Prions Found in Sheep Muscle

Scientists have found the first evidence that prions--thought to cause variant Creutzfeldt-Jakob disease (vCJD) in humans--accumulate in the meat of food animals. The group discovered prions in the muscle of sheep with the prion disease scrapie. But don't give up your lamb chops just yet. Scrapie is only known to afflict sheep, and the prion levels in muscle are thousands of times lower than in the brains of infected animals.


Hold the mutton. Prion proteins (red) accumulate in the muscle tissue of infected sheep, a new study finds.
CREDIT: O. ANDREOLETTI
The most famous prion disease is bovine spongiform encephalopathy, or mad cow disease. The concern for humans is that the prions from infected bovine nervous tissue could contaminate beef, which then might can infect people with brain-destroying vCJD. So far, though, there's no evidence that prions occur naturally in bovine muscle tissue itself.

Checking the situation in sheep, Olivier Andreoletti of the Toulouse National Veterinary College in France and colleagues examined the muscle tissue of six animals they had infected with scrapie. In three sheep, they found small amounts of the prion protein PrP(Sc) in abdominal as well as in leg muscles. Two of four sheep naturally infected with scrapie also had muscle tainted with prions, in one case 8 months before the onset of symptoms, the team reports online 23 May in Nature Medicine.

The finding "is certainly no cause to panic," says epidemiologist Rowland Kao of the University of Oxford's Department of Zoology, who notes that humans don't seem to get scrapie. In addition, the amounts of PrP(Sc) in muscle were much lower than those in the brains of infected animals, and the team estimates that muscle tissue would be 5000 times less infective than brain tissue. The study could have important implications, though, if evidence emerges that PrP(Sc) is in fact a threat to human health, or if the prions that cause mad cow disease are shown to occur naturally in sheep, warns Fiona Houston, who studies prions at the Institute for Animal Health in Berkshire, U.K.