What Does the Presence Or Absence of CWD Really Mean?
by Amanda Dougherty
Results of recent testing of 1,342 deer and elk in Arizona are all negative for the presence of CWD. Neighboring states, however, have confirmed cases of CWD. But what does this really mean?
It seems as if more and more cases of CWD are cropping up in the western states, which brings about the debate of whether CWD is spreading rapidly, or if better surveillance and improved testing methods are simply finding more cases of CWD in new areas. The answer could be: both.
Recent work in Wyoming on the ecology and epidemiology of CWD in White-tailed deer in Wyoming investigated genetic differences in deer in relation to susceptibility to CWD infection, how CWD impacts populations (growth rate, pregnancy rates, fawn recruitment etc.) and behavioral differences between positive and negative deer.
This work was a seven year study in which deer were captured and fitted with GPS collars as fawns and tracked throughout the year, with CWD testing performed on an annual basis. In this population, the CWD prevalence was higher in does (42%) than in bucks (28.8%). Pregnancy rates were similar in CWD positive and negative females, indicating that reproduction is not negatively affected by CWD. CWD-positive deer were 4.5 times more likely to die (from any cause of mortality) than CWD negative deer, while bucks were 1.7 times more likely to die than does. The overall population growth rate was 0.8960, which means that the population is not growing and is unsustainable (values over 1.0 equal population growth).
Further analysis revealed that eliminating the doe harvest would produce a sustainable population with this prevalence of CWD. The mortality due to harvest and CWD cause the remaining population of CWD negative animals to be dominated by fawns, yearlings, and 2.5 year old deer. Genetic analysis showed that there are some genetic changes that are allowing deer to delay disease progression and time from infection to death. CWD positive deer were less active throughout the year, however, the positive bucks migrated the same distance as negative bucks. Migratory males may be responsible for geographic spread via horizontal transmission. Does that are positive are less likely to migrate, contributing to CWD prevalence via horizontal transmission and contributing to environmental contamination. The nature of the does to remain in a small home range results in contamination of prime habitat where deer occur at high densities and where deer congregate and experience increased risk of transmission. These areas can then serve as sources for new infections of migrating deer which can spread infection to other populations. These densely contaminated areas of prime habitat can also serves as locations for transmission to other species such as mule deer, moose and elk.
Looking at maps of CWD spread in Wyoming and surrounding states show that CWD is being discovered in new hunt areas and herd units. Perhaps additional factors such as climate changes and temperature swings, oil and gas development and urban sprawl are influencing deer distributions, migration corridors and impacting where CWD is spread.
Unfortunately, I believe it is just a matter of time before deer in areas currently free of CWD will test positive. At this point, with no preventative measures or clear management solutions, wildlife agencies will have to monitor disease prevalence and adjust hunting seasons accordingly to prevent decimation of white-tailed deer populations in areas with CWD.
For more information, visit https://wgfd.wyo.gov/web2011/WILDLIFE-1000284.aspx.