Why humans don't need to fear cattle disease Mycoplasma bovis

by Jennifer Bowden / 14 May, 2018
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How can meat from cattle diagnosed with Mycoplasma bovis be fit for human consumption? Don’t the experts remember mad cow disease?

Although Mycoplasma bovis is common in many food-producing nations, it was detected in New Zealand cattle for the first time last July. Given the risk to cattle well-being and the fact this outbreak is relatively well contained, the Ministry for Primary Industries recently announced a cull of all 22,000-plus cattle on the 22 properties still infected in the hope of eradicating the disease from New Zealand. So, what does this mean for beef consumers?

More than 125 Mycoplasma types exist, and each typically infects only one species of animal. They all share the same characteristics that make treatment and detection of this bacterial infection difficult: they lack a cell wall, so certain antibiotics aren’t effective against them; they have an ability to hide away from the immune system, making it difficult for the animal to fight an infection; and they can create conditions that allow them to evade antibiotic treatment (for example, inhabiting large abscesses).

Mycoplasma bovis bacteria can cause a range of serious conditions in cattle, including treatment-resistant mastitis, pneumonia, arthritis and late-term abortions.

Internationally, it’s common practice for animals not showing clinical signs of infection to be processed for human consumption, which is what New Zealand is doing. Cattle that are visibly sick, severely injured or have medicine in their system will not be butchered and sold.

Is there any risk to human health from eating meat from an infected animal? Richard Laven, from Massey University’s School of Veterinary Science, says that raises two questions.

“First, is Mycoplasma bovis killed by normal food-preparation mechanisms? The answer is yes, it is effectively killed.”

In fact, it’s easier to kill than such bacteria as salmonella, campylobacter and E coli, says Laven, because Mycoplasma bovis has not evolved to survive outside its host’s body and it has little, if any, defences against the heat used in cooking. “Good food hygiene will control Mycoplasma bovis and it’s unlikely to be there.”

Richard Laven.

The second question is: if Mycoplasma bovis is present in our food, would it present a health risk to humans? “It’s a cow-adapted organism, [so] it’s unlikely to cause disease in humans. We know mycoplasmas: they’re designed to live in animals and they don’t tend to spread across species.”

Indeed, Laven has found just two reports internationally of humans testing positive for Mycoplasma bovis. In one case, there was no evidence of any significant symptoms or disease; in the other case, the individual had multiple infections of which Mycoplasma bovis was one.

“You can never say with 100% certainty that there’s no disease risk to human beings whatsoever, but you can say it’s about as low as any of the diseases we’re encountering.”

By contrast, Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is caused by a prion, says Laven. Prions are infectious agents primarily composed of a protein material that is abnormally folded up.

When prions enter a human through the food supply, they somehow get into the brain, where they induce normal prion proteins to fold abnormally. This leads to brain damage and the fatal symptoms of Variant Creutzfeldt-Jakob disease (vCJD).

“We’ve got a long way to go before we fully understand what’s happening,” says Laven. What we do know is that prions can’t be destroyed by boiling, alcohol, acid or radiation – they are relatively indestructible.

Cooking BSE-infected meat, in other words, won’t stop it from causing vCJD in humans. Mycoplasma bovis, on the other hand, is unlikely to survive butchering, cooking and consumption. And if it did, it’s highly unlikely to infect a human or cause any symptoms.

“You’re more likely to choke to death from eating a piece of meat than you are to get Mycoplasma bovis from an animal slaughtered at an abattoir in New Zealand in the next few months.”

This article was first published in the May 5, 2018 issue of the New Zealand Listener.

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