Heatwaves and cold snaps alter host-parasite population dynamics

Climate change is increasing the frequency and intensity of temperature extremes, yet the consequences of heatwaves and cold snaps for host-parasite interactions remain incompletely understood.Thermal pulses of ±6°C were experimentally tested to determine how disease dynamics are affected in populations of Daphnia magna infected with Ordospora colligata across four baseline temperatures (14, 17, 20, and 23°C).Both heatwave and cold snap effects were strongly temperature dependent and reflected how far temperature deviations shifted the system toward or away from the parasite's thermal optimum.At 14°C, heatwaves produced short-term increases in parasite burden, whereas no such effect occurred at 17°C.Cold snaps also generated short-term increases in parasite fitness at warmer baselines (23°C).Host population size also varied with baseline temperature and treatment, and parasite burden was linked to host density: burden and density effects uncoupled at the warmest baseline.