Plerocercosis by Triaenophorus spp. (Cestoda: Pseudophyllidea) in Northern Italy

Infections by Triaenophorus spp. (Cestoda: Pseudophyllidea) are well known parasitoses affecting pike (Esox lucius) as definitive host and several freshwater fish species as second intermediate host. In Italy previous reports on infections by larval stages (plerocercoids) of Triaenophoridae referred only the presence of T. nodulosus in perch (Perca fluviatilis) , tench (Tinca tinca) and rudd (Scardinius erythrophthalmus). A massive infection by plerocercoids of T. crassus, the other European species, has been recently observed in coregonids (Coregonus spp.) from a lake in Bolzano province (South Tyrol), Northern Italy. In order to evaluate the pathogenic effect of these two species and to clarify the introduction way of T. crassus in Italy, a parasitological survey was carried out. During 2005-2006 a total of 190 fish belonging to 9 species was sampled from 13 lakes located in South Tyrol and submitted to a complete parasitological examination. All the collected cestodes were identified by morphological characters. Infected organs were fixed in 10% buffered formalin for histological examination and stained by Ematoxylin & Eosin and Mallory’s Trichrome. Seventy-eight out of 190 fish examined were positive for Triaenophorus spp. In particular, T. nodulosus was found in 11 out of 37 perch and 5 out of 46 pikes from 5 lakes, T. crassus in 56 out of 65 coregonids, 2 out of 4 brown trout (Salmo trutta fario) and 4 out of 6 pikes from 2 lakes. Concerning T. nodulosus, histology revealed in the liver parenchyma (the only affected organ) the presence of cysts surrounded by a fibrotic wall and containing several plerocercoids. The liver showed fibrosis, diffuse necrosis and haemorrhages. Many plerocercoids appeared degenerated. In T. crassus plerocercosis, always detected in lateral muscle, large cysts surrounded by fibroblasts and containing the larval stage of the parasite were present in muscle, with large necrotic areas and inflammatory cells infiltration. Granulomas containing a necrotic central core and structure of the parasite in regressive state were also observed. Triaenophorids plerocercoses confirm to be a serious sanitary problem for wild fish populations living in South Tyrol lakes, where restocking activities should be primarily implicated in the recent introduction of T. crassus.