Chronic wasting disease in deer has emerged over the last 20 years as one of the most significant issues in deer hunting in modern times. The fact the disease is complex and difficult to manage has caused much disagreement and debate in the hunting world. Understanding which deer catch, spread and die of CWD fastest is important, especially if you hunt in a CWD zone, because you can help manage the disease and the deer population in affected areas. Regardless of where you hunt, the information below may help you understand why certain management steps are advisable in CWD zones.
Let’s look first at what we know about the demographics of CWD in deer. Which deer catch CWD more often, and why?
Whitetail Bucks Are Most Susceptible to CWD
In whitetails, prevalence is typically highest in adult bucks, followed by adult does, yearlings (1 ½-year-olds) of both sexes, and fawns, in that order. Infection rates in yearlings are similar, and does can transfer the disease to their fawns in the womb via placental tissue.
Adult bucks are often 1.5 to 3 times more likely to be infected with CWD compared to females among white-tailed deer, mule deer and reindeer – but not elk (more on that later). Higher prevalence in bucks has resulted in management suggestions to focus the majority of harvest effort in CWD zones on bucks. What many lose sight of, however, is even though an individual adult buck is about twice as likely to have CWD, there are usually more than twice as many adult does than adult bucks on the landscape.
So, even though prevalence is higher in bucks, there are often more CWD-positive does on the landscape. From a management standpoint, if you want to remove the most CWD-positive animals, you can’t focus the majority of harvest effort on bucks. You must focus at least an equal amount of effort on does.
Why are bucks more likely to be infected? Some research suggests transmission of CWD among bucks during the nonbreeding season may be a potential mechanism for producing higher rates of infection. Others agree the formation of bachelor groups contributes, as well as competition for breeding, and possibly due to their larger body sizes requiring the need to consume more resources.
Others suggest older bucks may be at the highest risk due to their broader home range, which increases their chance of interacting with infected deer or contaminated landscapes. Also, bucks have a less-cohesive social structuring than does, and they contact other deer at higher rates during the rut – such as fighting with other bucks, courtship grooming, copulation, and scent verification. Ultimately, increased prevalence in bucks appears to be more behaviorally than physiologically related.
However, a few studies have identified higher prevalence in female deer. This could potentially be explained by research from the U.S. Geological Survey’s National Wildlife Health Center regarding genetic relatedness and deer social behavior. In short, deer are very social with much interaction among related individuals, and USGS data showed does were 10 times more likely to be CWD-positive when there was one CWD-positive relative nearby. Additionally, while does have a lower rate of infection than bucks, they live longer and are much more likely to reach the end state of CWD before dying. This may increase the chance for direct transmission before death and increase deposition of CWD onto the landscape after death.
Deer to Deer And Across the Landscape
CWD transmission occurs primarily through direct contact between deer, but also through environmental contamination. Higher animal densities can lead to increased disease transmission, so state wildlife agencies typically advocate for reducing deer density in disease zones with high deer numbers. Contagions spread through urine, feces, saliva, blood, semen and antler velvet. CWD spreads especially via live deer in the captive deer industry, because there is not a reliable and practical live animal test for CWD. The disease spreads through movement of infected live animals, the high-risk parts (brain, eyes, spleen, lymph nodes, backbone) of harvested animals, and natural movements of wild animals, like dispersal, migration and excursions.
Within chronic wasting disease outbreak areas, CWD does not exist evenly across the landscape. Research in Wisconsin shows disease prevalence is much higher near the centers of each infection, and the rate of disease prevalence increase over time is higher in forested areas.
How Fast Does CWD Kill?
CWD is 100% fatal to all deer species, but the long incubation period means death does not occur immediately. Once an animal contracts CWD, the clock starts ticking. The neurodegenerative disease slowly begins to affect health and behavior, but the disease will incubate in a deer for an average of one to two years before outward signs of disease appear. Importantly, with an average incubation period of 18-24 months in deer and 18-48 months in elk, most animals with CWD die from another cause before symptoms are evident to hunters. Meanwhile, the deer is capable of spreading the disease to other deer.
During this long incubation period, CWD-positive animals are less healthy, less active, and less able to avoid hunters, predators, and vehicles. Research in Wisconsin showed CWD-positive deer were 8 times more likely to have poor body condition than CWD-negative deer. Wyoming research showed CWD-positive white-tailed deer were 4.5 times more likely to die annually than CWD-negative deer.
Elk vs Deer
CWD prevalence rates differ among deer species. For example, elk are often reported as being less susceptible than mule deer and whitetails, possibly due to a natural mutation in the prion protein gene. However, elk populations are also negatively impacted at lower CWD prevalence rates than deer (13 vs 20%). Regardless, population declines due to CWD have been identified in elk, mule and white-tailed deer.
Though white-tailed, mule and reindeer males are more likely to be infected than females, this is not the case with elk. Washington State University researchers state a sex difference in prevalence has not been documented in elk. This may be due to their social structure of harems and regular comingling with unrelated animals.
Chronic Wasting Disease (CWD) is an always fatal, contagious, neurological disease affecting most deer species, including elk, reindeer, moose, mule, red and white-tailed deer. It causes a characteristic spongy degeneration of the brain of infected animals, which after a long incubation, results in emaciation, abnormal behavior, loss of bodily functions and death. Unfortunately, there is no vaccine or cure for CWD.
As of August 2023, CWD has been identified in 31 U.S. states, five Canadian provinces (including the Toronto Zoo), Korea (from an elk imported from Canada in 1997), Norway (in free-ranging reindeer, moose and red deer), Finland (free-ranging moose), and Sweden (free-ranging moose). In the U.S., CWD has been confirmed in at least 495 counties, and the percentage of CWD-positive counties within a state ranged from 1.5% (1 of 67 counties) in Alabama and Florida to 100% in Wyoming (23 of 23 counties).
The amount of misinformation on CWD has hindered our ability to slow its spread, manage infection rates in many outbreak areas, and fight for solutions as a united community of hunters. The NDA has made it a major priority to provide hunters with reliable, science-based information about CWD that will help them engage in the fight as informed stewards of the deer resource.