more-or-less bell-shaped components added together. The second issue with SARS is that there is extreme heterogeneity in transmission. Patient #1 probably infected 21 other cases, patient #4 probably infected 62. The great majority of others infected none. Individuals who infect more than 10 others are what is known as “superspreaders.” There were five in the Singapore outbreak out of a total of 201 probable cases of SARS and 722 suspect cases. Finally, there is often a delay between when people get sick and when their cases are reported. This means that the trailing edge of the epidemic curve always looks like it’s closing off its bell shape. The full case report for the Singapore outbreak can be found here.
So, is the current Salmonellosis outbreak on the wane? Let’s hope so, but as CDC warns:
It can be difficult to say when the outbreak is over, because of the reporting delay. The delay means that the curve for the most recent three weeks always looks like the outbreak could be ending even during an active outbreak. The full shape of the curve is only clear after the outbreak is over.
With a vehicle-borne disease, we don’t have to worry about superspreaders, but the fact that we still don’t know the source of the infection or the ultimate cause of the contamination is troubling. Who knows how much contaminated (presumably) produce is lurking out there? I, for one, will make sure to wash my produce well!
An outbreak of Salmonella serotype Saintpaul has sickened 943 people since April. Nearly 14% of these cases required hospitalization. It has been hypothesized that tomatoes have been the vehicle for this food-borne infection. Here in Palo Alto, certain types of tomatoes (e.g., Roma and beefsteak) were taken off store shelves for a while. The latest report from CDC suggests that tomatoes may not, in fact, be the culprit. CDC epidemiologists are expanding their investigations to include food items that are “commonly consumed with tomatoes.”
Epidemiological evidence indicates that while people of just about any age can contract the infection (age range of cases is <1-99 years), the most likely age group to contract the infection are 20-29 year-olds. The least likely age classes are 10-19 year-olds and people over 80. What food do young adults commonly eat with raw tomatoes that is less commonly eaten by the young or very old? I’d say salad greens but if that were the case, I’d expect a sex bias in infections. 50% of the infections are women and my informed guess (based on my experience with largely middle-class college students) is that 20-29 year-old women eat more salad than 20-29 year-old men.
So what is it if it’s not tomatoes? Something having to do with consumption of alcohol? Some salsa ingredient like jalapeños or scallions? (note: another thing consumed in bars)