050 | Proliferative kidney disease: new insights for Piedmont region; correlation between clinical signs and proliferative kidney disease positivity in rainbow trout

Giorgia Zicarelli¹, Annalisa Cotugno¹, Sara Glorio Patrucco¹, Silvia Colussi^1|2, Giulia Milanese¹, Giulia Stilo¹, Martina Gini¹, Pier Luigi Acutis¹, Caterina Faggio^3|4, Giuseppe Esposito¹, Paolo Pastorino¹, Marino Prearo¹ | ¹Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d’Aosta, Turin, Italy; ²Department of Animal, Nutritional and Food Sciences, Università Cattolica del Sacro Cuore, Piacenza, Italy; ³Department of Chemical, Biological, Pharmaceutical and Environmental Sciences, University of Messina, Messina, Italy; ⁴Department of Eco-sustainable Marine Biotechnology, Stazione Zoologica Anton Dohrn, Napoli, Italy.

Abstract

Parasitosis represents a significant threat to farmed fish, both in freshwater and in marine aquaculture facilities. In this context, Proliferative Kidney Disease (PKD), caused by Tetracapsuloides bryosalmonae (Myxozoan: Malacosporea), is one of the main causes of mortality, showing mortality ranging from 95 to 100% in salmonid species. These parasites require two hosts to complete their life cycle: a freshwater bryozoan as the definitive host and a salmonid as the intermediate host. In the fish host, the parasites infect the gills and skin, migrate to the kidney via the vascular system, and undergo sporogenesis there. Among the principal clinical signs in the salmonid host are kidney proliferation, hepatomegaly, splenomegaly, gill anaemia, ascites, lethargy, and secondary infections, all of which are exacerbated by T. bryosalmonae replication. Due to global warming and water temperatures rising above 15°C for more months of the year, the geographic range of PKD has expanded, resulting in high mortality and economic losses for aquaculture facilities. As part of the RESILTROUT project, the present study aimed to evaluate disease spread in rainbow trout from the Piedmont region (northwest Italy) and to promote an integrated approach that correlates PKD detection with clinical signs and possible secondary bacterial infections. For this study, rainbow trout were sampled from three distinct sectors of aquaculture facilities in the Cuneo province (Piedmont, Italy) between July and December 2024. Necropsies were performed to collect spleen and kidney samples and to evaluate clinical signs associated with PKD. Parasite detection was assessed by endpoint PCR. Moreover, a bacteriological analysis of kidney samples was performed to evaluate possible coinfections. Overall, 34.43% of rainbow trout tested were positive for PKD, and 7.88% were low-level positive. The highest positivity rate was recorded in sector 2, where over 70% of animals tested were positive. The main clinical signs observed were kidney tissue proliferation, splenomegaly, and enteritis (with varying percentages across the sectors analysed). Nevertheless, most positive samples did not show macroscopic alterations. Sector 3 was the least affected by PKD, but positive samples displayed severe lesions, suggesting advanced disease or a strong immunopathological response. Bacterial coinfections were observed only in specimens from sectors 1 and 2, underscoring the immunosuppression and tissue damage caused by PKD. In conclusion, the present findings confirm the south-westward spread of PKD in the Piedmont region, highlighting the importance of an integrated diagnostic approach combining different analyses for early detection of the parasitosis. Moreover, early detection can enable the development of strategies to protect a sector particularly vulnerable to PKD outbreaks, such as the aquaculture sector.