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

由寄生虫感染（毒力）引起的宿主适应性降低取决于感染强度和每种寄生虫造成的损害程度。环境条件可以影响两种毒力成分，但是与感染强度相反，人们对环境对每种寄生虫危害的影响知之甚少。在这里，我们研究了环境温度对每个寄生虫危害的影响，这是由寄生虫诱导伤害的能力（每个寄生虫的致病性）和宿主限制伤害的能力（耐受性）共同决定的。我们实验性地暴露了两种鲑鱼种类，即大西洋鲑鱼（Salmo salar）和海鳟鱼（Salmo trutta），以暴露于fl虫Diplostomum pseudospathaceum的复制基因型。。在暴露后的最初12周内，在温水（16°C）中对健康造成损害（眼白内障）后，我们将鱼在5°C（冷水）或16°C下保持另外8周，并量化了白内障是寄生虫负荷的函数。我们发现，与温水相比，冷水降低了每寄生虫的危害，这表明低温可以改善寄主的健康状况。每个寄生虫的损害也受寄生虫基因型和寄主物种的影响，但这些影响不会随温度变化。我们的研究结果表明，由于感染风险低，宿主寄生虫研究中经常忽略的冷水季节可能使宿主恢复健康，因此可能对寄生虫感染的生态学和流行病学产生重要影响。