By Amy Horn-Delzer, DVM, Veterinary Program Manager, Wisconsin Department of Agriculture, Trade and Consumer Protection
In July, the Associated Press reported that “Midwestern wildlife officials wrapped up what was hailed as an unprecedented conference on chronic wasting disease Thursday without coming up with any new tactics for fighting the stubborn disease.”
Why is it that even Wisconsin, a state where officials discovered the disease within its borders 17 years ago, continues to fight the battle of CWD? The disease appears in new areas and at higher frequency in certain areas in both wild and farm-raised cervid each year. Since its first recognition 52 years ago in Colorado mule deer, CWD has found its way into 26 states, three Canadian provinces, South Korea, Norway, Finland, and most recently, Sweden. What is it about the disease that allows it to be so difficult to control and prevent?
About CWD
CWD is an infectious neurological disease of animals in the family Cervidae. It is one of a group of diseases known as transmissible spongiform encephalopathies (TSEs). Other diseases caused by TSEs include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats, and Creutzfeldt-Jacob disease (CJD) in humans. The causative agent of TSEs is a prion, a naturally occurring protein, that when it becomes incorrectly folded, turns infectious and deadly. In the body, the infectious prion then multiplies by causing the animal’s normal and healthy prion proteins to misfold, leading to the death of neurons. The incubation period can be long (months to years), and infected animals are likely to be in good body
condition until the final stages of the disease. Unfortunately, during the incubation period and the clinical stages, cervids are shedding the prion in bodily fluids, causing transmission of disease through direct contact and indirectly through fomites and contamination of the environment.
Animals in the late stages of CWD exhibit neurological symptoms such as incoordination, difficulty swallowing, erratic behavior, emaciation, polydipsia and polyuria. However, due to cognitive dysfunction and the slow advancement of the disease, deer infected with CWD frequently succumb to aspiration pneumonia, predation, vehicular death and hunting. The disease is ultimately fatal.
Disease Control Challenges
The nature of the causative agent of CWD itself creates a challenge for control. The infectious prion proteins, unlike most viral and bacterial agents, are resistant to degradation. Factors such as freezing, UV light, desiccation, chemical or heat will not break down a prion. In the environment, the protein will bind to clay soil particles, after which, research shows the prion becomes even more infectious than the unbound prion. The protein will cling to
the outside surface of plants and roots, and it has been discovered that certain plants are able to uptake the prion. The environment becomes contaminated by diseased animals shedding the prion protein in saliva, urine and feces. Infected deer carcasses and parts of carcasses can be laden with the infectious protein. Research shows that scavengers such as coyotes and crows which ingest CWD prion-infected tissue are not affected by the disease, but their excrement contains the prion, which may exacerbate the spread of CWD. The human action of moving infectious carcasses or acting as fomites also cannot be overlooked.
Wisconsin enjoys a large population of wild whitetail deer, which also poses a challenge for disease control. Disease doesn’t respect county or state boundaries, nor is a fence able to confine it. Any time a disease exists endemically in a wild population, it becomes very difficult to control the disease in farmed animals. For example, in parts of Michigan, tuberculosis (TB) in the wild whitetail deer population continues to infect nearby fenced cattle herds.
In addition to other complications of disease spread, epidemiologic investigations are often challenging with CWD. Prions are the only class of infectious agents that do not contain nucleic acids, RNA or DNA, nor does the host develop an immune response. Unlike diseases such as avian influenza and TB, origins of a prion cannot be mapped through nucleic acid sequencing or by antibody neutralization studies. Due to the slow nature of the disease, whatever incident occurred to introduce the prion to a population likely occurred years before the disease was identified.
Farm-raised deer keepers are especially challenged with controlling CWD, as prevention and treatment strategies are limited. There is no vaccine, no treatment and no live individual animal test. Additional research is needed; for all that scientists know about CWD, there remains much that is still unknown. CWD is a relatively new disease, unlike scrapie, which has been recognized for more than 250 years. Research of a prion disease is expensive and slow, since the disease moves slowly through the infected animal.
Looking Ahead
Why should we be concerned about CWD? This infectious disease is geographically expanding in both wild and farm-raised populations in Wisconsin and worldwide. In many areas where the disease is already established, the proportion of CWD-infected animals continues to increase. When infection reaches a certain point, herd populations decline. In CWD-infected farm-raised deer populations, unless aggressive purposeful management strategies are incorporated, the infection will continue to spread within the herd.
As the range and prevalence of CWD increase, so will the potential for human exposure. To date, there is no evidence of cross-species transmission of CWD to humans, however the Centers for Disease Control and Prevention and the World Health Organization recommend that you should not consume meat from animals infected with CWD.
Veterinary Care
How can veterinarians help in the battle against CWD? Equip farm-raised deer clients with knowledge and share your professional expertise of biosecurity and disease prevention measures. Work with your deer clients to develop a biosecurity plan and educate owners and farm employees to be active participants in complying with the plan. Conduct your annual herd examinations with care and integrity; ask specific questions in regard to herd management, animal additions and disease prevention strategies. Complete the required physical inventories, fill out the certificates of veterinary
inspection (CVIs) and test submission forms completely and accurately, verifying ear tag numbers, recording re-tags, and notifying the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) if animals do not complete the movement that is documented on a CVI. Although paperwork may seem tedious, DATCP relies on veterinarians for accurate data regarding farm-raised deer identification, location and testing. If you wish to become certified to TB test deer using the DPP blood test or to collect and submit samples for CWD, you must first complete
training. More information about training is available at https://datcp.wi.gov/Pages/Programs_Services/VeterinarianTraining.aspx.
To learn more about DATCP’s farm-raised deer program, visit https://datcp.wi.gov/Pages/Programs_Services/FarmRaisedDeer.aspx or contact DATCP at (608) 224-4872 for additional details.
For information regarding the wild deer population, visit the Department of Natural Resources at https://dnr.wi.gov/topic/wildlifehabitat/regulations.html.