Host and parasite thermal ecology jointly determine the effect of climate warming on epidemic dynamics

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Why is it important?

Temperature can regulate populations through direct physiological effects on organisms; however, these effects become hard to predict if they influence interconnected species with joint population dynamics. Quantifying how parasites and their wildlife hosts jointly respond to rising temperatures is of increasing importance, particularly given the public health burdens of pathogens such as malaria. Gehman et al. conducted the first study to experimentally derive thermal performance curves for an ectothermic host and its parasite across multiple components of parasite fitness. Gehman et al. then used these physiological data to parameterize an epidemiological model and evaluate effects of present and future temperature scenarios, including ones with seasonal variation, on infection prevalence. Through this approach, a robust and striking conclusion was realized that increased temperature should lead to local extirpation of the parasite. How parasites will response to climate warming has been debated extensively, exemplified by multiple review and opinion articles. Gehman et al. provides a unique empirical example of how parasites and their hosts can differentially respond to temperature changes. Such examples are needed because the nonlinear effects of temperature on multiple components of host and parasite biology can influence infection outcomes in complex and non-intuitive ways. The specific predictions for this system are striking because the rhizocephalan parasite was shown to be more sensitive than its crab host, exemplifying that a warmer world is not always a sicker world. Furthermore, the parasite was highly sensitive to temperature, with just a 2°C rise enough to cause its local extirpation. The study gives insight into a real system and exemplifies the importance of comparing host and parasite optimal temperatures and temperature ranges to sharpen disease forecasting.