Not all hosts are equal: creating relevant questions for wildlife disease management

In their latest research, Will Rogers and colleagues use an age- and sex-structured simulation model to explore harvest-based management of CWD under three different transmission scenarios that all generate higher male prevalence.

In many host-pathogen systems, males tend to be more diseased than females. This can be the result of many different factors such as the effects of testosterone on immune function and different social contact patterns. In addition, females are often more important to population growth than males. As a result, wildlife managers may preferentially want to focus any disease control efforts on males rather than females.

One such system where sex seems to have an important relationship with prevalence and demography is chronic wasting disease (CWD). CWD is a serious concern for deer, elk, reindeer, and other cervid species. The disease is chronic, and the severity of symptoms (staggering, drooling, odd movement) increases the longer an individual has been infected, typically around one to two years.

Male deer tend to have a higher prevalence of CWD than females. Whether this phenomenon is caused by greater susceptibility or exposure of males to CWD is unknown, and we cannot yet rule out that differences in the interactions and behaviors of males and females might lead to biases in transmission between sexes. How managers should respond to CWD is under active research, and we wanted to explore the role of sex in harvest-based management in our paper recently published in Journal of Applied Ecology.

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Photo: National Park Service / Jacob W. Frank

There is a gap in our understanding of how to link prevalence patterns to management strategies. Differences in prevalence can be created by any number of epidemiological asymmetries between demographic classes, whilst the most diseased class is not always the most infectious class. To interrogate this idea, we created a model incorporating both demography (split into age and sex categories) as well as CWD transmission and disease progression.

We then simulated three different scenarios (hypotheses) about how transmission might occur: males being generally more susceptible, males infecting mainly other males, or females infecting mainly other males. All of these scenarios can result in higher prevalence in males compared to females. Within each of these scenarios, we altered sex-specific harvest rates of adults to simulate how harvests affect both epidemiologic and demographic outcomes.

We found that most male-susceptibility or high male-to-male transmission scenarios could be addressed through minor to severe harvests of males. However, if females played serious roles in transmission, the tradeoff between population growth and disease outcomes became important – too little female harvest led to large epidemics and too much female harvest led to population declines. In such scenarios, there was no combination of male and female harvest rates that maintained both high populations and low CWD prevalence.

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Given that the harvest-based management differences were so stark, we also wanted to understand how managers can leverage prevalence patterns to better understand transmission dynamics. We found that monitoring populations with different levels of male and female harvest tend to lead to differences in prevalence patterns. Critically, even under male-susceptibility or male-dominated transmission scenarios, male-biased harvests may need to be aggressive to generate decreases in prevalence over time.

In general, male-biased prevalence can arise from many transmission and susceptibility scenarios. Relying on prevalence alone to guide host management is only useful in scenarios where the most diseased class is also the class most responsible for transmission. For CWD, harvest-based management could be a useful tool to explore transmission dynamics – particularly in exploring the demographic outcomes of disease- versus harvest-based mortality.

Read the full article Epidemiological differences between sexes affect management efficacy in simulated chronic wasting disease systems in Journal of Applied Ecology

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