The press releases came with distressing regularity this fall. All announced that Chronic Wasting Disease (CWD) was found in new locations in LaCrosse, Dodge and Milwaukee counties.
CWD was first identified in Wisconsin in 2001 in one county. Now, CWD has been identified in 42 counties. It was first identified in Colorado in 1967 and has now been found in 24 states and two Canadian provinces, South Korea and Norway.
CWD has reached epidemic proportions in some Wisconsin counties, where infection rates are nearing fifty percent in certain portions of the deer population. Scientists are predicting a decline in the whitetail population in these areas. Scientists are also stating that CWD is threatening mule deer populations in Wyoming with extinction in roughly 40 years.
Since CWD was first identified, scientist have been researching the disease and have discovered many important facts about the disease. Current research was recently summarized in the report, The Challenges of CWD: Insidious and Dire. CWD is a part of a group of diseases known as Transmissible Spongiform Encephalopathy (TSE). Some other diseases in this class include mad cow disease (Bovine Spongiform Encephalopathy, BSE) and scrapie in sheep.
Research provides answers
One of the first characteristics of CWD that scientist worked to determine is how old is CWD? Younger diseases tend to be more contagious and deadly, while older diseases spread slower and kill fewer victims. When CWD was first identified, scientists wanted to know if this was the beginning of the disease or had it been present in the wild but just not detected?
After extensive research and modeling, it has been determined with great confidence that CWD is a new disease that developed in the 1960s. Testing of deer herds has helped to determine this fact. If CWD was a long-standing disease it would show up in testing over wide geographic areas. Instead CWD only shows up in the areas where it is already known.
The fact that CWD is always fatal also helped to determine that it is a new disease and if animals had had a long history with CWD they should have developed ways to fight it. However, due to the fatality rate, defenses against the disease has not had enough time to develop.
Trial and error
Scientist have spent a lot of time and effort in trying to kill the misfolded prion that causes CWD. They have treated the prions with alcohol, disinfectants, formaldehyde, detergents, protein enzymes, disinfectants, radiation, freezing, and heat (up to 1100 degrees).
None of these methods were 100 percent effective in killing CWD. Scientists also ran CWD through a typical sewage treatment regimen and found that it also survived and can last a long time in the wild. Scientists are not sure how long CWD can persist in the wild, however. In one documented case, the related disease, scabies, persisted for 16 years in a sheep barn and reinfected animals after they moved back into the barn.
Since it’s nearly impossible to kill CWD, scientists want to know how it spreads so it can be contained. CWD has been spread by long range and local vectors. The major long-range vector for CWD is the transportation of infected deer from one deer farm to another.
One game farm in South Dakota shipped infected deer to Saskatchewan and ended up infecting deer in 21 different locations in the Canadian province. In Wisconsin, CWD in captive deer herds have been responsible for the spread in numerous counties including Eau Claire County. Studies have also shown that the stress and density of deer in deer farms increases the risk of contraction.
Avenues of contraction
Deer carcass transportation by hunters can also spread CWD if the carcass is not properly disposed in a landfill. Any deer harvested in a known infected area should be treated as a CWD positive animal until testing proves otherwise. Dumping untested deer carcasses in road ditches and on DNR land will eventually lead to the spread of the disease.
Once CWD is introduced into a local area, it is spread from animal to animal by a variety of means. Thus far, scientists have determined that CWD can be transmitted directly from deer to deer via saliva, urine, and feces. It can also be transmitted indirectly.
Recent research shows that plants absorb CWD prions through their roots, and then the prions can be ingested by deer and subsequently become infected. CWD can also be present in soil and water, resulting in carcasses of deer that die from CWD to spread the disease further. Multiple species of animals and birds feed on the carcass and then can potentially spread the prions via their feces.
Deer can have an active CWD infection for up to 15 months before they begin to show signs of the infection. They can spread the always fatal disease in that time. CWD is now the largest prion-based disease in recorded history and there is no known way to treat it.
Slowing down the spread of the disease is the only course of action currently available. Practices that help slow the spread of the disease include reducing deer density, baiting and feeding bans, double fencing at deer farms, and extensive testing for CWD in deer. Unfortunately, none of these options are being vigorously pursued in Wisconsin.