Hunter beware: infectious proteins found in deer muscle.

Infectious agents called prions that cause a mad cow-like disease in deer and elk are present in the infected animals' muscles, according to a new study. Prions had been shown to congregate primarily in the brains and spinal cords of infected animals, so the finding could increase concern among people who eat deer meat.

Researchers have found that prions are responsible for similar diseases in several species: for example, Creutzfeldt-Jakob disease in people; bovine spongiform encephalopathy, also known as mad cow disease, in cows; and chronic wasting disease (CWD) in deer and elk. These diseases start when a normal protein present in all mammals, called the prion protein, becomes misfolded. When normal prion proteins interact with the misfolded ones, they also become misfolded and propagate the disease.

Since transmission of these diseases relies partly on the similarity between the infectious prions and the normal proteins, each disease typically passes only among members of the same species. However, some prion diseases can pass between species--bovine spongiform encephalopathy has killed people, for example.

Earlier studies in cows had suggested that muscle and other tissues from infected animals are free of prions. Thus, hunters handling or eating muscle from deer with CWD hadn't previously been considered at risk for prion exposure (SN: 11/30/02, p. 346). However, some researchers suspected that muscles of infected deer carry prions, says Glenn Telling, a microbiologist at the University of Kentucky in Lexington.

Telling and his colleagues took samples from the brains and muscles from deer infected with CWD and from deer that didn't have the disease. The researchers injected these samples into the brains of mice that were genetically engineered to carry the deer form of the normal prion protein, making them susceptible to CWD.

As expected, mice injected with bits of CWD-infected deer brain readily came down with CWD, showing telltale neurological symptoms within 9 months. Mice injected with tissue from uninfected deer never showed signs of the disease. However, Telling's group found that mice injected with infected deer muscle also developed CWD, although symptoms took about 5 months longer on average to arise than they did in mice receiving infected brain matter.

The researchers report these results in an upcoming Science.

Telling notes that the findings don't necessarily mean that people who eat meat from infected deer will come down with CWD. "Whether or not humans will be susceptible to CWD is a gray area right now" he says.

However, says neurologist Patrick Bosque of Denver Health Medical Center, the new results "clearly support [the possibility] that humans who eat CWD-infected deer are exposed to prions. If humans can avoid consuming meat from these deer, they probably should."