What is Chronic Wasting Disease (CWD)?
CWD (Chronic Wasting Disease) is a contagious disease caused by prions. The disease appears in free-ranging deer, captive deer, moose and elk causing neurodegenerative damage in adult animals. As a member of the TSE (Transmissible Spongiform Encephalopathy) family of disease, it is very similar with Scrapie in sheep and goats and the Mad Cow Disease (Bovine Spongiform Encephalopathy).
Prions are unconventional pathogens that only consist of proteins. This protein is a misfolded isoform of a normal cellular prion and thus cannot be digested because of its resistance to protease digestion. Eventually, the prion accumulates in neuron a cell which causes neuronal death.
The first time the disease was identified was back in the 1960’s in Colorado among captive deer. A decade later the link between CWD and Scrapie were recognized.
So far the Chronic Wasting Disease has been found in animals in the USA (Mostly the western states) and Canada, while outside North America has been identified only once.
Transmission of Chronic Wasting Disease
CWD is generally horizontally transmitted disease and even vertical transmissions have been reported. The horizontal transmission between cervids can be through direct contact between healthy and diseased animal, contact or ingestion of fecal or urine contaminated grass or contact with a carcass with CWD.
The dense population of farm-raised cervids results in potentiating the spread of the disease. Prions are very resistant and can stay infective for long periods of time in nature.
This disease can be transmitted to humans as it is a zoonosis. However, there hasn’t been a reported case of diseased people. That’s mostly due to the species barrier protecting people from epidemics with Chronic Wasting Disease. Not many scientific facts have been discovered about the species barriers and human transmission, but caution must remain a priority.
Symptoms of Chronic Wasting Disease
The symptoms can be very unspecific at the early stages. They start to appear in animals that are older than 16 months of age and are in the form of weight loss and modified behavior at first. It’s hard to notice these subtle changes, especially in free-range animals.
Later on, the symptoms progress and result in somnolence, polyuria and polydipsia, changes in interaction with other herd members or caretakers, persistent walking and overexcitement when irritated. During this period tremors of the head muscles and ataxia can also appear.
The late stages of CWS disease include specific signs such as drooped ears, hypersalivation, low head carriage, teeth-grinding and fixed eye-gazing. Aspiration pneumonia is common and usually the main cause of death, except for death by misadventure as a result of the neurological damage.
Control and prophylaxis
Like any other member of the TSE family of diseases, this one also cannot be a cured. There are some regulative control standards in the USA and Canada which are performed on farmed herds of cervids mostly because controlling the free-ranging cervids is an extremely difficult task.
The farmed control is achieved with depopulation, individual animal identification, testing all animals on Chronic Wasting Disease if they die after a certain age and limiting the addition of new herd members from herds of higher CWD status.
The regulations for controlling CWD vary and mostly depend on location. Besides limiting the movement of wild cervids herds from endemic areas, attempts to control the disease included a reduction in population density, intensified surveillance of wild cervids and testing and removal of positive individuals.
Those animals positive on CWD must be properly disposed of using medical incinerators, certified landfills and special equipment for alkaline digestion.
Not-effective control management
The disease was supposed to be monitored and controlled for some time now, however, the latest result shows an enormous spread and growth of infected animals. The situation is so bad, that some locations risk deer and elk extinctions.
According to Dr. Todd E. Cornish, an associate professor who studies CWD at the University of Wyoming, a 10% decline per year of the deer population is noted in areas where the prevalence of the
disease is around 30%. That is a significant loss in wildlife.
The wild-life experts confess that for now, they don’t have any effective tools for eradicating the disease or even minimizing the spread. But that doesn’t mean that they don’t work hard to figure out how to do this with success.
Vaccine development was something many experts were talking about initiating, but the study program kept its status Q.
New methods of management Chronic Wasting Disease
The control efforts for CWD are quite limited and mainly cover activities of depopulation by eliminating infected animals and sometimes activities that alter the transmission rate of the disease.
The herds that have many infected individuals in particular areas will probably vanish in time because the transmission rate is too high and already uncontrollable. With this, the most important thing is to prevent contact of infected herds with herds that are free from the disease or have few infected animals and low transmission rate.
This can be achieved with preventing animal translocation and with establishing buffer zones between infected and non-infected populations.
Another point which requires great commitment during a course of many decades is early diagnosing of CWD. The early diagnosing of sub-clinical cases and removing them from a herd in time will lower the dispersion rate and the rate of newly infected cases.
Still, the main problem is that there aren’t any effective diagnostic field methods that make it possible to easily test wild-range deer.
Even though many resources have been spent in the last few decades for managing the spread of Chronic Wasting Disease, the ineffectiveness of the control, led to what seems to be an epidemic among cervids in Northern America, potentially harmful for humans.
Even though new methods of control of the disease are worked on, and some of them even put to test, no one is 100% sure if there will be any positive effect. Nevertheless, if something isn’t done fast the crisis with deer and other cervids in America can get much worse and eradicate an entire population of species.
Catharina is a Veterinary Medicine student from Uni of Copenhagen. When she isn’t making camp in the library, stuck to the books, she’s also a writer and avid photographer. Capturing everything from buildings to dogs – especially her poodle Bailey is a frequent subject.