Foodborne trematodes (FBTs) have a worldwide distribution (with particular prevalence in south-east Asia) and are believed to infect almost 75 million people, with millions more living at risk of infection. Although mortality due to trematodiasis is low, these infections cause considerable morbidity and some species are associated with the development of cancer in hyperendemic regions. Despite this, FBTs are often side-lined in terms of research funding and have been dubbed neglected tropical diseases by the World Health Organisation. Thus, the aim of this special issue was to provide an update of our understanding of FBT infections, to shine a light on current work in the field and to highlight some research priorities for the future. With contributions from leading researchers, many from endemic regions, we review the major FBT species. In doing so we revisit some old foes, uncover emerging infections and discover how outbreaks are being dealt with as a result of new approaches to parasite control. We also report advances in our understanding of the interactions of FBTs with their mammalian hosts and uncover new interplay between trematodes and host microbiome components. We hope that this article collection will stimulate discussion and further research on the FBTs and help raise them from their neglected status.

The title of this article refers to Table 1 in Zhou (2022, Infectious diseases of poverty: progress achieved during the decade gone and perspectives for the future. Infectious Diseases of Poverty 11, 1), in which it is indicated that Paragonimus species, like many other foodborne trematodes, are ancient pathogens that are also re-emerging to cause disease in modern times. This article provides a general overview of Paragonimus species and the disease they cause. This is followed by comments on several specific topics of current interest: taxonomy and distribution of members of the genus; details of the life cycle; global and regional prevalence of paragonimiasis; genomics of lung flukes and possible effects of global environmental change. Unresolved questions relating to these topics are discussed and gaps in knowledge identified.

Snails and digeneans have been associated for at least 200 million years. Their inter-relationships over such a time-span must have been complex and varied. Few studies have attempted to explore these relationships in the light of knowledge of the phylogeny of both host and parasite groups. Here we focus on two important families of digeneans, the Schistosomatidae and the Paragonimidae, for which molecular phylogenies are available. We investigate the types of evolutionary relationships between host and parasite, operating at different phylogenetic depths, that might explain current host specificity and distributions of both associates. Both families of parasites utilise a number of highly diverged gastropod families, indicating that host extensions have featured in their histories. However, schistosomatids and paragonimids show different patterns of association with their snail hosts. As befits the apparently more ancient group, schistosomatids utilise snails from across a wide phylogenetic range within the Gastropoda. The genus Schistosoma itself has experienced one long-range host switch between pulmonates and caenogastropods. By contrast, paragonimids are restricted to two superfamilies of caenogastropods. Despite these differences, modern schistosomatid species appear to be more host specific than are paragonimids and host additions, at the level of host family, are far less common among species of schistosomatids than among paragonimids. Some species of Paragonimus exhibit remarkably low levels of host specificity, with different populations utilising snails of different families. Existing knowledge relating to the phenomenon will be presented in the context of phylogenies of schistosomatids, paragonimids, and their snail hosts. Discussion focuses on the usefulness of current theories of snail–digenean coevolution for interpreting these findings. In the past, much emphasis has been placed on the idea that digeneans engage in a one-to-one arms race with their snail host. We consider that phylogenetic tracking rather than an arms-race relationship might be a common alternative. Not being bound by the restrictions imposed by an arms race, some digeneans might be able to extend to new host species more easily than the literature suggests. Switches into related host taxa are most likely. However, ecologically equivalent but unrelated gastropod hosts may also be exploited. Given the right ecological setting, digeneans are able to switch across considerable phylogenetic distances. Examples from the Paragonimidae and Schistosomatidae are given.