Cold water reduces the severity of parasite-inflicted damage: support for wintertime recuperation in aquatic hosts

The reduction in host fitness caused by parasite infections (virulence) depends on infection intensity and the degree of damage per parasite. Environmental conditions can shape both virulence components, but contrast to intensity, environmental impacts per-parasite are poorly understood. Here, we studied effect ambient temperature damage, which is jointly determined ability parasites induce harm (per-parasite pathogenicity) hosts limit (tolerance). We experimentally exposed two salmonid species, Atlantic salmon (Salmo salar) sea trout trutta), replicated genotypes eye fluke Diplostomum pseudospathaceum. After development health (eye cataracts) warm water (16 °C) during first 12 weeks post exposure, maintained fish at either 5 °C (cold water) or 16 for another 8 quantified changes cataracts as a function load. found that was reduced cold compared water, suggesting temperatures improved health. Per-parasite also affected genotype these effects did not change with temperature. Our findings suggest cold-water seasons, often neglected host-parasite studies due low risk, could allow recuperate thus, may have important implications ecology epidemiology infections.