Timing of a cold snap relative to exposure drives large shifts in disease proliferation

In recent years, the frequency and intensity of anomalous weather events (such as cold snaps) have increased due to climate change. While heatwaves have received substantial attention, the effect of cold snaps, particularly the effects of the timing of the event, has remained underexplored in host-pathogen systems. Here, we investigated the effect of 48 cold snap treatments using Daphnia magna infected with its microsporidian parasite Ordospora colligata. The amplitude and duration of cold snaps were manipulated across four baseline temperatures and at three distinct time points in relation to parasite exposure. Parasite burden varied up to 17-fold compared to constant baseline temperature conditions. Indeed, cold snaps can either enhance or suppress parasite fitness depending on the interplay between baseline temperature, cold snap treatment (amplitude and duration), and timing. Notably, cold snaps occurring before or after infection produced contrasting outcomes, likely due to mismatches in host and parasite developmental timing. These findings underscore the context-specific nature of extreme thermal variation and its capacity to alter host-parasite dynamics in non-linear ways. To better predict climate change impacts on disease ecology, it is crucial to incorporate timing and thermal variability, not just mean temperature, into forecasting models.