Study on the distribution of Tetracapsuloides bryosalmonae in wild trout in South Tyrol - Italy

Tetracapsuloides bryosalmonae is a myxozoan endoparasite responsible for Proliferative Kidney Disease (PKD) among salmonids. The myxozoan life cycle and the manifestation of clinical signs of infection in fish are water temperature related. PKD is considered an emerging disease, as climate changes could increase the possibility of outbreaks. In South Tyrol (Italy), the protection of the environment and ecosystems is considered of primary importance. The population of fish is therefore routinely monitored to verify the health status of the local waterways. Based on this knowledge, a two-year study (2022-2023) was conducted by the Institute for Animal Disease Control of Bolzano, with the contribution of the Wildlife Management Office of Bolzano. The aim of the study was to investigate the prevalence and the geographic distribution of T. bryosalmonae in trout from four of the main watercourses in South Tyrol: Isarco, Rienza and Adige rivers and the Passirio stream, with a particular focus on identifying clinical and subclinical forms of infection. A total of 143 trout was collected in 2022 (102 juveniles and 41 adults) and a total of 137 in 2023 (76 juveniles and 61 adults) in 30 different sites from the abovementioned watercourses and their adjacent irrigation ditches. All fish were subjected to anatomical inspections and kidney samples were processed to perform cytological, histological and molecular examinations. Results showed a positivity rate of T. bryosalmonae of 11 % within the sampled specimens, specifically affecting adult trout, and mainly distributed in a limited geographic area.