Proliferative Kidney Disease and Viral Pathogens in Wild Brown Trout (Salmo trutta) Populations in the Czech Republic.

Populations of wild brown trout (Salmo trutta) have been in long-term decline across Central Europe, including the Czech Republic, with multiple factors, such as habitat alteration, climate change, predation and emerging diseases, implicated. Among the diseases, proliferative kidney disease (PKD), caused by the myxozoan Tetracapsuloides bryosalmonae, has gained increasing attention. Between 2020 and 2024, we investigated the occurrence of T. bryosalmonae and selected viral pathogens in wild brown trout populations from 34 streams (51 sites) across the three major Czech river basins (Elbe, Morava, Odra). In total, 501 fish were examined using pathology and molecular techniques. While T. bryosalmonae DNA was detected in 54.7% of fish and at 74.5% of localities, with highest prevalence in the Elbe basin (63.5%), gross kidney changes consistent with PKD were observed in just 7.4% of individuals. Significant associations were found between parasite occurrence and mean summer water temperature, with positive sites showing higher water temperatures. No mortalities were observed in the wild populations, though subclinical infections were common. Our findings demonstrate that not only is the PKD agent widespread in Czech trout populations but that temperature plays an important role in parasite dynamics, highlighting increasing risks posed by climate change. We suggest that careful fish stocking strategies will be essential in preventing further spread. The viral pathogens Salmonid novirhabdovirus, Piscine novirhabdovirus and Aquabirnavirus salmonidae were not detected, and piscine orthoreovirus genotype 3 was confirmed in two samples from the Odra basin, showing high sequence identity with previously reported Czech isolates.