Officer, M. | Search bioRxiv

bioRxiv2026

Feeding the host reshapes virulence: nonlinear scaling in a microsporidian pathogen.

Resource availability is a central driver of ecological and evolutionary processes, yet its effects on infectious disease and virulence are not fully understood. A key limitation is that many studies consider only a narrow range of resource conditions or a limited subset of host and pathogen traits, potentially obscuring non-linear relationships. Here, we quantify how a gradient of six food levels simultaneously shapes host fitness and pathogen performance in the Daphnia magna- Ordospora colligata system. Across two laboratory experiments, we measured infection rates, pathogen burden, host fecundity, survival, and filtration rates. Increased food availability enhanced pathogen fitness, with both infection rates and spore burden increasing with provisioning. In contrast, host responses were trait-specific: while fecundity increased with food availability, pathogen-induced reductions in fecundity (i.e., virulence) peaked at intermediate resource levels, despite continued increases in pathogen load. This pattern indicates that resource availability alters host tolerance as well as pathogen growth, generating non-linear disease outcomes. Host survival was unaffected by either food provisioning or infection, further demonstrating that resource availability can simultaneously influence host and pathogen traits in different directions. Our results highlight the importance of integrating multiple fitness components across provisioning levels to understand disease dynamics and suggest that ongoing anthropogenic changes in resource availability may alter host-pathogen interactions.

zoology↗

bioRxiv2026

The duration and predictability of heatwaves shape host-parasite interactions under thermal stress

Anthropogenic climate change is expected to increase not only mean temperatures but also the magnitude and pattern of thermal variability, including the frequency, duration, and predictability of extreme events. While the effects of elevated mean temperatures on disease dynamics are well studied, far less is known about how different patterns of temperature variability shape host-parasite interactions, despite clear theoretical predictions from the climate variability hypothesis. Here, we used a controlled experimental system (Daphnia magna and two microsporidian parasites, Ordospora colligata and Hamiltosporidium tvaerminnensis) to disentangle the effects of thermal variability structure from mean temperature. Across two experiments, we exposed hosts to cyclic (predictable) and random (unpredictable) heatwaves under both non-stressful and stressful mean temperature regimes. Contrary to predictions from the climate variability hypothesis, temperature variability did not uniformly increase infection risk. Instead, infection outcomes depended on the interaction between mean temperature, duration of the heatwaves, the pattern of thermal variation, and parasite identity. Under non-stressful mean temperatures, thermal variability had negligible effects on infection. In contrast, under stressful mean temperatures, parasites responded in distinct ways: H. tvaerminnensis infection was sensitive to the predictability of heat events, whereas O. colligata responded primarily to heatwave duration. These results demonstrate that climatic variability can differentially alter host-parasite interactions rather than exerting consistent directional effects on disease. By showing that parasite-specific sensitivities to the pattern of thermal variation emerge under thermal stress, our study highlights a mechanism by which increasing climatic variability may reshape parasite communities and disease outcomes in a warming world.

ecology↗