First report on proliferative kidney disease (PKD) in marble trout (Salmo trutta marmoratus, Cuvier 1817)

Proliferative kidney disease is a hyperplastic response of the principal lymphoid tissue of salmonid fish infected by Tetracapsidoides bryosalmonae, a myxozoan parasite. This parasite affects many rainbow trout farms in Europe and North America. The disease has also been reported in other salmonids as well as in pike (Esox lucius) and grayling (Thymallus thymallus). In autumn 2000, an outbreak of PKD induced mortality in a group of marble trout (Salmo trutta marmoratus) juveniles reared in a farm in north-east Italy. The fish were intended to restock public waters. Diseased fish showed a lethargic behaviour, skin darkening, abdominal dilatation, gill anaemia and, after necroscopy, increase in volume and a pale colour of the kidney. All fishes subjected to histological examination showed a marked granulomatous interstitial nephritis, as well as foci of pancreatic and hepatic necrosis. The immunohistochemistry and PAS stain allowed visualisation of the extrasporogonic phase of the parasite Tetracapsuloides bryosalmonae in several tissues of the host. This is the first reported outbreak of PKD in marble trout, and should receive full attention since this species is potentially under risk of extinction.