The life cycle of the tapeworm Dibothriocephalus latus in Switzerland: what we know, what we miss, and how it is sustained

Viktor Kovalov, Barbora Trubenova

Dibothriocephalus latus is a broad fish tapeworm with a complex life cycle that requires three host stages - copepods, fish, and humans. Over the last 25 years, the prevalence of D. latus has noticeably declined in many parts of Europe, with the exception of the Alpine region. Recent studies have shown that D. latus is prevalent in fish from sub-alpine lakes, the second intermediate host, which suggests its presence in copepods and humans, hosts from other trophic levels. However, information on human cases, as well as cases from alternative definitive hosts, is limited to sporadic reports. In addition, no studies have investigated the natural occurrence of the tapeworm in its first intermediate host — copepods, as potential host species from this stage were shown only experimentally. All this raises a question on how and why the life cycle of D. latus is maintained in Switzerland, but not in other regions. In this manuscript, we follow the life cycle of D. latus, focusing on each host stage, summarising existing knowledge on species range, prevalence rate, and recent history of case reports. We identify knowledge gaps and propose new research directions that would address them. In addition, we discuss several aspects of D. latus life cycle that have not been investigated before, such as the transition of D. latus eggs into the aquatic environment through sewage, potential alternative definitive hosts, and triploidy in the Alpine population. Considering the life cycle as a whole, we want to emphasise the importance of the multi-faceted approach in studying parasites with complex life cycles. Such an approach can provide better insights into the ecology and epidemiology of D. latus, given also that this tapeworm exists in a human-mediated environment. Moreover, we believe that D. latus can serve as a model system of parasites with complex life cycles, as well as a model of the effects of polyploidy on host-parasite interactions. Suggesting D. latus as a model system goes well in line with the recent calls for large-scale programs of parasite monitoring due to the evident scarcity of information from this ecological group.