It's a worm-eat-worm world: Consumption of parasite free-living stages protects hosts and benefits predators

Outbreak of parasite-induced limb malformations in a declining amphibian species in Colorado

The detection of severe limb malformations in metamorphosing northern leopard frogs (Rana pipiens) from a Colorado pond in August 2022 prompted questions about the cause(s) and concern over the implications. Northern leopard frogs, which are a Tier 1 Species of Greatest Conservation Need in Colorado, have declined over much of their range in the state, particularly along the Front Range. Although malformations in amphibians have been reported in other parts of the USA, they are rare in Colorado, and the current case represents the most severe hotspot reported in the state for over 70 years. Across three survey events in late summer and early fall of 2022, approximately 68% of captured leopard frogs (late-stage larvae and metamorphic frogs) exhibited one or more malformations. Malformations exclusively affected the hind limbs and were dominated by skin webbings (51.7% of the total), bony triangles (32.2%), and extra limbs or digits (11%). Many animals had multiple malformations that limited the movement of one or both limbs (average of 2.3 malformations per malformed frog). Dissection of a subset of animals coupled with 28S rDNA genetic sequencing revealed the occurrence of the trematode Ribeiroia ondatrae at an average of 75.2 trematode cysts (metacercariae) per frog. The parasite was also detected in 2.6% of dissected snails (Helisoma trivolvis), which function as the trematode's first intermediate host. The relatively high loads of infection detected here - coupled with the similarity of observed malformations to those previously linked to R. ondatrae in experimental studies and from other malformation hotspots in the USA - offer compelling evidence that the current case is the result of parasite infection. Unresolved questions include why malformation prevalence was so high in 2022 and the degree to which such abnormalities will affect population persistence for local leopard frogs, particularly if malformations continue.

Cryptic carnivores: Intercontinental sampling reveals extensive novel diversity in a genus of freshwater annelids

Freshwater annelids are globally widespread in aquatic ecosystems, but their diversity is severely underestimated. Obvious morphological features to define taxa are sparse, and molecular phylogenetic analyses regularly discover cryptic diversity within taxa. Despite considerable phylogenetic work on certain clades, many groups of freshwater annelids remain poorly understood. Included among these are water nymph worms of the genus Chaetogaster (Clitellata: Tubificida: Naididae: Naidinae). These worms have diverged from the detritivorous diet of most oligochaetes to become more predatory and exist as omnivores, generalist predators, parasites, or symbionts on other invertebrates. Despite their unusual trophic ecology, the true diversity of Chaetogaster and the phylogenetic relationships within the genus are uncertain. Only three species are commonly referenced in the literature (Chaetogaster diaphanus, Chaetogaster limnaei, and Chaetogaster diastrophus), but additional species have been described and prior molecular data suggests that there is cryptic diversity within named species. To clarify the phylogenetic diversity of Chaetogaster, we generated the first molecular phylogeny of the genus using mitochondrial and nuclear sequence data from 128 worms collected primarily across North America and Europe. Our phylogenetic analyses suggest that the three commonly referenced species are a complex of 24 mostly cryptic species. In our dataset, Chaetogaster "diaphanus" is represented by two species, C. "limnaei" is represented by three species, and C. "diastrophus" is represented by 19 species. North American and European sequences are largely interspersed across the phylogeny, with four pairs of clades involving distinct North American and European sister groupings. Overall, our study demonstrates that the species diversity of Chaetogaster has been underestimated and that carnivory has evolved at least twice in the genus. Chaetogaster is being used as a model for symbiotic evolution and the loss of regenerative ability, and our study indicates that researchers must be careful to identify which species of Chaetogaster they are working with in future studies.

Consumption of trematode parasite infectious stages: from conceptual synthesis to future research agenda

Given their sheer cumulative biomass and ubiquitous presence, parasites are increasingly recognized as essential components of most food webs. Beyond their influence as consumers of host tissue, many parasites also have free-living infectious stages that may be ingested by non-host organisms, with implications for energy and nutrient transfer, as well as for pathogen transmission and infectious disease dynamics. This has been particularly well-documented for the cercaria free-living stage of digenean trematode parasites within the Phylum Platyhelminthes. Here, we aim to synthesize the current state of knowledge regarding cercariae consumption by examining: (a) approaches for studying cercariae consumption; (b) the range of consumers and trematode prey documented thus far; (c) factors influencing the likelihood of cercariae consumption; (d) consequences of cercariae consumption for individual predators (e.g. their viability as a food source); and (e) implications of cercariae consumption for entire communities and ecosystems (e.g. transmission, nutrient cycling and influences on other prey). We detected 121 unique consumer-by-cercaria combinations that spanned 60 species of consumer and 35 trematode species. Meaningful reductions in transmission were seen for 31 of 36 combinations that considered this; however, separate studies with the same cercaria and consumer sometimes showed different results. Along with addressing knowledge gaps and suggesting future research directions, we highlight how the conceptual and empirical approaches discussed here for consumption of cercariae are relevant for the infectious stages of other parasites and pathogens, illustrating the use of cercariae as a model system to help advance our knowledge regarding the general importance of parasite consumption.

Bulinus snails in the Lake Victoria Basin in Kenya: Systematics and their role as hosts for schistosomes

The planorbid gastropod genus Bulinus consists of 38 species that vary in their ability to vector Schistosoma haematobium (the causative agent of human urogenital schistosomiasis), other Schistosoma species, and non-schistosome trematodes. Relying on sequence-based identifications of bulinids (partial cox1 and 16S) and Schistosoma (cox1 and ITS), we examined Bulinus species in the Lake Victoria Basin in Kenya for naturally acquired infections with Schistosoma species. We collected 6,133 bulinids from 11 sites between 2014-2021, 226 (3.7%) of which harbored Schistosoma infections. We found 4 Bulinus taxa from Lake Victoria (B. truncatus, B. tropicus, B. ugandae, and B. cf. transversalis), and an additional 4 from other habitats (B. globosus, B. productus, B. forskalii, and B. scalaris). S. haematobium infections were found in B. globosus and B. productus (with infections in the former predominating) whereas S. bovis infections were identified in B. globosus, B. productus, B. forskalii, and B. ugandae. No nuclear/mitochondrial discordance potentially indicative of S. haematobium/S. bovis hybridization was detected. We highlight the presence of Bulinus ugandae as a distinct lake-dwelling taxon closely related to B. globosus yet, unlike all other members of the B. africanus species group, is likely not a vector for S. haematobium, though it does exhibit susceptibility to S. bovis. Other lake-dwelling bulinids also lacked S. haematobium infections, supporting the possibility that they all lack compatibility with local S. haematobium, thereby preventing widespread transmission of urogenital schistosomiasis in the lake's waters. We support B. productus as a distinct species from B. nasutus, B. scalaris as distinct from B. forskalii, and add further evidence for a B. globosus species complex with three lineages represented in Kenya alone. This study serves as an essential prelude for investigating why these patterns in compatibility exist and whether the underlying biological mechanisms may be exploited for the purpose of limiting schistosome transmission.

Critical role of parasite-mediated energy pathway on community response to nutrient enrichment

Parasites form an integral part of food webs, however, they are often ignored in classic food web theory or limited to the investigation of trophic transmission pathways. Specifically, direct consumption of parasites by nonhost predators is rarely considered, while it can contribute substantially to energy flow in food webs. In aquatic systems, chytrids constitute a major group of fungal parasites whose free-living infective stages (zoospores) form a highly nutritional food source to zooplankton. Thereby, the consumption of zoospores can create an energy pathway from otherwise inedible phytoplankton to zooplankton ("mycoloop"). This parasite-mediated energy pathway might be of special importance during phytoplankton blooms dominated by inedible or toxic primary producers like cyanobacteria, which are on the rise with eutrophication and global warming. We theoretically investigated community dynamics and energy transfer in a food web consisting of an edible nonhost and an inedible host phytoplankton species, a parasitic fungus, and a zooplankton species grazing on edible phytoplankton and fungi. Food web dynamics were investigated along a nutrient gradient contrasting nonadaptive zooplankton species representative for filter feeders like cladocerans and zooplankton with the ability to actively adapt their feeding preferences like many copepod species. Overall, the importance of the mycoloop for zooplankton increases with nutrient availability. This increase is smooth for nonadaptive consumers. For adaptive consumers, we observe an abrupt shift from an almost exclusive preference for edible phytoplankton at low nutrient levels to a strong preference for parasitic fungi at high nutrient levels. The model predicts that parasitic fungi could contribute up to 50% of the zooplankton diet in nutrient-rich environments, which agrees with empirical observations on zooplankton gut content from eutrophic systems during blooms of inedible diatoms or cyanobacteria. Our findings highlight the role of parasite-mediated energy pathways for predictions of energy flow and community composition under current and future environmental change.

Thinking "outside the box": The effect of nontarget snails in the aquatic community on mollusc-borne diseases

There is a great need to understand the impact of complex communities on the free-living parasite stages that are part of them. This task becomes more complex as nonnative species emerge, changing existing relationships and shaping new interactions in the community. A relevant question would be: Can the coexistence of nontarget snails with the target hosts contribute to trematodasis control? We used field and experimental approaches to investigate nonnative competitor-induced parasite dilution. During a three-year field study, we investigated digenean infection in Lymnaea stagnalis from eight Polish lakes inhabited or uninhabited by Potamopyrgus antipodarum. Additionally, we verified the presence of digenean infections in the populations of P. antipodarum. Moreover, we conducted an experimental infection of L. stagnalis with miracidia of Trichobilharzia szidati under increasing densities of P. antipodarum and aimed to infect P. antipodarum with them separately. The prevalence of avian schistosomes in lymnaeid snails was significantly higher in uninhabited lakes than in lakes inhabited by P. antipodarum. Our study indicates that waters with a higher density of invaders have a lower prevalence of avian schistosomes in lymnaeid hosts. The results of experimental studies confirmed that the presence of high densities of P. antipodarum reduces the probability of target host infection. Both field and experimental studies rule out the role of P. antipodarum as a source of avian schistosome cercariae. Here, a nonnative species was tested as a diluter, which in practice may be harmful to the local environment. This work is not a call for the introduction of nonnative species; it is intended to be a stimulus for researchers to continue searching for natural enemies of parasites because, as our results show, they exist. Finding natural enemies to the most dangerous species of human and animal parasites that will pose no threat to the local environment could be groundbreaking.

Scratching the Itch: Updated Perspectives on the Schistosomes Responsible for Swimmer's Itch around the World

Although most studies of digenetic trematodes of the family Schistosomatidae dwell on representatives causing human schistosomiasis, the majority of the 130 identified species of schistosomes infect birds or non-human mammals. The cercariae of many of these species can cause swimmer's itch when they penetrate human skin. Recent years have witnessed a dramatic increase in our understanding of schistosome diversity, now encompassing 17 genera with eight more lineages awaiting description. Collectively, schistosomes exploit 16 families of caenogastropod or heterobranch gastropod intermediate hosts. Basal lineages today are found in marine gastropods and birds, but subsequent diversification has largely taken place in freshwater, with some reversions to marine habitats. It seems increasingly likely that schistosomes have on two separate occasions colonized mammals. Swimmer's itch is a complex zoonotic disease manifested through several different routes of transmission involving a diversity of different host species. Swimmer's itch also exemplifies the value of adopting the One Health perspective in understanding disease transmission and abundance because the schistosomes involved have complex life cycles that interface with numerous species and abiotic components of their aquatic environments. Given the progress made in revealing their diversity and biology, and the wealth of questions posed by itch-causing schistosomes, they provide excellent models for implementation of long-term interdisciplinary studies focused on issues pertinent to disease ecology, the One Health paradigm, and the impacts of climate change, biological invasions and other environmental perturbations.