The impacts of environmental variability on parasite diversity within-hosts

Background/Question/Methods

Though we are beginning to understand the mechanisms that facilitate zoonotic spillover, we’ve left the mechanisms that promote parasite diversity within-hosts, creating opportunity for spillover events, understudied. Additionally, while attention has been paid to the effects of increasing temperatures on disease systems, we’ve neglected to explore the effects of environmental variability, despite evidence of its outsized impact on species diversity. Thus, understanding how climate change-induced environmental variability affects parasite diversity within-hosts will be vital in addressing the threat of zoonotic disease emergence. To investigate the impacts of environmental variability on parasite diversity within-hosts, we modified Antia et al.’s (1994) model of within-host population dynamics to include a secondary parasite which could indirectly compete with the primary parasite through the host’s immune response, and defined the growth rates of each parasite along nonidentical thermal performance curves. We simulated the model under constant, demographically stochastic, environmentally stochastic, and demographically and environmentally stochastic conditions, and compared the ability of the secondary parasite to invade a single-parasite system, the tendency of one parasite to outlast another, and the amount of time for which both parasites co-occurred.

Results/Conclusions

We’ve found that where constant and demographically stochastic conditions may prohibit or limit parasite co-occurrence, environmental stochasticity may allow for or increase the longevity of parasite co-occurrence, suggesting that environmental variability may promote parasite diversity within-hosts. Furthermore, we have been able to characterize the impact of environmental variability on parasite co-occurrence along continuums of frequency and intensity, and have found that more frequent and more intense variations in the thermal environment may enable increasingly common and lengthy periods of parasite co-occurrence, thus promoting parasite diversity, until a tipping point is reached. Climate change has made the next pandemic inevitable. Understanding how climate change-induced environmental variability impacts parasite diversity within-hosts will bolster our understanding of zoonotic disease emergence and allow us to make robust predictions and implement actionable mitigation strategies.