Abstract Short-term elevation of glucocorticoids (GCs) is one of the major physiological mechanisms by which vertebrates cope with challenging environmental or social factors (stressors). However, when exposure to stressors occurs repeatedly or over a prolonged period of time, animals may experience chronic elevation of GCs, which reduces the immune response efficiency and can lead to higher intensity of parasitic infection. Here, we used invasive gray squirrels Sciurus carolinensis introduced in Northern Italy and their 2 most prevalent gastrointestinal parasites, the nematode Strongyloides robustus and coccidia of the genus Eimeria, as a model to investigate relationships among macroparasite infection and concentrations of fecal glucocorticoid metabolites (FGMs), an integrated measure of circulating GCs. Our results revealed an association of FGMs with infection by St. robustus, but not with coccidia. Individuals with higher FGMs appear to be responsible for the greatest St. robustus egg shedding within gray squirrel populations, thus possibly acting as superspreaders. However, FGMs were negatively associated with adult St. robustus, suggesting that the abundance of adults of this nematode species does not induce elevation in FGMs, but is only affected by it through immune-mediated effects on its fecundity. Finally, the relationship between St. robustus (both eggs and adult parasites) and FGMs was not linear, suggesting that only high levels of physiological stress influence parasite infection. Our findings highlight that the direction and magnitude of the stress–infection relationship may depend not only on the specific host–parasite system, but also on the different life stages of the same parasite.

中文翻译：

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生理压力与自然种群内寄生虫感染之间的复杂关系

摘要 糖皮质激素 (GCs) 的短期升高是脊椎动物应对具有挑战性的环境或社会因素（压力源）的主要生理机制之一。然而，当反复或长时间暴露于压力源时，动物可能会经历 GC 的慢性升高，这会降低免疫反应效率，并可能导致更高强度的寄生虫感染。在这里，我们使用在意大利北部引入的侵入性灰松鼠 Sciurus carolinensis 及其 2 种最常见的胃肠道寄生虫，即粗壮线虫和艾美球虫属球虫，作为研究大寄生虫感染与粪便糖皮质激素代谢物 (FGM) 浓度之间关系的模型，循环 GC 的综合测量。我们的研究结果揭示了 FGM 与 St.robustus 感染的关联，但与球虫无关。具有较高 FGM 的个体似乎是灰松鼠种群中最大的 St.robustus 卵脱落的原因，因此可能充当超级传播者。然而，FGMs 与成虫 St.robustus 呈负相关，这表明该线虫物种的成虫丰度不会导致 FGMs 升高，而只会通过免疫介导对其繁殖力的影响而受到影响。最后，St.robustus（卵和成虫寄生虫）和 FGMs 之间的关系不是线性的，这表明只有高水平的生理压力会影响寄生虫感染。我们的研究结果强调，压力-感染关系的方向和程度可能不仅取决于特定的宿主-寄生虫系统，