Understanding the impact of disease in natural populations requires an understanding of infection risk and the damage that parasites cause to their hosts (= virulence). However, because these disease traits are often studied and quantified under controlled laboratory conditions and with reference to healthy control hosts, we have little knowledge about how they play out in natural conditions. In the Daphnia–Pasteuria host–parasite system, field assessments often show very low estimates of virulence, while controlled laboratory experiments indicate extremely high virulence. To examine this discrepancy, we sampled Daphnia magna hosts from the field during a parasite epidemic and recorded disease traits over a subsequent 3-week period in the laboratory. As predicted for chronic disease where infections in older (larger) hosts are also, on average, older, we found that larger D. magna females were infected more often, had fewer offspring prior to the onset of castration and showed signs of infection sooner than smaller hosts. Also consistent with laboratory experiments, infected animals were found in both sexes and in all sizes of hosts. Infected females were castrated at capture or became castrated soon after. As most females in the field carried no eggs in their brood pouch at the time of sampling, virulence estimates of infected females relative to uninfected females were low. However, with improved feeding conditions in the laboratory, only uninfected females resumed reproduction, resulting in very high relative virulence estimates. Overall, our study shows that the disease manifestation of P. ramosa, as expressed under natural conditions, is consistent with what we know from laboratory experiments. However, parasite induced fecundity reduction of infected, relative to uninfected hosts depended strongly on the environmental conditions. We argue that this effect is particularly strong for castrating parasites, because infected hosts have low fecundity under all conditions.

中文翻译：

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浮游甲壳动物自然种群中的寄生虫毒力评估

要了解疾病在自然种群中的影响，就需要了解感染的风险以及寄生虫对其寄主造成的损害（即毒力）。但是，由于经常在受控实验室条件下并参考健康对照宿主对这些疾病特征进行研究和定量，因此我们对它们在自然条件下如何发挥作用的了解甚少。在水蚤-巴氏杆菌宿主-寄生虫系统中，现场评估通常显示出极低的毒力估计值，而受控实验室实验则显示出极高的毒力。为了检查这种差异，我们在寄生虫流行期间从田间采样了大型蚤（Daphnia magna）宿主，并在随后的3周时间内在实验室中记录了疾病特征。正如对慢性疾病的预测，在该疾病中，平均年龄较大（较大）的宿主也受到感染，年纪较大时，我们发现较大的D. magna雌性感染率更高，在去势开始前的后代较少，并且比较小的寄主更早地显示出感染的迹象。与实验室实验一致的是，在性别和所有大小的宿主中都发现了受感染的动物。被感染的雌性在捕获时被cast割，或在不久后被cast割。由于该田间大多数雌性在采样时都没有在卵囊中产卵，因此相对于未感染雌性而言，受感染雌性的毒力估计值很低。然而，随着实验室饲养条件的改善，只有未感染的雌性才恢复繁殖，从而导致很高的相对毒力估计值。总体而言，我们的研究表明，在自然条件下表达的ramosa P. ramosa的疾病表现，与我们从实验室实验中了解到的一致。然而，相对于未感染的宿主，寄生虫诱导的繁殖力降低，很大程度上取决于环境条件。我们认为，这种效应对于rating割寄生虫特别强，因为感染的宿主在所有情况下的繁殖力都较低。