Penn Veterinary Medicine | University of Pennsylvania

Penn Vets Keep Watch on Chronic Wasting Disease

By Susan I. Finkelstein

Because of the extensive media coverage in the past several years, almost all of us are familiar with the ravages of bovine spongiform encephalopathy (BSE)—or, as more commonly known, mad cow disease. Through November 2003, more than 183,000 cases of BSE were confirmed in the United Kingdom alone, in more than 35,000 herds. Millions of cattle were destroyed in attempts to halt the disease, which is linked to approximately 140 human deaths worldwide. The first U.S. case was found in December 2003 in a dairy cow in Washington state. 

Conversely, few of us—mostly scientists, wildlife commissioners, hunters and others—are aware of a similar disease spreading among cervids, that is, deer, caribou, elk and moose. Chronic wasting disease (CWD), like mad cow disease, is a neurological disease that attacks the brain and produces small lesions ultimately resulting in the infected animal’s death. Although how the disease spreads is not exactly known, experts believe the agent responsible for the disease may be transmitted both directly (animal-to-animal contact) and indirectly (soil or other surface to animal). Saliva and feces are believed to be the most common carriers of the CWD agent.  

Outbreaks of CWD have been reported in wild deer and elk in Colorado, Wyoming, Nebraska, South Dakota, Wisconsin, New Mexico and Saskatchewan. It has been diagnosed in game ranches in Colorado, Nebraska, South Dakota, Montana, Oklahoma, Kansas, Alberta and Saskatchewan. So far, the disease has not been detected east of Wisconsin. 

New Bolton assists commonwealth’s efforts

While no known cases of CWD exist in the commonwealth, the Pennsylvania Game Commission, joined by veterinarians at New Bolton Center and laboratory technicians from the Pennsylvania and U.S. departments of agriculture, has been diligent in its efforts to test for the disease. In Pennsylvania in 2003, samples from more than 2,000 hunter-killed deer and 55 hunter-killed elk tested negative for CWD at the Pennsylvania Animal Diagnostic Laboratory System in Harrisburg and at the diagnostic pathology laboratory at New Bolton Center. This was the third year for testing hunter-killed elk, and the second for hunter-killed deer.

While there are no known cases of CWD in the Commonwealth, the Pennsylvania Game Commission, joined by veterinarians at New Bolton Center and laboratory technicians from the Pennsylvania and U.S. departments of agriculture, has been diligent in its efforts to test for the disease. 

Beginning December 2004, which marked the start of Pennsylvania’s two-week concurrent rifle deer season, Game Commission officials started collecting 4,000 deer heads randomly at meat-processing facilities around the Commonwealth. The CWD tests on the samples will be conducted in Harrisburg and at New Bolton Center. Currently, testing live animals remains impractical, and no vaccine against the disease has been developed, nor is there a cure for animals that become infected.

The New Bolton laboratory has designated personnel to perform these specific tasks, in the extremely low possibility of personal health risks. Necropsy prosector Ralph Conti trims the tissue that is placed in the processing cassette. Suzy Hindman is the histotechnician; Jackie Ferracone is the immunohistochemistry technician; and Perry Habecker, V’90, staff pathologist, is the designated reader. 

Preventing the spread of CWD

The interagency taskforce is also focusing on ways to prevent CWD from spreading to Pennsylvania and to ensure early detection should it enter the commonwealth, as well as planning a comprehensive response to contain and eradicate CWD should it be found here. Effective August 1, 2002, the Game Commission closed Pennsylvania’s borders to all importation of any live cervids from any state or nation. In January 2003 the commission enacted regulations, among other things, to restrict shipments of live deer and elk from states and Canadian provinces with CWD monitoring programs. Other actions being considered to prevent CWD from entering or being spread in the Commonwealth, include a ban on feeding deer. Current Pennsylvania law prohibits the feeding of elk. Also, several states have banned the importation of certain body parts from infected states, and Pennsylvania is considering doing the same. Commonwealth officials also require deer and elk breeders to cooperate by submitting the heads of any adults that die on their premises.

Animals infected with CWD may not show any symptoms in the early stage; however, as the disease progresses, the animals begin to lose bodily functions and stagger or stand with poor posture. Animals may carry the head and ears lowered. Infected animals become emaciated (thus wasting disease) and will appear in very poor body condition. Animals with CWD also often will stand near water and consume large amounts of it. Drooling or excessive salivation may be apparent. Although these symptoms may also indicate other diseases, you should note the animal’s location and immediately contact the nearest Game Commission Regional Office if you see a deer or elk displaying these symptoms. Do not attempt to disturb, kill, or remove the animal. (In Pennsylvania, you can find the office nearest you on the Game Commission’s Web site.)

Dr. Habecker explains how the samples are tested. “Specifically, the test samples are the brainstem and lymph nodes at the back of the pharynx. One lymph node will be frozen, and the remaining node and stem are put into formalin [a water-formaldehyde solution]. The formalin-fixed specimens are subjected to application of an antibody to an exquisitely thin piece of tissue. If the CWD agent is present, the antibody binds to it, and subsequent chemical steps cause a color reaction to occur: positive samples have a bright red ‘crust’ surrounding cells. We will only be testing one lymph node. If an inconclusive test result occurs, the brainstem will then be subjected to the above test and the frozen lymph node tested via an ELISA [enzyme-linked immunosorbent assay] test.”

Can CWD infect people? Researchers have seen no evidence that chronic wasting disease is transmissible to humans or to other non-cervid livestock under normal conditions; however, the transmission of mad cow disease to humans as Variant Creutzfeldt-Jakob disease indicates that, provided sufficient exposure, the species barrier may not completely protect humans from CWD. Because the disease has occurred in a limited geographic area for decades, an adequate number of people may not have been exposed to the CWD agent to result in a clinically recognizable human disease, but the level and frequency of human exposure to the agent may increase with the spread of CWD in the United States.