Urban habitats can shape interactions between hosts and parasites by altering not only exposure rates but also within-host processes. Artificial light at night (ALAN) is common in urban environments, and chronic exposure can impair host immunity in ways that may increase infection. However, studies of causal links between this stressor, immunity, and infection dynamics are rare, particularly in migratory animals. Here, we experimentally tested how ALAN affects cellular immunity and haemosporidian parasite intensity across the annual cycle of migrant and resident subspecies of the dark-eyed junco (Junco hyemalis). We monitored an experimental group exposed to light at night and a control group under natural light/dark cycles as they passed through short days simulating early spring to longer days simulating the breeding season, followed by autumn migration. Using generalized additive mixed models, we show that ALAN increased inflammation, and leucocyte counts were greatest in early spring and autumn. At the start of the experiment, few birds had active infections based on microscopy, but PCR revealed many birds had chronic infections. ALAN increased parasitaemia across the annual cycle, with strong peaks in spring and autumn that were largely absent in control birds. As birds were kept in indoor aviaries to prevent vector exposure, this increased parasitaemia indicates relapse of chronic infection during costly life-history stages (i.e. reproduction). Although the immunological and parasitological time series were in phase for control birds, cross-correlation analyses also revealed ALAN desynchronized leucocyte profiles and parasitaemia, which could suggest a general exaggerated inflammatory response. Our study shows how a common anthropogenic influence can shape within-host processes to affect infection dynamics.

中文翻译：

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夜间人造光会加剧广泛分布的鸣禽中血孢子虫寄生虫的季节性复发


城市栖息地不仅可以改变暴露率，还可以改变宿主内部的过程，从而影响宿主和寄生虫之间的相互作用。夜间人造光 (ALAN) 在城市环境中很常见，长期暴露会损害宿主免疫力，从而可能增加感染。然而，对这种压力源、免疫和感染动态之间因果关系的研究很少，特别是在迁徙动物中。在这里，我们通过实验测试了 ALAN 如何影响黑眼灯笼草 (Junco hyemalis) 迁徙和常住亚种的细胞免疫和血孢子虫寄生虫强度。我们监测了一个在夜间暴露在光线下的实验组和一个在自然光/黑暗循环下的对照组，他们经历了模拟早春的短日照，到模拟繁殖季节的长日照，然后是秋季迁徙。使用广义加性混合模型，我们发现 ALAN 会增加炎症，并且白细胞计数在早春和秋季最高。实验开始时，根据显微镜观察，很少有鸟类存在活跃感染，但 PCR 显示许多鸟类患有慢性感染。艾伦在整个年度周期中增加了寄生虫血症，在春季和秋季出现了强烈的高峰，而对照鸟类基本上没有这种高峰。由于鸟类被饲养在室内鸟舍中以防止媒介暴露，这种寄生虫血症的增加表明在代价高昂的生活史阶段（即繁殖）慢性感染复发。尽管对照鸟的免疫学和寄生虫学时间序列是同步的，但交叉相关分析也揭示了 ALAN 不同步的白细胞谱和寄生虫血症，这可能表明普遍夸大的炎症反应。 我们的研究表明，常见的人为影响如何塑造宿主内的过程，从而影响感染动态。