In their new article, Gilbertson et al. discuss how combining preventative and reactive disease interventions synergistically reduces disease-induced mortalities in a simulated carnivore population, whilst at the same time preventing unexpected negative impacts associated with inadequate vaccination.

As the COVID-19 pandemic has demonstrated, controlling outbreaks of infectious diseases is incredibly challenging—and that is no less true in wildlife. In fact, wildlife management faces significant hurdles in the form of limited disease management tools and often a great deal of uncertainty about the effectiveness of the tools that are available. For example, most wildlife diseases do not have appropriate vaccines, and what vaccines are available may be adapted from use in other species with uncertain efficacy in the target wildlife species. In some cases, human interventions can even have unexpected negative effects on the diseases we are trying to manage. These kinds of uncertainties are especially problematic for very small populations of wildlife that are of high conservation concern. Such populations may have limited resiliency to recover from disease outbreaks, such that infectious diseases pose significant threats to conservation goals.

A key example of the challenges associated with mitigating infectious disease risk in wildlife is the Florida panther (Puma concolor coryi), an endangered subspecies of mountain lion found exclusively in southern Florida. Panthers experienced an outbreak of the viral disease feline leukemia virus (FeLV) in 2002-04, which caused significant sickness and mortality among their small population (only about 80-120 panthers remained in their single population at that time).

This outbreak appeared to result from spillover of FeLV from domestic cats, and, with the persistent threat of additional outbreaks, panthers are still managed for FeLV today. A domestic cat vaccine has been used in an attempt to protect against FeLV, but, as is common among wildlife, the actual efficacy of this vaccine in panthers is unknown. This uncertainty leaves us with the common but significant gap in understanding how best to use finite management resources to protect panthers from a very real and persistent infectious disease threat.

In our paper, recently published in the Journal of Applied Ecology, we used simulations to determine optimal FeLV control strategies for Florida panthers. Simulations can be a key tool for ethically testing out different management regimes and identifying optimal approaches, potential negative consequences, and key areas of uncertainty to prioritize for future research.

We tested the effectiveness of management interventions deployed prior to the start of an outbreak (i.e., preventative vaccination) and those initiated after the start of an outbreak. Our simulations also factored in important limitations experienced in real-world wildlife management, such as delays in outbreak detection.

Importantly, our simulations allowed us to investigate an understudied type of imperfect vaccine efficacy. Typically, vaccine efficacy is represented in modelling studies as “binary immunity”, where vaccinated individuals are either completely protected or completely susceptible. However, vaccine efficacy can also be represented as “partial immunity” where all vaccinated individuals are partially protected. We modeled vaccine efficacy in our simulations as partial immunity to understand what implications this understudied type of vaccine imperfection has on disease management strategies.

We found that the most efficient FeLV management strategies used vaccination prior to an outbreak in combination with reactive interventions (i.e., vaccination, test-and-temporary removal) initiated after an outbreak has started. This combined approach to disease management reduced simulated panther mortalities more than using any intervention in isolation, even in light of uncertainties about vaccine efficacy.

We also found that vaccine efficacy had important impacts on outbreak outcomes. Without accompanying reactive interventions, inadequate vaccination prior to an outbreak sometimes increased the cumulative number of mortalities, when compared with not intervening.

This counter-intuitive effect appears to be the result of simulating partial instead of binary vaccine immunity. Partially immune individuals were essentially semi-susceptible to infection and thereby appeared to allow outbreaks to persist in what might otherwise be rapid fade-out outbreak scenarios. This is not to say that proactive vaccination was ineffective, as this approach frequently reduced mortalities and the probability of outbreaks occurring.

Additional research is needed to fully delve into how imperfect vaccines—and how we model these imperfections—affect disease management efforts. Rather, our results support the value of using a combination of preventive and reactive strategies to mitigate the risks associated with imperfect vaccines, promote synergies among management approaches, and ultimately protect wildlife health and conservation.

Read the full article Paradoxes and synergies: Optimizing management of a deadly virus in an endangered carnivore in Journal of Applied Ecology