Parsing the Effects of Demography, Climate and Management on Recurrent Brucellosis Outbreaks in Elk

Zoonotic pathogens can harm human health and well‐being directly or by impacting livestock. Pathogens that spillover from wildlife can also impair conservation efforts if humans perceive wildlife as pests. Brucellosis, caused by the bacterium Brucella abortus, circulates in elk and bison herds of the Greater Yellowstone Ecosystem and poses a risk to cattle and humans. Our goal was to understand the relative effects of climatic drivers, host demography and management control programmes on disease dynamics. Using \u3e20 years of serologic, demographic and environmental data on brucellosis in elk, we built stochastic compartmental models to assess the influences of climate forcing, herd immunity, population turnover and management interventions on pathogen transmission. Data were collected at feedgrounds visited in winter by free‐ranging elk in Wyoming, USA. Snowpack, hypothesized as a driver of elk aggregation and thus brucellosis transmission, was strongly correlated across feedgrounds. We expected this variable to drive synchronized disease dynamics across herds. Instead, we demonstrate asynchronous epizootics driven by variation in demographic rates. We evaluated the effectiveness of test‐and‐slaughter of seropositive female elk at two feedgrounds. Test‐and‐slaughter temporarily reduced herd‐level seroprevalence but likely reduced herd immunity while removing few infectious individuals, resulting in subsequent outbreaks once the intervention ceased. We simulated an alternative strategy of removing seronegative female elk and found it would increase herd immunity, yielding fewer infections. We evaluated a second experimental treatment wherein feeding density was reduced at one feedground, but we found no evidence for an effect despite a decade of implementation. Synthesis and applications. Positive serostatus is often weakly correlated with infectiousness but is nevertheless used to make management decisions including lethal removal in wildlife disease systems. We show how this can have adverse consequences whereas efforts that maintain herd immunity can have longer‐term protective effects. Climatic drivers may not result in synchronous disease dynamics across populations unless vital rates are also similar because demographic factors have a large influence on disease patterns

Winter Feeding of Elk in the Greater Yellowstone Ecosystem and its Effects on Disease Dynamics

Providing food to wildlife during periods when natural food is limited results in aggregations that may facilitate disease transmission. This is exemplified in western Wyoming where institutional feeding over the past century has aimed to mitigate wildlife–livestock conflict and minimize winter mortality of elk (Cervus canadensis). Here we review research across 23 winter feedgrounds where the most studied disease is brucellosis, caused by the bacterium Brucella abortus. Traditional veterinary practices (vaccination, test-and-slaughter) have thus far been unable to control this disease in elk, which can spill over to cattle. Current disease-reduction efforts are being guided by ecological research on elk movement and density, reproduction, stress, co-infections and scavengers. Given the right tools, feedgrounds could provide opportunities for adaptive management of brucellosis through regular animal testing and population-level manipulations. Our analyses of several such manipulations highlight the value of a research–management partnership guided by hypothesis testing, despite the constraints of the sociopolitical environment. However, brucellosis is now spreading in unfed elk herds, while other diseases (e.g. chronic wasting disease) are of increasing concern at feedgrounds. Therefore experimental closures of feedgrounds, reduced feeding and lower elk populations merit consideration

R code. Function to create a smoother line using generalised additive models. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This file is called in to other code files and is necessary for the generation of smoothed brucellosis prevalence estimates, including in years for which there is no data

R code for seroprevalence of unfed herds (fig 3) from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

Reads in serology data from 3 unfed elk populations and generates a plot illustrating brucellosis prevalence, sample sizes, and trends over time

Test and slaughter serology data. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This is the serology data used to generate figure 4 in .csv format

R code. Function to create prevalence estimates by location and year. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This file is called in to other code files and is necessary for the generation of brucellosis prevalence estimates by site and year (and 95% confidence intervals)

Unfed elk herd serology data. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This is the serology data used to generate figure 3 in .csv format

R code for seroprevalence at test and slaughter feedgrounds (fig 4) from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

Reads in serology data from 3 sites and generates a plot illustrating brucellosis prevalence, strength of the estimates, and trends over time in relation to when experimental treatment occurred