First Detection and Seasonal Dynamics of Proliferative Kidney Disease in Northwestern Italy

Proliferative kidney disease (PKD), caused by the myxozoan parasite Tetracapsuloides bryosalmonae, represents a major threat to salmonid populations and aquaculture, particularly under warming environmental conditions. In Italy, its distribution remains poorly documented. This study aimed to update the epidemiological status of PKD by investigating its occurrence in a trout farm (Piedmont, northwestern Italy), a previously unreported area. A total of 787 individuals of Oncorhynchus mykiss were sampled between May and October 2024 across three seasonal periods. Fish were examined using necropsy, bacteriological, and molecular analyses. Water temperature ranged from 12.4 to 20.3 °C, showing a clear seasonal trend. Overall PKD prevalence was 42.3%, with significant temporal variation, increasing from 23.0% in early summer to 73.0% in late summer, and declining to 3.9% in autumn. A multivariable logistic regression model showed that PKD-positive fish had a lower probability of presenting macroscopic lesions, while sampling period was the main predictor of infection. These results revealed a temporal mismatch between pathological and molecular findings, highlighting the need for further histopathological validation of the observed seasonal patterns. This study provides the first evidence of PKD in southwestern Piedmont, extending the currently documented distribution of the disease in Italy and highlighting the need for enhanced surveillance under climate change.