A survey on plerocercosis by Diphyllobothrium latum (Cestoda: Pseudophyllidea) in Northern Italy

AIM. Diphyllobothriasis is a parasitic fish-borne zoonosis desrcibed in Italian sub-alpine lake districts due to the consumption of raw, cold-smoked or undercooked freshwater fish parasitized by the plerocercoid larvae of the pseudophyllidean cestode Diphyllobothrium latum. In recent years there has been documented a recrudescence of cases of human Diphyllobothriasis in Switzerland, France and Italy, indicating the persistence of this parasite in fish populations from lake environments. In order to update the epidemiological data on D. latum plerocercosis in fish populations from Italian sub-alpine lakes, a parasitological survey has been carried out on perch (Perca fluviatilis), pike (Esox lucius), burbot (Lota lota), whitefish (Coregonus lavaretus) and shad (Alosa fallax lacustris) from Como, Iseo, Maggiore, Garda, Monate and Comabbio Lakes. MATERIALS AND METHODS. From July 2008 to June 2011 a total of 1,606 fish were examined: 684 from Como Lake (509 perch, 13 pike, 29 burbot, 96 whitefish and 37 shad), 319 from Iseo Lake (271 perch, 2 pike, 5 burbot, 7 whitefish and 34 shad), 306 from Lake Maggiore (195 perch, 24 burbot, 64 whitefish and 23 shad), 212 from Garda (185 perch, 5 pike, 2 burbot, 20 whitefish), 57 perch from Comabbio Lake and 28 whitefish from Monate Lake. All the fish were weighed, measured and subjected to parasitological examination by visual inspection of fillets (by naked eye and candling) and of visceral organs. The parasites referable to cestode plerocercoid larvae were isolated and fixed in 70% ethanol. Identification was carried out at genus level by morphological study on the basis of the key proposed by Andersen KI, Gibson DI (1989, Syst Parasitol, 13: 6-9), then at species level by molecular analysis. The primers used were 82F (CAG TAG TCA 5.-TAT GCT TGT CTC AG-3) and 81R (TTC ACC TAC 5.-GGA AAC CTT GTT ACG-3), amplifying a fragment of 2500 bp of the 18S rDNA. The positive samples were sequenced and then compared with those deposited in GenBank using BLAST software. Prevalence (P) and Mean Intensity (MI) values were calculated as suggested by Bush AO et al (1997, J Parasitol, 83: 575-583). RESULTS. Plerocercoid larvae were found from Maggiore, Como and Iseo Lakes in perch, pike and burbot, in both fillets and visceral serosa. All the fish examined from the two small lakes of Monate and Comabbio as well as from Lake of Garda were negative for the presence of Diphyllobotrhium spp. plerocercoids. All the plerocercoid larvae found were identified as Diphyllobothrium latum. CONCLUSIONS. The results of this survey indicate a widespread presence of D. latum plerocercoids in perch populations of Lake Como and, to a lesser extent, in perch from Iseo and Maggiore Lakes, confirming this species as elective second intermediate host of D. latum in Italy. The occurrence of D. latum plerocercoid larvae in the lateral muscle of pike and burbot caught in Lake Como is an important finding since these two paratenic hosts usually show infections by Diphyllobothrium larvae only at the level of visceral serosa. This factor may greatly increase the risk level in consuming these two fish species as undercooked dishes. Furthermore it should be emphasized that all the examined whitefish and shad, among the most traded fish species on the local market and often consumed smoked and/or marinated, were negative and then could be considered safe with regard to D. latum transmission.