About fourteen years ago, Rocky Mountain Laboratory sponsored a program at the Hamilton Performing Arts Center on Chronic Wasting Disease research. As part of the National Institutes of Health, RML has been involved with research into prion related diseases like CWD for a long time. Now that CWD has been identified in Montana wildlife, RML once again sponsored a public presentation at the PAC last week to give the community some more insight into the disease and how Montana is attempting to manage it.
CWD was first found in Montana in 2017. A distant cousin of Mad Cow disease, CWD has serious implications for wildlife and hunters in Montana. In 2018, FWP detected 26 new cases of CWD among wild deer, including 21 cases along the northern border in every county from Liberty County east to the North Dakota border, and five cases within the CWD-positive area south of Billings.
Bruce Chesebro, Chief of Laboratory of Viral Diseases at RML, has been working on prion diseases at RML since 1985. His group was the first group to describe the molecular cloning of the prion protein gene that set this field in motion in the magazine Nature in 1985.
The question many people, especially hunters, are asking is: Can CWD be transmitted from wildlife to humans like Mad Cow disease?
The good news, according to Chesebro, is that all the laboratory research to date suggests that CWD is not readily transmissible to humans. On the other hand, the research results do leave open the possibility, however slight, that it may in a very few cases be possible. More testing is required to rule it out. At this point scientists are advising hunters not to consume the meat of any deer, elk or moose that has tested positive for CWD.
Chesebro compared CWD to three other prion diseases that involve mis-folding proteins. They are all different in terms of transmissibility to humans.
Sheep Scrapie, known since the 1700s, is present in almost all countries except a couple that have eliminated it, like Australia and New Zealand. People can live with these sheep and eat them and apparently remain unaffected by the disease.
Ones that are transmissible to humans include Creutzfeldt-Jakob Disease (CJD) and Kuru, discovered in the 1920s and 1940s respectively.
Creutzfeldt-Jakob Disease (CJD) is a rare, degenerative, fatal brain disorder. It can be sporadic, hereditary or acquired. It affects about one person in every one million per year worldwide; in the United States, there are about 350 cases per year. It belongs to a family of human and animal diseases known as the Transmissible Spongiform Encephalopathies (TSEs) or prion diseases. Spongiform refers to the characteristic appearance of infected brains, which become filled with holes until they resemble sponges when examined under a microscope. It can be transmitted from one human to another by contaminated medical instruments.
Kuru was identified in people of an isolated tribe who practiced ritual cannibalisms in Papua, New Guinea and has now almost disappeared. Kuru is considered an acquired prion disease.
CWD is found in deer, elk, moose, and reindeer. Despite its spread across the West, over the last decade there have been no reports of human infection.
Chesebro explained that proteins are long chains of amino acids that fold into different shapes. The sequence of the folding determines the function of the protein. In the case of these prion diseases, the protein is mis-folded and does not function correctly. It spreads by exposure to infected tissue and the misfolded protein, or prion, appears to help replicate the mis-folding “like seeding an ice crystal.”
Ordinary cleaning and disinfecting may not eliminate the infectious material which can last in the environment for years. It is resistant to harsh chemicals and can take a lot of heat. It is very difficult to sterilize a laboratory to the extent required to eliminate it.
There is also a lot we don’t yet understand about it. Symptoms can show up within weeks or months or even take decades from the time of exposure.
Brent Race, a veterinary scientist at RML, spoke about the specific experiments that the lab has been conducting over the last few years. He said that as a hunter and a parent with a daughter who hunts, he’s concerned about whether CWD can transmit to humans.
“Fortunately for those of us consuming venison, nearly all the laboratory research performed so far suggests that there is a strong species barrier preventing cervid CWD from infecting humans,” he said. He said epidemiological studies in Colorado, Wyoming and Wisconsin, where CWD has been present on the landscape for a long time, were also encouraging in that they did not indicate a history of transmissibility. Historical populations of middle age and younger people are not showing signs of prion disease increase.
Race described the various ways prion infection was tested in monkeys and “transgenic” mice, including injection into the brain and oral ingestion. There were a few inconsistent results in 4 out of 88 tests involving sub clinical examination of brain tissue for prions, however, indicating that further testing is required.
Montana Department of Fish, Wildlife and Parks disease ecologist Emily Almberg said that CWD was first identified in the south-central part of the state in 2017. They returned to the area and began intensive monitoring in 2018. They also began to monitor the Philipsburg area and the High Line. So far, they have sampled nearly 2,000 animals and had 26 detections.
In these designated management zones, FWP is doing the monitoring by taking samples at hunter check stations, at regional offices and by taking samples form road kills. Outside the designated management zones hunters are asked to get their own deer analyzed and pay the cost of that themselves.
The goal, she said, was to identify every area where infection was greater than 1% with a 95% accuracy. They also have a plan for management. That is to try and keep the infection below 5% of the population.
She said the agency had plans to manage the situation as well. There are restrictions on carcass transport, for instance, within the CWD Management Areas. CWD is particularly stable in environment worry about transmission for up to two years. Neither the full spinal cord nor the skull can be transported. Any taxidermy done must occur within the management zone.
The agency is also looking at managing the herds through hunting by increasing the harvest of mule deer bucks, which show an infection rate 2 to 3 times higher than the does. They may also direct hunters to “hot spots” where infection rates are climbing.
According to Almberg, there are a couple of study areas in Wyoming and Colorado where CWD has brought on population declines in mule deer. In Wyoming, one population suffered a 21% annual decline and in one heavily infected herd in Colorado saw a 45% decline.
Overall, however, the disease was a very slow-moving phenomenon, she said.
“We know that these declines are a possibility,” she said, “but overall this disease is very slow moving. We may not see any huge impact in our generation, but maybe our grandchildren will if we don’t do anything about it.”
Asked about the symptoms of an infected deer, she said in the terminal stages it could be emaciated, have a drooping head or drooping ears, or be walking in circles, stumbling, or other erratic behavior. It could be unresponsive to humans. But because symptoms can take so long to show, it is not a good way to judge infection. So far, all the symptomatic deer that have been analyzed turned out negative, and all the positives they have gotten looked in pretty good shape.