Listeria in Ice Cream?

Blue Bell Ice Cream–a cautionary tale

Unfortunately, yes, this is a possibility. See, Listeria monocytogenes lives in soil, decaying leaves, animal guts, raw milk, and lakes, ponds, and streams. It’s also really good at surviving in cold places (refrigerator temperatures) and forming biofilms to avoid getting scrubbed off or killed by sanitizers (they can’t easily penetrate biofilms).

So how does this lead to Listeria in ice cream? After all, ice cream is made from pasteurized milk, and pasteurization kills Listeria. No Listeria in milk should mean the ice cream is safe, right?

The best investigators can figure out, what happens is that Listeria gets onto the floors of ice cream processing facilities or into the drains or other surfaces, and from there can cross-contaminate surfaces the ice cream actually contacts. Since there’s no heat treatment at the end of ice cream making and antibacterials aren’t used, whatever gets into ice cream during processing will stay. Being frozen keeps the Listeria from growing, but it doesn’t kill it. So it’s ready to wake up and cause problems as soon as someone eats the ice cream.

Listeria growing on a metal chip (C. Assisi, unpublished)

Ok. Here’s where we press pause–most of the time, what happens next is that there aren’t many Listeria in the ice cream, and those that are present are mostly killed by the consumer’s stomach acid, or detected by their immune system before they reach the numbers to cause illness. However, if there are a lot of Listeria, enough may survive to cause illness.

And again, pause. Much of the time, the person consuming ice cream is older than six, younger than 65, isn’t pregnant, and doesn’t have an immunocompromising disease (such as cancer, diabetes, an organ transplant, or HIV). So this person may get sick, but it’ll be minor. They’ll have diarrhea and/or mild flu-like symptoms and then they’ll recover. In fact, it’ll probably be so minor, they won’t even go to a doctor.
Unfortunately, sick people also eat ice cream. Ice cream is a valued treat in nursing homes and hospitals. Pregnant women like ice cream. People over the age of 65 like ice cream. So sometimes, people do get sick–and even die–from ice cream.

So what can we do? Should all high-risk individuals stop eating ice cream?

That’s probably* an overkill. Ice cream companies are required to test their products for Listeria. In fact, the Bluebell outbreak was largely a result of the company violating protocol by shipping out ice cream before they got the test results back. The other good news is that the news story that inspired this blog post indicates that ice cream facilities are being more closely monitored now, which should make ice cream safer in the future. Happy ice cream eating!

*But use your judgment and talk to your doctor about this one if you’re worried. Keep in mind, fresh fruit and vegetables and anything that isn’t cooked before eating carries a risk of Listeria.


FDA Bad Bug Book 2013. pp 99-103.

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