FRESHWATER FISH-BORNE PARASITIC ZOONOSES IN ITALY

Fish-borne parasitic zoonoses have aroused an increasing interest in Italy over recent years for the changing of culinary habits towards an increasing consumption of raw/undercooked/marinated fish, in association with the expansion of international commercial trade in fishery products, globalization and migration flows across world, all factors which have driven to an increased risk of acquiring fish- borne zoonoses. Several zoonotic helminths can be transmitted through consumption of raw and/or undercooked fish products to humans, since fishes actively participate to biological cycles of heteroxenous parasites that involve man as definitive or accidental host, causing different degrees of pathology. Although currently WHO include, with regard to zoonotic fish helminthes, only Opisthorchiidae (Opisthorchis spp. and Clonorchis spp.) in the priority list of the foodborne parasites “that could produce a substantive burden of disease” (Torgerson et al., 2014, Trends Parasitol., 30: 20-26) it is undeniable that other fish parasites such as Anisakid nematodes and Diphyllobothriid cestodes should be taken into account when considering the zoonotic risks linked to European fish products consumption. In fact, according to EFSA Scientific Opinion on risk assessment of parasites in fishery products (European Food Safety Authority, 2010, EFSA J., 8, 1543) all wild fish should be considered at risk of containing any viable zoonotic parasites if these products are to be eaten raw or almost raw, pointing out the need to carry out epidemiological surveys on presence/diffusion of zoonotic parasites in all fishery grounds. Respect to infections due to Anisakid larvae in marine fish, until recently in Italy a lower attention has been devoted to the study of zoonotic helminths in freshwater fish populations, such as in primis the cestode Diphyllobothrium latum and the digenean Opisthorchis felineus. The recrudescence of cases of human Diphyllobothriasis registered in Switzerland, France and Italy during the last decades (Scholz et al., 2009, Clin.Microbiol.Rev., 22: 146-160) and the recent outbreaks of human Opisthorchiasis observed in Italy, with more than 200 autochthonous human cases reported from 2004 up to 2011 (Pozio et al., 2013, Acta Tropica, 126: 54-62), have determined a renewed interest in epidemiology of infections due to zoonotic parasites in freshwater fish. Concerning Diphyllobothriasis, recent surveys pointed out a widespread presence of D. latum plerocercoids in perch (P. fluviatilis) populations of Como Lake 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 was also found in pike (Esox lucius) and burbot (Lota lota), main paratenic hosts of D. latum, coming from the same sub-alpine lakes, while fish from Garda lake did not show D. latum infections (Gustinelli et al., 2013, Atti XIX Conv. SIPI, Siracusa, 75). With regard to Opisthorchiasis, although several species of cyprinids have been described as suitable second intermediate hosts of O. felineus from different endemic countries, so far in Italy only tench (Tinca tinca) has been found positive from Bolsena, Bracciano and Vico lakes (De Liberato et al., 2011, Vet.Parasitol., 177: 67-71; Gustinelli et al., 2011, 15th Conf. EAFP, Split, HR, 424), always with high prevalence and intensities. The consumption of raw perch (“perch carpaccio”) and marinated tench in the positive areas has been identified as the main food source of viable infective stages of D. latum and O. felineus respectively. In the case of O. felineus, it has to be pointed out that the microscopical size of metacercariae make their detection impossible through the visual inspection procedures commonly applied for the purpose of detecting parasites in fishery products, requiring more destructive and time consuming techniques. Anyway, both for Diphyllobothrium plerocercoids and Opisthorchiid metacercariae, morphology is not sufficient for identification at species level needing the support of molecular methods. The analysis of risk factors influencing the epidemiology of these zoonotic helminths indicate the lack of efficient urban wastewater depuration plants around lake shores as the main aspect influencing the maintenance of D. latum cycle, being humans the elective definitive hosts, together with the influence of abiotic parameters such as temperature and oxygen on development of Diphyllobothrium first stages (Wicht et al., 2009, Bull.Eur.Ass.Fish Pathol., 29: 58-65). Concerning O. felineus, the presence of parasitised stray cats, main reservoirs of the infection, and of Bythinia spp. gastropods, suitable first intermediate hosts, seem to be the key factors in the subsistence of this zoonotic helminth described in Italy since XIX century in dogs and cats (Rivolta, 1884, Giorn. Anat. Fisiol. Patol. An., 16: 20–28), although the role of fox and other wild animals as reservoirs and the possible involvement of cyprinids other than tench as second intermediate hosts have to be better studied. Since the maintenance of the complex biological cycles of these zoonotic helminths is influenced by several biotic and abiotic factors, investigating the interplay of these factors may be crucial in next future not only for a better understanding of parasite transmission patterns, but also for predictive studies on presence/ maintenance of zoonotic parasites in different fish species in order to map the risks linked to consumption of freshwater fishery products. At this purpose, further surveys aimed at investigate the possible occurrence of larval stages of other zoonotic helminths such as the Opisthorchiid Metorchis sp. and the Heterophyid Metagonimus sp. in Italian freshwater fish populations should be carried out in order to increase the knowledge on epidemiology of fish-borne zoonotic parasites and to prevent outbreaks of new zoonoses. Furthermore, although so far in Italy the presence of zoonotic helminths has been documented only in wild fish populations, extensive epidemiological surveys should be conducted on parasites of public health importance in farmed fish species, as required by EFSA and by current EU food hygiene regulations, in order to confirm as null or negligible the zoonotic risk linked to consumption of national aquaculture products.