Infection prevalence and density of a pathogenic trematode parasite decrease with stream order along a river continuum

Abstract

In lotic ecosystems, the River Continuum Concept (RCC) provides a framework for understanding changes in environmental factors and free-living communities, yet how parasite populations shift along river continua remains less clear. We quantified infections by a pathogenic trematode parasite (Nanophyetus salmincola) in >14,000 host snails across 130 stream reaches spanning 165 km in the Willamette River Basin in western Oregon, USA. Environmental factors—including flow volume, temperature, benthic algae, canopy cover, woody debris, and land cover—changed predictably with stream order, consistent with the RCC. From first- to eighth-order reaches, infection prevalence decreased by ˜42-fold and infected snail density, a measure of disease risk to fish hosts, decreased ˜3-fold. Infected snail density, but not prevalence, was positively associated with snail biomass density, and individual infection probability increased strongly with host size. Shifts in snail population characteristics across stream orders, however, did not explain the observed changes in N. salmincola populations, suggesting that environmental variables and corresponding changes in non-snail hosts explain the downstream decrease in infections. Our findings show predictable spatial variation in disease risk to vertebrate hosts from N. salmincola and indicate the RCC can help explain shifts in parasite populations in lotic ecosystems.