Chronic wasting disease discovered in wild moose

By Linda Bell
Correspondent

A September announcement by the Colorado Division of Wildlife of the first confirmed case of chronic wasting disease to be found in a moose puts a new and different puzzle piece on the table for CWD researchers.

The moose was harvested by a bow hunter in Jackson County, south of Cameron Pass. "We were disappointed about finding the moose," said Kathi Green, disease management coordinator for the DOW, "and consider it a rare occurrence because the opportunities for transmission are so much less."

Moose are the least gregarious of the native deer family. Historically never prevalent in Colorado, the DOW began reintroducing them into the state in 1978 and the total herd size is now estimated at more than 900, according to Mark Vieira, DOW wildlife biologist for area 4. Moose are mainly browsers and prefer to forage woody plants like willows and quaking aspen, but during summer they graze on emerging woody plants and forbs and eat aquatic vegetation in ponds and streams.

Green said the DOW knew moose were susceptible to CWD because a controlled laboratory experiment in Wyoming in 2004 produced a positive diagnosis on a captive moose injected with infected brain tissue. It has been mandatory for harvested moose to be checked by the DOW for CWD since 2003. In 2004 hunters harvested 103 moose in the state, out of 133 available licenses.

Chronic wasting disease is classified as a transmissible spongiform encephalopathy, or prion disease, along with other animal diseases such as scrapie in sheep and bovine spongiform encephalopathy in cattle. Until the free-ranging moose was diagnosed, the only known natural hosts for CWD were deer, both whitetail and mule, and Rocky Mountain elk.

Most epidemiological research on CWD focuses on elk and deer herd density and contaminants left in soils from urine, feces and carcasses. Green said controlled experiments have shown CWD to be more prevalent in deer than in elk, probably because of their social habits.

A June 2004 paper presented in "Emerging Infectious Diseases," a publication of the Center for Disease Control and Prevention, written by Belay, Maddox, Miller, Williams, Gambetti and Schonberger, states "the mode of CWD transmission among deer and elk is not fully understood; however, evidence supports lateral transmission through direct animal-to-animal contact or as a result of indirect exposure to the causative agent in the environment, including contaminated feed and water sources." This article quotes an earlier study on mule deer by Michael Miller and the late Elizabeth Williams, lead scientists in CWD research at the Colorado DOW and the University of Wyoming, respectively.

Since the disease can jump between members of the deer family, research is focusing on whether it can jump to other animals, including domestic livestock and, more importantly, humans. The 2004 paper quoted above is titled: "Chronic Wasting Disease and Potential Transmission to Humans." The study's researchers looked carefully at various reported cases of prion disease in humans and came to the conclusion that there is a "lack of evidence of a link between CWD transmission and unusual cases of Creutzfeldt-Jakob disease, despite several epidemiologic investigations, and the absence of an increase in CJD incidence in Colorado and Wyoming suggest that the risk, if any, of transmission of CWD to humans is low."

The study does acknowledge, however, that even though CWD does not appear to occur naturally outside the deer family, it has been transmitted experimentally in controlled conditions to mice, ferrets, mink, squirrel monkeys and goats. Conversely, Green said, a captive herd of fallow deer at the DOW research facility show no signs of becoming infected with CWD even after many years of exposure. Fallow deer are not native to North America and have origins in Persia.

Isolated incidences of CWD in deer in previously unknown areas of infection are increasing, Green said; however, more animals are also tested now for CWD. She said a recent road kill in West Virginia tested positive and there have also been positive isolated cases in New York, Utah and New Mexico, far from endemic regions or commercial captive herds.

Green said even before the DOW knew what CWD was, they saw it in infected animals that were wasting away. She said it is hard to pinpoint the origins of the disease, whether it is an historic condition or was introduced into the environment more recently.

Michael Miller at the Colorado DOW and research associates at the University of Wyoming and CSU have over $4.7 million in grants from the National Science Foundation, National Institutes of Health, the USDA and the Department of Defense to study the various dynamics of CWD.

In addition, CSU scientists with the Prion Research Laboratory in the College of Veterinary Medicine and Biomedical Sciences were awarded an $8.4 million grant in 2002 by the National Institutes of Health. The seven-year grant is in its third year. Dr. Edward Hoover, CSU distinguished professor and director of the laboratory, is the lead researcher.