Researchers discovered chronic wasting disease in Colorado and 42 years later the Centennial State remains at the center of studies into the fatal neurological disease found in deer, elk and moose.
For example, a team of scientists from the Colorado Division of Wildlife’s Wildlife Research Center and the University of California at San Francisco reported on a potential break-through in how the prion disease is transmitted.
Reporting in the journal Nature, the team said it discovered that deer asymptomatic for chronic wasting disease excrete the infectious prions that cause the disease in their feces. Further, they found, the deer with CWD shed prions in their feces months before they show clinical symptoms of the disease.
Deer, elk and moose routinely eat feces and soil as they graze in the wild. The research team wanted to know if such inadvertent consumption could result in the transmission of CWD. They went about measuring the amount of prions in the feces of orally infected deer up until they became symptomatic and then calculated whether the prolonged exposure to the concentration of prions in the feces would be enough to cause the disease.
“Prion levels in feces samples of asymptomatic deer were very low compared to those in the brains of the same deer at time of death,” says Dr. Erdem Tamguney, the lead author of the study. “However, the total number of prions excreted over time was sufficiently high enough to cause the disease in other deer.”
Chronic wasting disease is now found in 14 states and two Canadian provinces, mostly in the West. In wild herds like those in Colorado’s Rockies, as many as 30 percent of the animals can be affected. In captive herds, it can spread to every animal.
The susceptibility of animals to infection might be increased by the simultaneous ingestion of clay soil, which scientists think enhances the infectivity of prions by slowing their clearance from the gastrointestinal tract.
Photo: White Tail Deer, Wisconsin Department of Natural Resources.
The UCSF-CDW study did not examine whether chronic wasting disease would be transmitted to humans from prions in deer feces. No evidence exists that CWD can be transmitted to humans, but scientists have not ruled it out.
“We can only say that prions of chronic wasting disease have not transmitted to mice genetically engineered to carry the normal, healthy form of human prion protein in earlier studies, ” says Dr. Stanley B. Prusiner, UCSF professor of Neurology.
The prion is an infectious form of the normal prion protein, which has been found in all mammals, including humans. When it strikes, the normal protein twists and turns initiating a disease process that ravages the brain.
Prion diseases in cervids (deer, elk, and moose), sheep, cows, and humans are also called spongiform encephalopathy.
So-called “horizontal transmission” of Bovine Spongiform Encephalopathy (BSE) from one cow to another is rare. Other studies have shown the prion diseases can only be spread orally, and deer, elk, and moose are neither carnivores nor cannibals.
That’s why the rapid spread of CWD through herds was such a mystery, until now.