A Superbug Salmonella Sickened Hundreds of People Through Cheese, Beef, and Raw Milk

Photo: Phil Walter (Getty Images)

According to a new report from the Centers for Disease Control and Prevention, an outbreak of superbug salmonella sickened hundreds of people and hospitalized dozens over the past two years. But what makes the incident especially scary is that the outbreak was transnational, having likely come from cattle in both the U.S. and Mexico. It may very well be a harbinger of even worse outbreaks to come.

The first cases, according to the report, emerged in June 2018 in the U.S. These cases, found during an investigation into another, unrelated outbreak of salmonella, were caused by a strain of Salmonella Newport that is resistant to multiple drugs. From June 2018 to March 2019, state and local health officials documented a total of 255 cases across 32 states traced to that same strain. Of the roughly 200 cases they had concrete information on, 60 people ended up hospitalized, 10 developed a life-threatening blood infection, and two died (the deaths involved people with other health complications, but the infections played a role).

Near as the disease detectives can tell, many victims contracted their infection from eating Mexican-style soft cheese sold in Mexico. Nearly half of those with available travel information had visited Mexico in the week before they got sick, while those who reported what they ate often ate cheese processed from raw, unpasteurized milk. A sample of cheese bought in Mexico and kept by one patient tested positive for a genetically identical Salmonella strain to those found in the cases; it also matched a strain found in a sample of Mexican beef tested in 2016.

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Not everyone who got sick visited Mexico or ate Mexican cheese, but they did eat beef. And in September 2018, the same strain was found in a Texas cattle steer, meant to be processed as beef; the strain was detected twice more in samples of beef collected from two processing facilities from Texas in November 2018 and March 2019.

Antibiotic resistance doesn’t automatically make infections harder to treat, since it depends on which drugs are commonly used for that bacteria. But in this outbreak, a third of people treated with antibiotics were likely given ineffective drugs, the authors noted.

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The CDC was never able to find any common suppliers of contaminated beef or cheese. But the most likely scenario is that this superbug is popping up in cattle throughout the U.S. and Mexico. What’s more, the superbug is likely a new emerging menace; prior to 2016, strains of Salmonella Newport resistant to these specific antibiotics in livestock, meat, or people hadn’t been detected by routine testing.

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“CDC is concerned about an increase in human illness from a new strain of multidrug-resistant (MDR) Salmonella Newport, that appears to have spread from cattle in the United States and Mexico,” the agency wrote in an advisory to doctors and health officials on Friday.

Though the outbreak seems to have ended for the time being, it’s likely that many more people were infected and sickened than was reported. Most people infected with a harmful strain of salmonella do develop symptoms, like diarrhea, fever, and stomach cramps, but most recover without treatment so they never visit a doctor. And it’s possible that a similar strain will wreak havoc in the future.

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If that’s not bad enough, all of the genes found in the strain that made it resistant were found on a plasmid—mobile bits of DNA that can be swapped between different species of bacteria. So it’s likely this bug can teach its tricks to other troublesome germs. And because livestock today are fed more antibiotics than needed, the conditions for further antibiotic resistance among foodborne bacteria are still ripe.

In light of all this, the authors say there needs to be more surveillance of this particular strain, as well as research done to figure out how to stop its spread among cattle. The average person, meanwhile, should follow the same steps for keeping themselves safe as usual from these germs: Avoid dairy products made from raw milk and properly cook meat.

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The CDC recommends using a meat thermometer and cooking until your steaks or roasts reach 145 degrees Fahrenheit (62.8 degrees Celsius), then letting it sit for three minutes before eating. Ground beef or hamburgers should be cooked until they reach 160 degrees Fahrenheit (71.1 degrees Celsius).

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About the author

Ed Cara

Science writer at Gizmodo and pug aficionado elsewhere