Seasonal and ontogenetic dynamics in trophic transmission of parasites

Transmission rates from the intermediate (amphipods) to the definitive hosts (fish) were quantified for two helminth species (Cyathocephalus truncatus, Cestoda, and Cystidicola farionis, Nematoda) both seasonally and through the ontogeny of the final hosts (arctic chart, Salvelinus alpinus, and brown trout, Salmo trutta). Amphipods (Gammarus lacustris) were important prey for both fish species, especially in the autumn. Both parasite species had low infection levels in amphipods compared to high abundance in fish. The seasonal variations in transmission rate of C. truncatus procercoids from amphipods to fish were in accordance with the observed abundance in fish hosts, being highest in the autumn and lowest during late winter and early summer. During summer, however, the estimated monthly transmission rates of C. truncatus were higher than the observed infection levels in the fish, suggesting restricted establishment success and shorter development time and longevity of the cestode in fish at higher temperatures. The accumulated transmission of C. farionis over the ontogeny of arctic charr was similar to the observed age-specific infection levels, reflecting a high establishment success and longevity of this parasite in chart. In contrast, brown trout exhibited an infection level that was much lower than the estimated transmission rates, suggesting a high resistance against C. farionis in these fish. The magnitudes of the estimated transmission rates were sufficient to explain the paradoxical contrast between low infection levels in the intermediate and high in the final hosts.