In the last column, the spread and contagious of Chronic Wasting Disease (CWD) was discussed. This column will explore the symptoms of CWD, how it effects diseased animals, and the possible spread to other species.
CWD is in a group of diseases that include Mad Cow Disease and Scrappies in sheep. All three are caused by a misfolded prion that infects the animal and leads to the degeneration of the brain and other nervous system tissue.
According to the CWD Alliance, the prion promotes the conversion of normal cellular protein to the abnormal form. Because the CWD prion is smaller than most viruses, the presence of the prions is not detected by the host animal’s immune system and hence is able to slowly spread.
CWD has an incubation period that can last 16 to 24 months. Cases of the disease occur most commonly in adult animals, but also in yearlings. After the disease takes hold in an animal, it is usually dead within a year. The disease is 100 percent fatal.
Symptoms and diagnosis
A typical CWD infected deer is severely malnourished with ribs protruding through its hide. Chunks of hair are often missing, the animal is drooling and shows no sign of fear of people or other predators and usually stays near a water source.
Since other deer diseases have similar symptoms — and deer in the incubation phase show no visible symptoms — the only way to positively determine if an animal has CWD is to clinically test it by analyzing tissue from the brain, lymph nodes, or tonsils. Therefore, deer with CWD infections are be killed and eaten by a wide variety of animals and people.
Research shows that some species that regularly consume CWD contaminated venison includes, but is not restricted to, coyotes, dogs, crows, raccoons, cats, small rodents and people. One of the biggest questions in CWD research is: can or will CWD spread to species other than deer, elk and moose?
The research so far has shown no inclusive results in trying to get an answer to this question, but it does demonstrate some disturbing trends.
Infection in other animals
Dogs and other canine animals seem to be resistant to prion infections. However, the prions can survive the canine’s digestive tract and are then spread through the canine’s feces. There does not seem to be any significant research on the effects of CWD prions on birds.
Mink can and do contract a specific form of prion disease called Transmissible Mink Encephalopathy. In lab experiments, Syrian golden hamsters do not get CWD if contacted directly, but if the prions pass though a ferret first, then the hamsters can contract the disease.
Also, scientists demonstrated that hamsters that were fed wheat plants that had absorbed CWD prions from the soil also developed the disease. Hence eating plant-based food could be another possible means of contracting CWD.
Experiments were conducted on both squirrel monkeys and on macaques. The squirrel monkeys were susceptible to developing CWD while initially the macaques were not. However, in later experiments, some macaques did develop the disease after eating CWD contaminated venison. Since macaques are genetically closer to humans than the squirrel monkey, this finding shows there is a possibility that CWD could cross the species barrier and ultimately effect humans.
Animal to human cross-over
There is also the example of BSE — or Mad Cow disease — which crossed over to infect humans in the 1990s. To date, BSE has killed over 200 people. Creutzfeldt Jakub disease is a prion disease that is specific to humans. So, there is precedent and a strong possibility that people can contract CWD.
Currently, a massive experiment is being conducted in North America, and in Wisconsin, to determine if CWD can jump the species barrier and infect humans. Every year an estimated 7,000 to 15,000 CWD infected deer are consumed by humans. It is estimated that 4,000 of those deer are consumed in Wisconsin.
If a deer is harvested in a known CWD area, the only way to know if the deer is infected is through testing. Yet in Wisconsin, the number of CWD tests have been steadily declining since Governor Walker was elected.
In 2006, 20,000 deer were tested and in 2014 3,665 tests were run. The number of deer testing positive has increased, so the percent of the deer herd that has an active CWD infection had also greatly increased. As infection rates increase, it is estimated by the Alliance for Public Wildlife that the number of CWD infected deer consumed by people will double every three years.
As the number of animals with CWD consumed by people increases, will it make it more likely that the CWD prion will change into a form that will infect people? Another possibility is that the CWD prion will pass through an animal like a mouse and then be absorbed by a plant and then move on to infect humans.
What can be done to stop the spread of CWD and protect people from a potential CWD jump from deer to people and to protect the uninfected deer herd? Science and sound public policies are the keys to combating CWD.
Unfortunately, both have been in short supply in Madison since 2011. The science section in the Department of Natural Resources has nearly been destroyed and efforts to stop the spread of CWD have been ignored.
The amount of CWD testing must be vastly expanded, the science section must be rebuilt, and the University of Wisconsin system needs additional funds and support to research into this disease.