Jan 31, 2006 (CIDRAP News) – Meat from deer infected with chronic wasting disease (CWD) has been shown to transmit the prion disease to genetically altered mice, according to a report published Jan 26 in Sciencexpress, part of Science magazine.
The experimental findings "raise the possibility that dietary exposure to prions might occur through meat consumption," writes researcher Glenn Telling at the University of Kentucky, along with six other colleagues.
CWD, a rare brain-wasting disease that affects deer, elk, and moose in parts of North America, is similar to bovine spongiform encephalopathy (BSE), or mad cow disease. It has not been known to infect humans, and the skeletal muscle meat from deer and elk was not previously believed to contain infective levels of abnormal prion protein. However, state game agencies now advise hunters to take precautions when handling carcasses, and some states test harvested animals for CWD.
"The emergence of CWD in an increasingly wide geographic area and the interspecies transmission of bovine spongiform encephalopathy (BSE) to humans as variant Creutzfeldt Jakob disease (vCJD) have raised concerns about zoonotic transmission of CWD," Telling and colleagues write.
"We don't know if people will develop a human prion disease as a result of exposure to CWD but we know people developed prion disease as result of exposure to BSE," Telling said in a Reuters news service story Jan 27. "Obviously the most likely route of exposure would be via meat, and that is why we addressed this."
"It's difficult to predict how prions will behave when they cross species barriers," Telling told Reuters.
To test the infectivity of skeletal muscle, the researchers used specially adapted mice that expressed cervid prion protein (CerPrP), which made them susceptible to CWD. Groups of these mice were inoculated intracerebrally with extracts of leg muscle from either CWD-infected or CWD-free mule deer, while other mice were inoculated with central nervous system (CNS) materials from both infected and uninfected deer.
The researchers found that all skeletal muscle samples from infected deer led to the progressive mental dysfunction associated with transmissible spongiform encephalopathies (TSEs), with mean incubation times ranging from 360 to roughly 490 days. The incubation period after infection with CNS material was shorter, from about 230 to 280 days.
The skeletal muscle and brain tissue from uninfected deer didn't cause disease in the special mice.
The researchers note that similar studies involving skeletal muscle from BSE-infected cattle failed to show such high levels of prion infectivity. The study also shows that prion levels were lower in muscle than in the CNS and that the muscle prion levels may vary. Studies to assess the prion titers are under way, the authors write.
The report says the risk of CWD infection following consumption is reduced by relatively inefficient prion transmission, but that the findings show the muscle tissue most likely to be consumed by humans is a significant source of prion infectivity.
"Humans consuming or handling meat from CWD-infected deer are therefore at risk to prion exposure," the report concludes.
Both the findings and the methodology of the study are significant, said Srinand Sreevatsan, DVM, MPH, PhD, an associate professor in the University of Minnesota's Department of Veterinary Population Medicine.
The presence and infectivity of CWD prion in muscle hasn't been demonstrated with a bioassay before, Sreevatsan told CIDRAP News. The bioassay allows researchers to know if the misfolded prions are present and if they are capable of causing the disease in an animal model. "It's only now that we are realizing the sensitive assays we can use."
"The presence of infectivity in the muscle tissue in CWD does raise some issues. That it's there and it is infectious is a concern," he said, but added that it is important to do a risk assessment as well. "Is it present in enough quantity to be of public health significance? We don't know. That needs to be determined."
"It may not be in sufficient quantities to cause infection [if people consume it]," he added.
Telling G, Angers R, Browning S, et al. Prions in skeletal muscles of deer with chronic wasting disease. Sciencexpress 2006 Jan 26 (early online publication) [Abstract]
Oct 26, 2005, CIDRAP News story "Chronic wasting disease found in moose"
Institute of Food Technologists scientific summary report "Transmissible Spongiform Encephalopathies," published Jun 2005