Science Corner: Chronic Wasting Disease

Chronic Wasting Disease (CWD) is a fatal neurological disorder that occurs in deer, elk, and moose. While the infectious agents of most diseases are viruses, bacteria, or protozoans (along with some parasitic worms and fungi), these diseases are caused by proteins called prions. While all mammals produce normal prions that are used by cells, then degraded and eliminated after use, disease-causing prions are mis-folded and are not readily broken down. Because they are not broken down and eliminated, they accumulate in and damage both lymphatic and nervous tissue, including the brain. Collectively, these diseases are known as transmissible spongiform encephalopathies (TSEs). Loosely translated, this means “disease that turns a brain into a sponge”. When a disease-causing, misfolded prion encounters a normally-folded, functional prion, it induces the normal prion to change shape and fold up incorrectly, and because each of the disease-causing prions have a long lifespan, each can in turn cause an increasing number of other normal prions to misfold, with the number of prions increasing in an exponential growth pattern. 
  The exponential increase in the number of disease-causing prions present in an animal is reflected in the long asymptomatic period (where few or no symptoms of infection are present) of 18 to 24 months. After symptoms begin, death is likely to occur within only a few months. Symptoms of CWD in deer begin with weight loss and disinterest in eating and progress through lethargy, reduced social interactions with other deer, and an otherwise wild deer showing no fear of humans. The disease progression ultimately leads to death, and there is no cure.
  CWD was first detected in captive deer herds in Colorado and Wyoming in the 1960s and spread to wild herds in the 1980s. It is thought to have originated from a related prion-caused disease that occurs in sheep called Scrapie. The initial spread of CWD outside of Colorado and Wyoming has been linked to the transport of captive deer and elk from those states and between states. Remember that animals typically appear asymptomatic for as much as two years before showing symptoms and that initially tests were not widely available to detect CWD in these animals. 
  CWD is now known to be present in deer herds (both wild and captive) in 32 US states. Infection occurs by contact (typically nose-to-nose) between individual deer, but also may occur from the environment, as disease-causing prions can also persist in the soil where infected deer have been. For these reasons, it is a difficult, if not impossible, disease to eradicate from wild deer populations. The question of whether or not CWD can be transmitted to humans remains unanswered.
  Similar diseases, such Creutzfeldt-Jakob disease (CJD) and Kuru, occur naturally in human populations. CJD rarely can occur (or seems to occur) spontaneously, with no known contact, while Kuru occurs in certain areas of New Guinea and is associated with unusual funeral practices in a few indigenous groups. Bovine spongiform encephalopathy (BSE or “Mad Cow Disease”) has been documented as being transmissible to humans, causing what is known as variant-CJD. In the mid-1980s, BSE emerged in the United Kingdom, and over the span of about a decade, 178 who were thought to have eaten BSE-infected beef developed variant-CJD and died. The chain of infection was later determined to have spread through the UK cattle herds through feed tainted with BSE prions and then to people. This raises the possibility that other prion diseases like CWD might infect humans as well. CWD is the most transmissible of the prion disease family, showing highly efficient transmission between deer, elk, and moose. However, a 2024 study from the National Institute of Health using cultured human brain tissues suggested that there is a strong species barrier preventing CWD from being transmitted to humans. These scientists were able to readily infect these tissue cultures with human CJD prions, but following similar protocols, were unable to infect these cultures with CWD prions.
 While studies such as these provide optimism that CWD may not pose a human health concern, the possibility remains that some individuals may be more susceptible to CWD prions OR that CWD prions could mutate into new strains that COULD infect humans. Consequently, eating CWD infected venison should still be avoided. More specifically, avoid eating or coming into contact with the eyes, brain or spinal tissue, lymph nodes, and the spleen.
  So what is being done to monitor the spread of CWD in Texas? From the TPWD website: “To detect and manage this disease, the department has designated CWD Zones. If you harvest a deer in a CWD zone with mandatory testing, hunters are required to bring the deer to a CWD check station within 48 hours. When mandatory sampling is required in a CWD zone hunters must check each animal harvested and receive a CWD receipt before taking any part of that animal to the final destination.”
  In our general area, there are Mandatory testing zones in northern Bexar County, in Medina County, and within a 2-mile radius of several deer breeding facilities (where CWD-positive deer have been detected previously) in Zavala and Frio Counties. A monitoring/voluntary testing zone is present in southern Val Verde County. Additionally, there are voluntary check stations in Uvalde, Rocksprings, Leakey, Tarpley, and at the Chaparral Wildlife Management area in Dimmit County. There are also carcass disposal restrictions now in place requiring disposal of bones and unused parts of the carcass to either be disposed of on the property where the deer was harvested, disposed of in a commercial trash service, or buried three feet underground (judging by the number of deer bones that I see randomly scattered across the landscape, enforcement and compliance with these regulations is lacking). The state also monitors and samples road-killed deer for CWD, particularly near monitoring zones (painted with an orange “X” - note: painting carcasses also is used in monitoring/counting associated with vehicle-deer collision studies, among other reasons, so a carcass with other paint markings could have been marked for other purposes).