| Summary: | Fish-borne cestodes capable of infecting humans are represented almost exclusively by so called broad tapeworms, i.e. members of the order Diphyllobothriidea. These large-sized human tapeworms have three host life-cycles, in which teleost fishes (except in the case of Spirometra) play a role of the second intermediate hosts and represent a source of human infection. Although the broad fish tapeworms (genera Adenocephalus, Diphyllobothrium and Diplogonoporus) have been recognized as human parasites for a long time, many aspects of their biology and epidemiology, including species composition of individual genera, their clinical relevance and geographical distribution have been noticeably understudied. The overriding obstacle preventing clarification of the diversity, origin and host-associations of diphyllobothriids is the poor state of systematics of the group. Even though diphyllobothriosis itself is not a life-threatening disease, it is considered the most important fish-borne zoonosis caused by a cestode with up to 20 million people estimated to be infected worldwide, with an affinity to colder climates including subarctic and arctic areas of the North and partly South Hemisphere. Moreover, several species seem to (re)-emerge in the most developed countries. Current (re)-emergence of diphyllobothriosis and the introduction of its agents into new geographical regions are mainly fuelled by: (i) increased preference of human societies to consume raw food, (ii) globalized trade with fish products, (iii) human migration. Dozens of nominal species have been described so far, but only 15 species currently recognized as valid have been reported to infect humans. Moreover, the validity of species described from Alaska, which have been reported from man (Diphyllobothrium alascense, D. dalliae, D. ursi) should be confirmed using molecular data. Yet, we still lack information about the intraspecific variability and species boundaries of the most important broad tapeworm species including those occurring in cold climates, such as Diphyllobothrium latum, D. dendriticum and D. nihonkaiense. Moreover, molecular data indicate paraphyly of the genus Diphyllobothrium, which may explain different host associations and morphology of species in distantly related lineages. For the first time in the literature, data on all human-infecting broad fish tapeworms, most of which occur in cold climates, are summarized, with focus on rare or uncommon species that were largely neglected in the literature or appeared in hardly accessible papers.
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