Individual fish infection data from metapopulation epidemics used to calculate resistance, tolerance and competence

Host competence, defined as the likelihood that a host will transmit infection, may be affected by an individual's resistance to infection and its ability to withstand damage caused by infection (tolerance). Host competence may therefore be one of the most important factors to impact host-parasite dynamics, yet the relationships among resistance, tolerance, and competence are poorly understood. The objective of the present study was to determine whether individual host resistance (ability to resist or minimize infection) and/or tolerance (ability to withstand or minimize reduction in fitness due to infection) contributed to the competence (ability to spread infection) of hosts using guppies infected with the ectoparasite, Gyrodactylus turnbulli. This individual-fish level analysis used data collected from a previous metapopulation experiment that had tracked host-parasite dynamics at the metapopulation scale using individually marked guppies that were moved among experimental tanks within replicate metapopulations. Fish tolerance was measured as the residual from a fish's expected survival post-infection for a given parasite burden. Fish resistance was measured as the peak parasite load ( - log-transformed). Host competence was measured as the incidence (number of new infections over two days after the arrival of a fish to a tank) weighted by the density of available uninfected fish in the tank. In contrast to the assumption of a trade-off between resistance and tolerance, individual fish tolerance and resistance were both negatively associated with competence. Connectivity (the number of fish with which an individual came into contact) was not associated with competence. Our results indicate that resistance and tolerance are both important to disease spread. These findings highlight the importance of understanding how individual defence against parasites may contribute to its competence as a host, and therefore impact metapopulation-level dynamics.宿主传播能力（Host competence）被定义为宿主完成感染传播的可能性，其可受个体对感染的抵抗力以及耐受感染所致损伤的能力（耐受性，tolerance）影响。因此，宿主传播能力或许是影响宿主-寄生虫动态的关键因素之一，但目前学界对抵抗力、耐受性与传播能力三者间的关联仍知之甚少。本研究以感染了外寄生虫（ectoparasite）特氏三代虫（Gyrodactylus turnbulli）的孔雀鱼为研究对象，旨在明确个体宿主的抵抗力（抵抗或抑制感染的能力）以及/或耐受性（耐受感染或降低感染所致适合度（fitness）下降的能力），是否会对宿主的传播能力（完成感染传播的能力）产生影响。本项基于个体鱼类水平的分析，所用数据源自此前一项集合种群（metapopulation）实验：该实验以带个体标记的孔雀鱼为研究对象，在重复设置的集合种群内的实验水槽（tanks）之间移动这些个体，以追踪集合种群尺度下的宿主-寄生虫动态。鱼类耐受性通过感染后特定寄生虫负荷下的预期存活残差进行量化。鱼类抵抗力以峰值寄生虫负荷（经负对数转换）进行衡量。宿主传播能力则以感染发生率（某尾鱼迁入水槽后的两天内出现的新感染数量），结合水槽内未感染宿主的密度进行加权计算。与抵抗力与耐受性之间存在权衡的预设假设相反，个体鱼类的耐受性与抵抗力均与传播能力呈负相关关系。连通性（connectivity，即个体接触的鱼类数量）与传播能力并无关联。本研究结果显示，抵抗力与耐受性均对疾病传播具有重要作用。上述发现凸显了厘清个体针对寄生虫的防御机制如何影响宿主传播能力，进而影响集合种群尺度下的疾病动态这一问题的重要性。