Artificial light at night amplifies seasonal relapse of haemosporidian parasites in a widespread songbird

Abstract

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.

Long-term exposure to artificial light at night in the wild decreases survival and growth of a coral reef fish

“…Approximately one-tenth of the world's population (600 million people) live in coastal areas that are less than 10 m above sea level, resulting in considerable anthropogenic light pollution [ 7 ], which is expected to increase in parallel with global human population increases along the world's coastline [ 8 ]. Light pollution is a recognized threat for wildlife and biodiversity worldwide [ 9 , 10 ], directly affecting biological and ecological processes across taxa, including changes in key life-history traits, such as immune function [ 11 , 12 ], survival [ 13 ], ageing [ 14 ], and fecundity [ 15 ], however, the impacts of ALAN have rarely been assessed for marine species in the wild.…”

Exploring the thermal limits of malaria transmission in the western Himalaya

“…infection risk increases with elevation; however, most of these infections were submicroscopic in high elevation in breeding season (April–May). This contrasts with studies on the ecology of haemosporidia in temperate regions where birds with latent infections return to the breeding grounds and experience a relapse, with increase in parasites visible in the blood stages (Applegate, 1970 ; Becker et al, 2020 ). In general, parasite intensity showed a significant decline with elevation in the breeding season (April–May) and an increase across mid‐elevations in the nonbreeding season.…”

Reactivation of latent infections with migration shapes population-level disease dynamics

“…Such resources could dampen or amplify pathogen relapse at spring departure depending on their nutritional quality or effects on host density and crowding [75]. Other stressors in urban habitats, such as artificial light at night, could further amplify the likelihood of relapse [76]. These consequences of urban habituation could be especially relevant for human health in the context of wildlife reservoirs of relapsing zoonoses, such as flying foxes and henipaviruses [77].…”