Department of Biology, University of Bergen, Bergen, Norway.
J Evol Biol. 2012 May;25(5):856-61. doi: 10.1111/j.1420-9101.2012.02474.x. Epub 2012 Feb 22.
Models of virulence evolution for horizontally transmitted parasites often assume that transmission rate (the probability that an infected host infects a susceptible host) and virulence (the increase in host mortality due to infection) are positively correlated, because higher rates of production of propagules may cause more damages to the host. However, empirical support for this assumption is scant and limited to microparasites. To fill this gap, we explored the relationships between parasite life history and virulence in the salmon louse, Lepeophtheirus salmonis, a horizontally transmitted copepod ectoparasite on Atlantic salmon Salmo salar. In the laboratory, we infected juvenile salmon hosts with equal doses of infective L. salmonis larvae and monitored parasite age at first reproduction, parasite fecundity, area of damage caused on the skin of the host, and host weight and length gain. We found that earlier onset of parasite reproduction was associated with higher parasite fecundity. Moreover, higher parasite fecundity (a proxy for transmission rate, as infection probability increases with higher numbers of parasite larvae released to the water) was associated with lower host weight gain (correlated with lower survival in juvenile salmon), supporting the presence of a virulence-transmission trade-off. Our results are relevant in the context of increasing intensive farming, where frequent anti-parasite drug use and increased host density may have selected for faster production of parasite transmission stages, via earlier reproduction and increased early fecundity. Our study highlights that salmon lice, therefore, are a good model for studying how human activity may affect the evolution of parasite virulence.
水平传播寄生虫的毒力进化模型通常假设传播率（感染宿主感染易感宿主的概率）和毒力（感染导致宿主死亡率的增加）呈正相关，因为繁殖体的更高产生率可能会对宿主造成更多的损害。然而，对这一假设的经验支持很少，并且仅限于微生物寄生虫。为了填补这一空白，我们研究了鲑鱼虱（Lepeophtheirus salmonis）的寄生虫生活史和毒力之间的关系，鲑鱼虱是一种水平传播的桡足类外寄生虫，寄生在大西洋鲑鱼（Salmo salar）上。在实验室里，我们用等量的传染性鲑鱼虱幼虫感染幼年鲑鱼宿主，并监测寄生虫首次繁殖的年龄、寄生虫的繁殖力、对宿主皮肤造成的损害面积以及宿主的体重和体长增加情况。我们发现寄生虫繁殖的开始越早，寄生虫的繁殖力就越高。此外，寄生虫繁殖力越高（作为传播率的一个指标，因为随着水中释放的寄生虫幼虫数量增加，感染概率也会增加），宿主的体重增加越低（与幼年鲑鱼的存活率降低相关），支持存在毒力-传播的权衡关系。我们的研究结果在集约化养殖不断增加的背景下是相关的，在这种情况下，频繁使用抗寄生虫药物和增加宿主密度可能通过更早的繁殖和增加早期繁殖力，选择了更快的寄生虫传播阶段的产生，从而影响寄生虫毒力的进化。我们的研究强调，鲑鱼虱因此是研究人类活动如何影响寄生虫毒力进化的一个很好的模型。
