Cystidicola farionis (Nematoda: Cystidicolidae) from wild salmonids in Italy.

A parasitic nematode was found to occur in large number in the swim bladder of wild rainbow trout (Oncorhynchus mykiss) caught in Adige River, Northern Italy. In order to identify the parasite and study its diffusion and pathological effect on the infected fish, a total of 31 salmonids belonging to 4 different species were sampled from the wild. In particular, 15 rainbow trout (O. mykiss), 8 brown trout (Salmo trutta fario), 6 marble trout (Salmo trutta marmoratus) and 2 brook trout (Salvelinus fontinalis) were submitted to parasitological examination. The parasites collected from swim bladder were rinsed in dechlorinated tap water and fixed in 70° ethanol. Swim bladders from heavily infected fish were fixed in 10% buffered formalin for histology. Parasites were clarified by Amman’s Lactophenole and observed to detect the main morphological features. The presence of nematodes was detected in the swim bladder of 13 out of 15 rainbow trout, 3 out of 8 brown trout and 2 out of 6 marble trout. On the basis of the morphological features the parasites were all identified as Cystidicola farionis (Nematoda: Cystidicolidae), species widely spread among European salmonids but not yet described in Italy. Gross pathological observations revealed in heavily infected fish the presence of worms free in the lumen or strongly penetrating the swim bladder wall, with large haemorrhagic lesions. Histopathological observations were consistent with previous descriptions, showing sloughing off of the mucosa and necrosis with pyknosis and caryoressis of epithelial cells. Necrotic lesions extended to the subepithelial connective tissue, where diffuse inflammatory cells infiltrations were present too. In some cases focal lesions were observed in the muscular layer. This report points out the importance of associating a parasitological control to the mandatory analyses requested for restocking activities, with the aim of avoiding the introduction of pathogenic parasites that could represent a great sanitary risk for wild populations