Thursday, September 29, 2011

What’s Listeria?

Image Credit: Clay Irving.

By now you’ve surely heard about the latest food-borne illness freak-out: cantaloupes contaminated by Listeria monocytogenes. Originating in Colorado’s Jensen Farms, the fruits have already caused 72 illnesses and 13 deaths in a total of 18 U.S. states. And those are just the numbers as of this writing, they’ll probably be higher by the time you read this. For reasons that I’ll explain momentarily, cases are expected to keep rising for some time.

Listeria doesn’t make headlines very often. Usually it’s the more familiar pathogens E. Coli and Salmonella that are responsible for outbreaks of food poisoning. As with these more famous cooties, Listeria is a bacterium with a propensity for turning up in animal feces (though it can also be found in soil), which then comes in contact with our food. But it has a few additional qualities that distinguish it from our friends E. Coli and Salmonella. For one thing, Listeria can reproduce in colder conditions, rendering it unfazed by all our fancy refrigeration technology. A dash of Listeria on a cantaloupe (or on the cheese or cold cuts that more typically serve as its intermediate home*) will gleefully multiply into a teeming hoard of bacteria without ever being improperly stored.

Another important difference is the Listeria bacterium’s lengthy incubation period in the gut of its human host. It can take several weeks for someone who ingested the microorganisms to manifest symptoms of Listeriosis – the actual illness caused by Listeria. The good news is that most people who eat Listeria-tainted food won’t get this malady; a healthy immune system generally destroys these pathogens before they have a chance to do any real damage. But in those with weaker internal defenses – older adults, pregnant women (and their newborn babies) and anyone with a compromised immune system – Listeria can escape the intestinal tract and cause serious illness. Symptoms include fever, muscle aches and diarrhea, but life-threatening complications like meningitis can also occur.

Given that the initial symptoms of Listeriosis are best described as “flu like”, we’re ensured an extra special flu season this year, as every under-the-weather individual with recollections of cantaloupe consumption rushes to the ER. Not that they shouldn’t. Listeria may not account for many food-borne disease outbreaks, but when it does cause illness, the fatalities are high, even when treated with antibiotics. When dealing with E. Coli and Salmonella, we’re accustomed to hearing figures in which hundreds or thousands of people are sickened but only a handful die. Listeriosis, on the other hand, can be fatal in as many as 25% of cases. If I’m doing my math correctly, this current outbreak is at about 18%. Pretty scary.

The textured rind of a cantaloupe. Image Credit: Bruno Girin.
And what can you do to protect yourself from killer cantaloupe? Clearly peeling it doesn’t do the trick. The trouble with produce of this sort is specifically that it does have a thick rind. People tend to view it as safe and forgo the more thorough washing they would give to something like lettuce. I’ve been scrubbing fruits with peels for ages (except bananas, one has to draw the line somewhere) and others in the kitchen act like I’m insane. But as I’ve explained again and again, the peel touches things that touch other things that end up in your mouth. It touches the knife that slices through the rind and right into the fruity center, it touches the plate onto which you place the sliced fruit, it touches your hands. Ugh, cooties everywhere! Just wash your fruit, okay. And if you fall into any of the high risk groups mentioned above, I’d recommend also having a look at the CDC’s prevention page for Listeria for a complete list of foodstuffs to avoid. It’s a pathogenic world out there.

* This is the first case of Listeria being found on cantaloupe.


  1. Thank you for the suggestion/reminder to was all fruit even that which will be pealed. When I was living in Central American it was always very apparent why one needed to do this: the fruit LOOKED very dirty. I think that at best the fruit gets a hosing down of sorts that does not necessarily remove germs. I think that if the fruit companies did not hose down the fruit so much and left some dirt on then maybe people might be more inclined to wash it more thoroughly like they do in Central America.

  2. Do you have any more infomation on how listeria reproduces?

  3. Haha! Interesting =) I always wash my fruit, but really? Washing cantelope seems so strange--like one of those things only a true germaphobe would do. It's apparent that the fruit needs some washing before consumption though!