Bird trematodes in the era of rapid environmental change

Rapid climate warming, hydrological regulation, eutrophication, pollution, and habitat conversion are reshaping aquatic ecosystems in which bird trematodes complete complex life cycles. Because birds act as definitive hosts, dispersal agents, and trophic links among habitats, we provide a bird-focused review aimed at improving our understanding of how environmental change affects trematode transmission, community structure, and biogeography.

We reviewed evidence on avian trematodes across freshwater, coastal, artificial wetland, agricultural, and high-latitude systems. The review integrates mechanistic studies of temperature-dependent intramolluscan development, cercarial emergence, survival, and infectivity with field studies documenting changes in hydrology, intermediate-host availability, bird phenology, habitat use, and trematode community composition.

Environmental change affects bird trematodes through multiple interacting pathways. Warming can accelerate cercarial production and shift emergence phenology, but effects are nonlinear and stage-specific because higher temperatures may also reduce cercarial survival or disrupt synchrony with bird hosts. Hydrological alteration, eutrophication, and pollution alter snail and second intermediate host communities, creating spatially heterogeneous transmission mosaics. Changes in bird distribution, migration timing, diet, and use of artificial habitats can either maintain transmission, increase exposure to generalist trematodes, or interrupt complex life cycles. Long-term and molecular studies indicate that disturbed systems may lose specialist trematodes, become functionally homogenized, or conceal cryptic lineage turnover under apparently stable morphology-based species lists.

Bird-trematode systems respond to environmental change in context-dependent ways rather than through uniform increases or decreases in transmission. Birds can amplify, disperse, buffer, or interrupt trematode life cycles depending on host competence, movement, habitat connectivity, and intermediate-host availability. Future surveillance should combine stage-specific parasite sampling, molecular identification, bird movement data, and remote sensing to improve forecasts of trematode redistribution and transmission risk under ongoing environmental change.