Use of environmental sites by mule deer: a proxy for relative risk of chronic wasting disease exposure and transmission

Prions that cause chronic wasting disease (CWD) in cervids can remain infective for years outside the host. Infectious cervids shed prions for a long time, consequently depositing prions in frequently used areas. These environmental prions are important in CWD epidemiology. Unfortunately, effective tools for quantifying CWD prions in soil, water, and other environmental sources are not currently available. Our goal was to investigate relative differences in visits by mule deer (Odocoileus hemionus hemionus) to various environmental site types as an indicator of the relative risk of prion contamination and disease transmission. For this, we deployed a system of triggered-by-movement cameras at eight site types in a CWD-endemic area in Saskatchewan, Canada. We first assessed whether the relative differences among site types in the frequency of visits by mule deer of any sex-and-age class, males, and females varied by season and site type. We then assessed whether the rate of behaviors with a high risk of environmental prion transmission (either contamination or acquisition) differed by season and site type. Finally, we assessed whether the intensity of visitation, based on the number of animals per picture, differed by season and site type. We found that grain sources and beds were key attractants for mule deer: (1) The greatest number of pictures with mule deer per camera-day occurred at grain sources across all seasons, except in fawning, when beds were the most visited sites; (2) during pre-rut and early gestation, mule deer visited grain sources at least twice as often as most other sites; (3) females were more likely to visit beds and grain sources, but there was no significant site preferences for males after accounting for season; (4) mule deer were most likely to be pictured contacting the environment at grain sources in early gestation; and (5) beds and grain sources were the most intensively visited sites. We also found that environmental contacts at waterholes were more frequent during spring. We discuss the potential importance of various sites in the transmission of CWD and how their modification could potentially reduce the risk of prion environmental exposure among mule deer.