Pennsylvania’s abundant deer population is one of its most striking natural features.
According to Penn State University, the state averages about 30 deer per square mile of land. However, an infectious disease poses a growing threat to Pennsylvania’s deer herds, one that eventually destroys deer’s brains, making them weaker, dumber and eventually killing them outright.
Chronic wasting disease, or CWD, is a contagious neurodegenerative disease, similar to mad cow disease, and affects the family of animals that includes deer, elk, moose and caribou.
CWD was discovered in 1967 and first detected in Pennsylvania in 2012, although the state began testing for it in 1998. It is currently known to be in 25 states and multiple countries across the globe, and its prevalence continues to grow.
When CWD is detected somewhere in the state, the Pennsylvania Game Commission establishes something called deer management areas or DMAs, where special rules are implemented to reduce the spread and expansion of the disease.
There have been no confirmed cases of CWD in the Lehigh Valley proper, but a deer management area covers parts of Carbon, Schuylkill, Luzerne, Montour, Northumberland and Columbia counties to the north and a different one includes much of Lancaster County to the southwest. There are nine active DMAs in the state, and most of southcentral Pennsylvania is included in a management area.
How chronic wasting disease spreads
The first confirmed cases of CWD in Pennsylvania were among captive deer, and the disease appears to arise most commonly in captive and livestock populations before first spreading to wild deer. The state Department of Agriculture is in charge of managing CWD in captive herds.
Travis Lau, spokesperson for the game commission, said CWD is passed to deer either through direct contact or indirectly via contact with infected matter.
“CWD is 100% fatal for animals that are infected. Part of the challenge with that, and part of the challenges of managing CWD, is that an infected, CWD-positive deer could look absolutely normal. It can take up to 18 months for deer to develop clinical symptoms of CWD,” Lau said.
Visible symptoms of CWD can include disorientation, loss of body weight, excessive salivation, excessive urination, some loss of motor control and lack of fear of humans. Lau said looking for deer with CWD based on symptoms is largely impractical, though, due to how long it can take for symptoms to manifest as well as the fact that many Pennsylvania deer will not live long enough for symptoms to show.
Andrea Korman, Chronic Wasting Disease Section supervisor for the game commission, said other conditions can cause similar outward symptoms to CWD as well.
Like mad cow disease, CWD is caused by prions, which are misshapen proteins that can arise naturally in the body, Korman said. These prions can spread to the nervous system and begin damaging the brain.
“They form plaques in the brain, which causes neural cell death, and as those neural cells start to die, it causes holes in the brain. And obviously you can’t really survive with holes in your brain,” Korman said.
Korman said that this brain damage will eventually progress to the point where it kills the deer on its own, but as it progresses it also makes it harder for the deer to survive.
“They’re more likely to get hit by a car. They’re more likely to be harvested by a hunter because it’s making them less aware,” Korman said. “But they’re also dying with a co-infection of pneumonia, and what that seems to be is because that excessive salivation, they’re aspirating it, kind of inhaling that saliva, and it causes a pneumonia infection. And so a lot of times, if they die at the end stage of CWD, we will we will find they also have pneumonia.”
At the same time, they also become infectious. A deer can transmit prions to another deer directly, or through infected urine or feces. Some studies have shown that CWD also can spread through the digestive systems of crows and coyotes and spread to new areas this way. Ticks and plants also can carry CWD infectious prions.
What causes chronic wasting disease?
Unfortunately, the mechanism behind the disease still is not well understood. It is not known why proteins misfold, causing CWD or other conditions, Korman said. It isn’t clear why prions specifically damage the nervous system. What is known is that they are extremely durable, capable of withstanding freezing, thawing, temperatures above 1,000 degrees Fahrenheit, and persist in soil and groundwater without denaturing. During that time, they will remain infectious, unless exposed to synthetic enzymes specifically created to break them down. Prions also appear to be highly infectious.
“There’s still a lot of research being done on what is an infectious dose. It seems that it’s not a high dose that would be required. But they don’t know. Is it a one-time exposure? Is it multiple exposures? Does it matter?” Korman said. “There’s just still a lot that they don’t know.”
She added that there is no immune system response to prions even when they are transmitted, though some deer appear to be more resistant to chronic wasting disease than others.
“There are certain genotypes that seem to be less susceptible. They can still get it, but they might live a little bit longer with it. So genetics can play a little bit of a part. But in terms of an immune response, there does not seem to be one,” Korman said.
What hunters should know about chronic wasting disease
Lau said even though deer with chronic wasting disease may lose their natural fear of humans, he isn’t aware of documented cases of CWD infected deer attacking humans; the greater concern is if humans could be infected. The No. 1 cause of deer death in Pennsylvania is hunters, and preventing CWD from becoming a risk to humans as well as safeguarding the state’s deer herds are the top priorities of the state’s CWD management.
There is no indication that CWD can pass to other non-cervid animals, including humans at this time, but humans are susceptible to prion diseases, such as Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease and Kuru, all of which are fatal. Eating meat from a cow infected with mad cow disease can cause humans to develop a variant of Creutzfeldt-Jakob disease. There is no cure for any prion disease.
Out of general caution and because of guidance from the Centers for Disease Control and Prevention, the Pennsylvania Department of Health recommends people not consume meat from CWD-infected deer and dispose of it.
Lau said the game commission’s surveillance is thorough, but public assistance plays a critical role. Anyone who suspects they see a sick deer should contact the state game commission so the suspected deer can be killed and tested for the disease. The game commission also requests that hunters in DMAs submit the heads of deer they kill so that the specimens can be tested. The owners of captive herds also are expected to report signs of CWD.
The state game commission prohibits the feeding of wild cervids and rehabilitating of white-tailed deer in active DMAs. Removing high-risk cervid parts, which include the head, the spinal column and the spleen, from deer within a DMA or disposing of high-risk parts out in the wild are also forbidden. However, high-risk parts can be taken directly to a game commission-approved cooperator or disposed of with commercial trash service within that DMA.
Lau said that through surveillance and proper disposal, CWD can be eliminated in an area. For example, the state eliminated all evidence of CWD in the first DMA it ever declared, though in this case, only captive deer were affected. But he added that detection of CWD across the state has increased over the last decade, not decreased. It is not certain that CWD spreading across the state is inevitable.
“The Game Commission continues to see value in regulating CWD and the rules that are established within DMAs, trying to limit the disease and to keep it out of areas where it’s not,” Lau said. “Whether it’s headed that direction, [spreading across the state] I don’t know. In a world where we continue to see expansion of these DMAs and then some expansion or disease, maybe that is the case.”