Fire-associated declines in laboratory-measured cercarial emergence from trematode-infected freshwater snails

Climate change is intensifying wildfire activity across western North America, altering disturbance regimes with far-reaching consequences for aquatic ecosystems that support high biodiversity. However, the effects of wildfire on host-parasite interactions, as key components of food webs and ecosystem functioning, remain poorly understood. We investigated whether wildfire influences trematode infection prevalence and cercarial emergence by sampling Juga plicifera snails in 16 headwater streams in Oregon’s Cascade Range, 1 year after the 2020 Labor Day Fires. We also evaluated the influence of abiotic and biotic factors on parasite dynamics using concurrent field data.

Trematode infections were widespread, with prevalence ranging from 2 to 40% (mean 12.4%), and cercarial emergence averaging 190 parasites per snail. Morphological and genetic analyses identified at least four trematode taxa, including Nanophyetus salmincola and Metagonimoides oregonensis. Zero-inflated negative binomial models indicated that fire extent was negatively associated with laboratory-measured cercarial emergence, while showing no relationship with infection prevalence. Additionally, cercarial emergence was negatively associated with chlorophyll a concentration, amphibian biomass, and snail mass, while light availability, fish biomass, and macroinvertebrate density had positive associations. Infection prevalence was negatively related to the proportion of scraper macroinvertebrates, 7-day maximum stream temperature, and snail mass, but positively associated with amphibian biomass.

Wildfires can potentially suppress parasite transmission by disrupting host-parasite-environment interactions in stream ecosystems. The divergent drivers of infection prevalence and cercarial emergence highlight the complex interplay of biotic and abiotic factors shaping parasite dynamics in fire-affected freshwater habitats. These findings underscore the need to integrate parasite ecology into assessments of wildfire impacts on aquatic ecosystems.