Parsing the effects of demography, climate and management on recurrent brucellosis outbreaks in elk

Abstract

1. 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.2. Using >20 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.3. 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. 4. 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 longerterm protective effects. Climatic drivers may not result in synchronous disease 380 | Journal of Applied Ecology COTTERILL ET aL. K E Y W O R D SCervus, disease ecology, metapopulation dynamics, partially observed Markov process, pathogen transmission, POMP model, supplemental feeding, test and remove, Yellowstone, zoonotic | 381 Journal of Applied Ecology COTTERILL ET aL.Additional supporting information may be found online in the Supporting Information section. How to cite this article: Cotterill GG, Cross PC, Merkle JA, Rogerson JD, Scurlock BM, du Toit JT. Parsing the effects of demography, climate and management on recurrent brucellosis outbreaks in elk.

Reproductive Fate of Brucellosis-Seropositive Elk (Cervus Canadensis): Implications for Disease Transmission Risk

“…Additionally, we only detected B. abortus at three abortions and two live births of 60 promptly sampled events, adding further credence to the limited risk seropositive elk, as a group, may pose. The current focus on managing seropositive elk and their removal to curtail transmission risk may not be as effective as once thought, particularly since positive serostatus is weakly correlated with infectiousness (Cotterill et al 2020). If the greatest risk of shedding occurs with abortion events that typically occur the year after first exposure (Thorne et al 1978), then seropositive elk that were exposed years ago and still retain antibodies may have a degree of immunity to becoming infectious again and may be less susceptible than naïve seronegative elk to having a brucellosis-induced abortion.…”

“…Although abortion is considered the hallmark clinical sign of recent brucellosis infection (Cheville et al 1998), the longer-term reproductive effects and risk of shedding infectious materials are unknown. Studies that try to document active infection through culture of B. abortus from seropositive elk tissues are limited and the correlation between positive serostatus and infectiousness is generally weak (Cotterill et al 2020).…”

Effects of culling vampire bats on the spatial spread and spillover of rabies virus

“…Moreover, culls can comprise a single component of multifaceted management plans. Co-occurring interventions, such as vaccination of humans and domestic animals or food provisioning of wildlife reservoirs, can obscure how culling affects cross-species transmission risk or transmission dynamics in the reservoir ( 10 ). Last, data collected alongside culls are generally restricted to incidence of infection.…”

A pseudomolecule assembly of the Rocky Mountain elk genome

“…This can be problematic for ranchers occupying areas near elk populations like the Greater Yellowstone Area. Elk are known reservoirs for brucellosis, (Brucella abortus) a disease that is highly contagious and poses a risk to livestock and humans [8][9][10]. Because of the potential for causing abortion in cattle, the USDA used vaccines and serologic testing to nearly eradicate B. abortus from domestic herds [11].…”