Heatwaves and cold snaps alter host-parasite population dynamics in the Daphnia magna-Ordospora colligata system

Climate change is driving more frequent and severe temperature extremes, including heatwaves and cold snaps, with growing implications for ecology and disease. Yet, our understanding of how heatwaves and cold snaps influence disease dynamics remains underexplored. Using the host Daphnia magna infected with its microsporidian microparasite Ordospora colligata, pathogen fitness and host population size were measured in experimental populations using a factorial design at four baseline temperatures (14, 17, 20 and 23{degrees}C). A heatwave or cold snap treatment with an amplitude of {+/-}6{degrees}C was administered four weeks after measurements began and lasted for ten days. The effect of heatwaves is dependent on baseline temperature but can induce long-lasting increases in burden (>4 weeks). The impact of cold snaps were also temperature-dependent, leading to short-term increases in parasite fitness at higher temperatures. Host population size also varied in response to temperature and treatment. Importantly, burden and host density were interdependent, jointly shaping infection patterns. At lower temperatures, parasite burden and host population size were positively correlated, whereas at higher temperatures, increased host population size corresponded with reduced burden. These patterns were consistent at both individual and population levels, underscoring how individual physiological responses can scale up to impact disease dynamics across populations. Thus, extreme temperature variation can have complex, context-specific outcomes on disease dynamics. As climate extremes become more frequent, understanding these nuanced responses is critical for predicting and managing disease risk in natural populations. Author SummaryWe are experiencing more extreme weather events around the world, including heatwaves and cold snaps, but we dont fully understand how these temperature extremes will affect wildlife diseases. In our study, we tested how heatwaves and cold snaps influence both parasite success and host population size using a small aquatic animal, the water flea, and its naturally occurring gut parasite. We ran experiments at four average temperatures and simulated a heatwave or cold snap by raising or lowering the temperature by 6{degrees}C for ten days. We found that heatwaves often led to long-lasting increases in parasite burden, while cold snaps caused short-term spikes in parasite fitness, especially at warmer average temperatures. These effects also depended on the baseline temperature and were linked to changes in the host population. At cooler temperatures, parasite levels increased as host populations grew, but at warmer temperatures, the opposite happened, resulting in negative density-dependence. This suggests that the impact of extreme weather on disease isnt straightforward; it depends on when and where the event occurs. As extreme temperatures become more common with climate change, understanding these complex interactions is important for predicting disease outbreaks in the wild.