Negative covariance between parasite load and body condition in a population of feral horses

In wild and domestic animals, gastrointestinal parasites can have significant impacts on host development, condition, health, reproduction and longevity. Improving our understanding of the causes and consequences of individual-level variation in parasite load is therefore of prime interest. Here we investigated the relationship between strongyle fecal egg count (FEC) and body condition in a unique, naturalized population of horses that has never been exposed to anthelmintic drugs (Sable Island, Nova Scotia, Canada). We first quantified variation in FEC and condition for 447 individuals according to intrinsic (sex, age, reproductive status, social status) and extrinsic (group size, location, local density) variables. We then quantified the repeatability of measurements obtained over a field season and tested for covariance between FEC and condition. FECs were high relative to other horse populations (mean eggs per gram +/- SD = 154328 +/- 20994). FECs generally decreased with age, were higher in lactating vs non-lactating females, and unexpectedly lower in males in some part of the island. FECs and condition were both spatially structured, with patterns depending on age, sex and reproductive status. FECs and condition were both repeatable. Most notably, FECs and condition were negatively correlated, especially in adult females.

在野生动物和家畜中，胃肠道寄生虫会对宿主的发育、体况、健康、繁殖和寿命产生重大影响。因此，增进我们对寄生虫负荷个体水平差异的原因和后果的理解是至关重要的。在此，我们研究了在一个从未接触过驱虫药物的独特的自然化马群（加拿大新斯科舍省塞布尔岛）中，圆线虫粪便虫卵计数（FEC）与体况之间的关系。我们首先根据内在（性别、年龄、繁殖状态、社会地位）和外在（群体大小、位置、局部密度）变量，对447个个体的FEC和体况的差异进行了量化。然后，我们量化了在一个野外季节所获得测量值的重复性，并检验了FEC和体况之间的协方差。与其他马群相比，FEC较高（每克粪便平均虫卵数±标准差 = 154328 ± 20994）。FEC通常随年龄增长而降低，哺乳期雌性的FEC高于非哺乳期雌性，并且在该岛某些区域雄性的FEC出乎意料地较低。FEC和体况都具有空间结构，其模式取决于年龄、性别和繁殖状态。FEC和体况都具有重复性。最值得注意的是，FEC和体况呈负相关，尤其是在成年雌性中。