Chronic wasting disease is getting a lot of press lately. The illness – which only affects deer, elk, and related species – is spreading into more areas, spurring discussions about how to limit its spread.
A chronic, fatal condition, chronic wasting disease (CWD) affects the nervous system of the infected animal. This causes signs such as progressive weight loss, abnormal behavior, and incoordination. The slow progression to death can take years.
That slow progression makes it challenging for those tasked with figuring out where animals became infected. But the most challenging aspect of CWD control lies in the germ that causes the disease: not a fungus or a bacteria or a virus, but a mystifying molecule called a prion.
Other germs are amazingly complex compared to prions. Viruses and other germs contain nucleic acids (DNA or RNA) that regulate how the germ survives and reproduces, but prions are even simpler than a DNA molecule: they’re just plain protein molecules.
Interestingly, healthy animals and people already have prion proteins. They stick out from cells in the lymphoid system (glands throughout our body involved with our immune system) and nervous system. These normal prions don’t cause any trouble at all; in fact we’re not quite sure what they do!
But the prion associated with CWD is different. The CWD prion is folded up in a different way. When these abnormally-folded prions are taken into a deer’s body and find their way to the nervous system, they cause the normal prions to become abnormally folded themselves, in a chain reaction. When enough of these abnormal prions build up in the brain or spinal cord, they create holes that crowd out and destroy the normal cells. Then we see the abnormal behavior, loss of appetite, and wasting away, depending on where in the brain these holes are formed.
If that was the end of the story, it would be bad enough. But while the affected animal is alive, these abnormal prions leave the body through saliva, urine, and other excretions to potentially infect another animal.
For most infectious animal diseases, the infected and vulnerable animals have to be pretty close to each other to spread, since the germ can’t live outside the animal for very long.
Unfortunately, the opposite is true for the CWD prion. These abnormal prions can stay dangerous for a long time – years, even – outside the animal. Most of the things that inactivate germs out in the open – disinfectants, heat, sunlight – work by denaturing proteins (mess them up so they can no longer help the germ reproduce or survive), not destroying them. To make things worse, the way the abnormal CWD prion is folded makes it even tougher than normal proteins to destroy. In fact, this prion can survive rendering temperatures and all but the harshest disinfectants. High temperature incineration is about all that will inactivate it.
This is not good for disease control. A deer infected with CWD can slobber the prion everywhere it goes, where it can stay infective for months or years. Worse yet is when the brain and spinal cord of a deer that died from CWD decomposes into the soil because those body parts contain a lot of the CWD prion.
The potential of the CWD prion to survive in the environment is the guiding principle behind efforts to reduce its spread. Since infected animals can still harbor the CWD prion while appearing healthy, what happens to a deer or elk carcass (especially its brain and spinal cord) from a CWD-affected area becomes very important. Hunters are being advised (and beginning next year, regulated) to properly dispose of deer carcasses from the CWD area in South Dakota (the southwestern-most 4 counties), as well as from other states. These carcasses should be picked up by a garbage hauler that uses a regional landfill – and not just thrown out where a vulnerable deer could be exposed to CWD.
A further spread of CWD would have severe effects on our wild deer and elk populations. Hunters and others who deal with deer and elk carcasses can play a big role in limiting that spread. Understanding the unique nature of the prion that causes CWD is important in crafting those strategies.