I’ve been away at a couple of very interesting conferences — more on those soon — so I’m late to this story; on the other hand, the story hasn’t even peaked yet, and thus there’s plenty of time for us to catch up.
So: An outbreak of foodborne illness that appears to be spread by fresh cantaloupes has sickened 72 people so far, in 18 states, and 13 have died. According to investigators, the source of the contamination has not yet been found. And also, according to a media briefing today, the contaminated cantaloupes were also shipped overseas, to countries that investigators would not identify. And, as an extra bonus, the tally of cases and deaths is likely to keep rising, because the particular illness in this outbreak has an incubation period of up to two months.
That’s the highlight reel: Dead from eating melon. Now, the details:
The outbreak, which has been building for several weeks, involves melons from a single grower in Granada, Colo. called Jensen Farms. The first cases occurred at the beginning of August and authorities began to be concerned when the outbreak crossed state lines in early September. On Sept. 14, the growers did the right thing and launched a recall of all the whole cantaloupes they shipped between July 29 and Sept. 10. To their knowledge, they had sold cantaloupes to wholesalers and distributors in 17 25 states.
At this point, I can practically hear foodborne-disease geeks — as well as almost anyone who has taken a tropical vacation — thinking to themselves: “Wait. Weren’t we told it’s safe to eat fruit if it has a rind and you don’t eat the rind? You don’t eat cantaloupe rind. What gives?” And that’s correct, generally. The advice you get, if you want to eat anything raw that might have been contaminated, is to choose something with a peel, wash it, and then peel it yourself. But there’s an aspect of melon that makes this problematic: Unlike a banana, you don’t peel a melon with your fingers. You slice it, and the knife blade can carry any organisms on the outside of the melon into the flesh. Take a look at the image of cantaloupe skin at the top of this post. It has lots of nooks and crannies; even if you scrubbed it — and who scrubs a melon? — you would be unlikely to get it clean.
There’s a further complication in this outbreak. The organism that is causing the illnesses and deaths is Listeria monocytogenes. It’s uncommon in produce; in fact, this is the first time the bacterium has been found in cantaloupe. And in the annual toll of foodborne illness, Listeria is a bit player. It causes perhaps 1,600 cases per year in the United States, compared to more than 1 million cases annually of Salmonella. But pretty much alone among the foodborne organisms, Listeria possesses a unique adaptation to modern life: It reproduces very well in the cold. So anything that is contaminated with it and then is refrigerated — fruit in this outbreak, and deli meats, fresh cheeses and milk in past ones — will come out of the refrigerator bearing a bigger burden of bacteria than when it went in.
But even things that are refrigerated eventually go bad — and in the case of these cantaloupes, federal investigators said today (transcript not yet online) that they believe the shelf life of the fruit must be ending. That doesn’t mean however that the outbreak is over, even though the epi curve (at right) shows a clear downward trend. That’s because Listeria illness, listeriosis, has an incubation period of at least one to three weeks and in rare cases up to two months. So someone who bought a cantaloupe a while ago, and doesn’t know about the recall or doesn’t recognize the fruit’s provenance, could eat it today and not become ill until well into October.
The question bedeviling investigators is what caused this outbreak: How did Listeria, so uncommon in produce, get onto these cantaloupes? In the media call today, Sherry McGarry of the Food and Drug Administration said:
…We will look at various parameters, environmental in particular, that may have contributed to that contamination and spread. And some of those things that we’ll be looking at is any potential animal intrusion. We’ll be looking at water quality. We’ll be looking at the growing practices, the harvesting practice. We’ll also be looking at the process within the facility for packing and potentially rinsing the cantaloupes themselves and how they were stored and whether there’s amplification in that process.
A Denver-area TV station has reported that a fertilizer company applied biosolids — treated sewage — to a field not far from the farm where the cantaloupes were grown, but that the application occurred several years ago. The farm itself, Jensen, said in the same report: ““Jensen Farms uses two types of commercial-grade fertilizer: heat-treated or pasteurized organic fertilizer and phosphorus-based non-organic fertilizer. Both are approved for use on cantaloupe, among many other crops.”
A final question (so far) is whether there was anything the farm could have done, or the FDA could have required them to do, to reduce the likelihood of this outbreak happening. The answer seems to be, not yet. The Food Safety Modernization Act that passed last year does require that the FDA come up with guidelines and regulations to minimize risk from fruits and vegetables that have been associated with outbreaks in the past. But those new rules aren’t due until next year at the earliest, with a final version a year after that. (Here’s an explainer of the impact from the produce processors’ trade association.) And it’s an open question whether they would have applied to this outbreak if they existed today, since past cantaloupe outbreaks have been caused by Salmonella and the “winter vomiting sickness” norovirus, but never by Listeria before.
Update: Google+ reader David Tribe, a bacterial geneticist at the University of Melbourne, points out that there has been a case of Listeria in cantaloupe in Australia, as captured in this Australian government report from 2010.
