Chronic wasting disease (CWD) is a contagious neurological disorder that affects deer, elk, reindeer, and moose that leads to degeneration of brain tissue. Symptoms in infected animals include emaciation, abnormal behavior, loss of bodily functions, and ultimately death. CWD is a member of the transmissible spongiform encephalopathies (TSEs) which include scrapie and mad cow disease.
Chronic wasting disease (CWD) is a neurodegenerative disorder that affects deer, elk, reindeer and moose. It is caused by a misfolded protein known as a prion. Credit: Colorado State University
CWD can has been detected in animals in North America, Canada, Norway, and South Korea. As of September 2017, the CDC reported that at least 21 states in the continental US found CWD in free-ranging deer. While affected areas continue to expand, the overall incidence of CWD is relatively low.
Fortunately, there has been no reported incidence of CWD in humans however; studies have shown that it is possible for CWD to spread from infected deer or elk to non-human primates such as monkeys. The disease could potentially spread to humans through the consumption of contaminated meat or through contact with the brains or bodily fluids of CWD-infected animals. The World Health Organization advises it is important to prevent agents of prion diseases from entering the food chain.
Detection of prion diseases is difficult due to the nature of this atypical pathogen. CWD is caused by the misfolded form of a normal protein. As proteins are made within the cell, they fold into a specific configuration that allows them to perform a specific function. When proteins are misfolded, they are unable to perform their function, leading to disease.
A group of researchers from Colorado State University and the Alaska Fisheries Science Center used predictive modeling to discriminate the shedding of prions and the detection of prions in saliva. According to the authors of the study, this distinction is important for understanding the role of prion shedding in disease transmission as well as diagnosis of the disease. Sensitive detection methods are difficult for samples containing a high level of microbial diversity, such as saliva.
Their statistical analyses revealed that shedding of prions in saliva increases from the time the animal becomes infected. Shedding of prions can occur before the animal even shows symptoms of CWD which can occur for up to a year or more. Shedding was not influenced by the sex of the animal or by the specific genotype of the prion. Interestingly, authors of the study found that the source of the prion by which animals were experimentally inoculated played a large role in the quantity of prion shedding. Specifically, animals who were inoculated with prions from the saliva of CWD-infected animals shed more prions than those inoculated with brain tissue from CWD-infected animals.
This research provides insight into the horizontal transmission of CWD in animals as well as factors that influence prion shedding in the wild. More sensitive detection methods for better surveillance and diagnosis of the disease are warranted.
Sources: Chronic Wasting Disease Alliance, Journal of Clinical Microbiology, CDC