Large-scale spatial analyses reveal hotspots of proliferative kidney disease in brown trout and interactive effects of temperature and parasite load

Proliferative kidney disease (PKD) poses a threat to wild salmonids, yet its spatial patterns remain poorly understood, particularly at large scales and across varied ecosystems. We combined the largest nationwide screening of brown trout (Salmo trutta), consisting of 1072 fish from 155 locations, spanning a 1480 km latitudinal gradient, with process-based stream-temperature modelling. From this data, we map infection by the parasite Tetracapsuloides bryosalmonae (T.b), quantify parasite load (using qPCR from kidney tissue), and calculate renal hyperplasia across Sweden under different thermal regimes. PKD emerged if study-period mean water temperatures approached a threshold of approximately 15.4 °C and renal hyperplasia peaked near 17 °C; however, warm water did not always cause disease: asymptomatic individuals were common above the temperature threshold. Spatial mixed-effects models revealed that parasite load and temperature interacted to determine disease severity, whereby severe PKD was associated with lower parasite loads under warmer conditions. Furthermore, deviations from broad latitudinal patterns were observed, where distinct coastal hotspots in central Sweden would be overlooked by assuming a simple temperature-driven latitudinal gradient. Our model shows that such patterns remain after accounting for temperature and parasite load, indicating that local conditions and additional environmental drivers are likely affecting epidemiology at relatively small spatial scales. Management actions that moderate stream temperatures, such as riparian shading or the removal of impoundments, or reduce suitable habitat for bryozoan hosts, may therefore mitigate disease impacts under a warming climate.