Immune variation and host ontogeny constrain pathogen virulence and transmission in a helminth parasite

Host immune strategies vary widely, yet how this variation shapes parasite virulence and transmission in natural systems remains unresolved. We tested whether host immune strategy governs macroparasite transmission timing and magnitude in Schistocephalus solidus, a cestode with a three-host life cycle. Over 13 months, we surveyed infection and immune activation (fibrosis) in second-intermediate host populations (threespine stickleback) from eight lakes. Host immune strategy (ranging from resistance to tolerance) strongly influenced whether and when parasites reached transmissible sizes, and shifted the synchrony between transmissible-stage peaks and definitive host availability. Across populations, transmission potential peaked at intermediate parasite burdens, consistent with virulence-transmission trade-off theory. These findings demonstrate that host immune phenotype alters epidemiological outcomes in complex life cycle parasites, by shifting both the magnitude and phenology of parasite growth to structure the effective transmission window.