Field Observations Used to Establish Relationships of Illinois Female White-tailed Deer (Odocoileus virginianus) to Model Chronic Wasting Disease

Abstract

White-tailed deer (Odocoileus virginianus) female family groups consist of a matriarchal female, associated related females and fawns. Females most commonly interact with members of their family group. Females interact at the highest frequency in the winter-spring and the lowest during fawning. The contact structure of relationships and associations among female deer and female family groups has been linked to disease transmission. Chronic wasting disease (CWD) is caused by a prion primarily affecting deer and elk that causes degenerative brain disease. Prions are transmitted by direct and/or indirect (i.e. plants, soil, feces) contact with a slow onset of visible signs in clinically infectious animals. A single moment of proximity or direct contact of two hosts does not always indicate disease transmission; but an increased frequency in contacts logically increases the risk for prion exposure.

From 1990-1999 in rural and suburban Illinois, multiple deer ear-tagging and radio-telemetry studies were conducted, and my study expanded this research to examine female deer relationships and associations of related and unrelated animals. This research utilized 2,590 observations from 156 female rural deer and 1,431 observations based on 129 female suburban deer. Relationships were determined as mother, daughter, son, sister or half-sister. Deer that were commonly observed together but relationships could not be determined, were termed associated. Variations in interactions between related and associated, marked and unmarked deer were analyzed by chi-square (X2) tests. A significant difference was identified for all marked and unmarked deer from all study sites for relationships (marked) and associations (marked and unmarked) by age and season (p<0.001).

I incorporated data from the rural female family groups in a stochastic contact network using EpiModel in R to evaluate population level impacts of CWD under various infection probability rates. A contact network was constructed utilizing 100 female family groups into a one-mode process for susceptible or infected individuals. The simulations ran for 20 years and CWD infection probability rates included 1%, 1.5%, 2% and 2.5%. Time steps analyzed were 5, 10 and 20 years with summary statistics and box-whisker plots. A factorial ANOVA was conducted to compare the main effect variables (infection probability rate, simulation duration) and the interaction between the main effects on CWD prevalence. At 5 years, 1% and 1.5% infection probability rates had similar median prevalence, as did 2% and 2.5%. For 10 and 20 years, the higher the CWD infection probability, the higher the prevalence. The two main effects in the model, infection probability rate (F(3, 108) = 59.424, p<0.001) and simulation duration (F(2, 108) = 174.493, p<0.001) were statistically significant, as well as the interaction of main effects on CWD prevalence (F(6, 108) = 26.965, p<0.001).

We identified differences in white-tailed deer relationships and associations from observation data in rural and suburban Illinois. While modeling CWD transmission for female family groups in a rural white-tailed deer population, infection probability rate was identified as an important variable. The variable informs wildlife managers and epidemiologists about the likelihood of individual deer contracting the disease and how quickly CWD will spread through a population.