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Debates over the relationship between biodiversity and disease dynamics underscore the need for a more mechanistic understanding of how changes in host community composition influence parasite transmission. Focusing on interactions between larval amphibians and trematode parasites, we experimentally contrasted the effects of host richness and species composition to identify the individual and joint contributions of both parameters on the infection levels of three trematode species. By combining experimental approaches with field surveys from 147 ponds, we further evaluated how richness effects differed between randomized and realistic patterns of species loss (i.e. community disassembly). Our results indicated that community-level changes in infection levels were owing to host species composition, rather than richness. However, when composition patterns mirrored empirical observations along a natural assembly gradient, each added host species reduced infection success by 12–55%. No such effects occurred when assemblages were randomized. Mechanistically, these patterns were due to non-random host species assembly/disassembly: while highly competent species predominated in low diversity systems, less susceptible hosts became progressively more common as richness increased. These findings highlight the potential for combining information on host traits and assembly patterns to forecast diversity-mediated changes in multi-host disease systems.

To predict global warming impacts on parasitism, we should describe the thermal tolerance of all players in host–parasite systems. Complex life-cycle parasites such as trematodes are of particular interest since they can drive complex ecological changes. This study evaluates the net response to temperature of the infective larval stage of Himasthla elongata , a parasite inhabiting the southwestern Baltic Sea. The thermal sensitivity of (i) the infected and uninfected first intermediate host ( Littorina littorea ) and (ii) the cercarial emergence, survival, self-propelling, encystment, and infection capacity to the second intermediate host ( Mytilus edulis sensu lato) were examined. We found that infection by the trematode rendered the gastropod more susceptible to elevated temperatures representing warm summer events in the region. At 22 °C, cercarial emergence and infectivity were at their optimum while cercarial survival was shortened, narrowing the time window for successful mussel infection. Faster out-of-host encystment occurred at increasing temperatures. After correcting the cercarial emergence and infectivity for the temperature-specific gastropod survival, we found that warming induces net adverse effects on the trematode transmission to the bivalve host. The findings suggest that gastropod and cercariae mortality, as a tradeoff for the emergence and infectivity, will hamper the possibility for trematodes to flourish in a warming ocean.

A combination of extensive field surveys and realistic experiments involving an amphibian disease system reveals that biodiversity reduces pathogen transmission due to a predictable link between species richness and the ability of communities to support infection. How biodiversity combats disease Several lines of evidence have suggested that biodiversity loss in an ecosystem can affect pathogen transmission and host disease, with the pathogens gaining the upper hand. Pathogens tend to infect multiple host species, which can vary in their ability to maintain and transmit infections, and for biodiversity to protect against disease risk, resistant hosts would need to be the last to be added to an ecosystem and the first to be lost. Species-poor communities would then dominated by highly susceptible hosts. This study combines wetland field surveys with experimental mesocosms to show that, in an amphibian community experiencing infection with the parasitic flatworm Ribeiroia ondatrae , the negative correlation between disease and diversity is the result of this 'dilution effect'. These findings highlight the value of biodiversity as a cost-effective approach to minimizing the spread of infectious disease. Accelerating rates of species extinctions and disease emergence underscore the importance of understanding how changes in biodiversity affect disease outcomes 1 , 2 , 3 . Over the past decade, a growing number of studies have reported negative correlations between host biodiversity and disease risk 4 , 5 , 6 , 7 , 8 , prompting suggestions that biodiversity conservation could promote human and wildlife health 9 , 10 . Yet the generality of the diversity–disease linkage remains conjectural 11 , 12 , 13 , in part because empirical evidence of a relationship between host competence (the ability to maintain and transmit infections) and the order in which communities assemble has proven elusive. Here we integrate high-resolution field data with multi-scale experiments to show that host diversity inhibits transmission of the virulent pathogen Ribeiroia ondatrae and reduces amphibian disease as a result of consistent linkages among species richness, host composition and community competence. Surveys of 345 wetlands indicated that community composition changed nonrandomly with species richness, such that highly competent hosts dominated in species-poor assemblages whereas more resistant species became progressively more common in diverse assemblages. As a result, amphibian species richness strongly moderated pathogen transmission and disease pathology among 24,215 examined hosts, with a 78.4% decline in realized transmission in richer assemblages. Laboratory and mesocosm manipulations revealed an approximately 50% decrease in pathogen transmission and host pathology across a realistic diversity gradient while controlling for host density, helping to establish mechanisms underlying the diversity–disease relationship and their consequences for host fitness. By revealing a consistent link between species richness and community competence, these findings highlight the influence of biodiversity on infection risk and emphasize the benefit of a community-based approach to understanding infectious diseases.

Infectious diseases are intimately associated with the dynamics of biodiversity. However, the role that infectious disease plays within ecological communities is complex. The complex effects of infectious disease at the scale of communities and ecosystems are driven by the interaction between host and pathogen. Whether or not a given host–pathogen interaction results in progression from infection to disease is largely dependent on the physiological characteristics of the host within the context of the external environment. Here, we highlight the importance of understanding the outcome of infection and disease in the context of host ecophysiology using amphibians as a model system. Amphibians are ideal for such a discussion because many of their populations are experiencing declines and extinctions, with disease as an important factor implicated in many declines and extinctions. Exposure to pathogens and the host's responses to infection can be influenced by many factors related to physiology such as host life history, immunology, endocrinology, resource acquisition, behaviour and changing climates. In our review, we discuss the relationship between disease and biodiversity. We highlight the dynamics of three amphibian host–pathogen systems that induce different effects on hosts and life stages and illustrate the complexity of amphibian–host–parasite systems. We then review links between environmental stress, endocrine–immune interactions, disease and climate change.

Trematode metacercariae are the most abundant and frequently encountered helminths in freshwater fish. Yet, accurate species identification remains challenging, potentially leading to an underestimation of trematode diversity. Using data from parasitological examinations of 1,030 fish (47 species) collected from diverse freshwater habitats in Lithuania (2022–2024), we assessed the contemporary diversity of trematode metacercariae, host associations, microhabitat preferences, and changes in diversity patterns and transmission dynamics. Through integrated morphological and molecular techniques, we identified metacercariae belonging to 51 species from eight families, more than doubling previously reported diversity (25 species). While trematode family composition remained largely unchanged – the Diplostomidae and Strigeidae remained the most diverse families – notable differences were observed at the species level. Metacercariae of the Echinochasmidae and Echinostomatidae were detected for the first time, while previously reported Clinostomidae were absent. Fish of the Leuciscidae hosted the highest trematode diversity. Host specificity of metacercariae was generally low, with most species being euryxenous. At the microhabitat level, eyes harboured the highest number of species, while muscles showed the highest metacercarial density. Notably, we detected species first genetically characterised in North America (Echinoparyphium sp. 2 and Ichthyocotylurus sp. 2) and species potentially belonging to the genus Neogogatea, previously known only from Asia and North America, highlighting potential invasion risks and suggesting that European trematode diversity remains substantially underestimated. Future efforts should obtain molecular data from correctly identified adult specimens to resolve the identity of species currently identified only to the genus or family level, thereby enabling assessment of their geographical distributions and ecological roles.

While the importance of changes in host biodiversity for disease risk continues to gain empirical support, the influence of natural variation in parasite diversity on epidemiological outcomes remains largely overlooked. Here, we combined field infection data from 2,191 amphibian hosts representing 158 parasite assemblages with mechanistic experiments to evaluate the influence of parasite richness on both parasite transmission and host fitness. Using a guild of larval trematode parasites (six species) and an amphibian host, our experiments contrasted the effects of parasite richness vs. composition, observed vs. randomized assemblages, and additive vs. replacement designs. Consistent with the dilution effect hypothesis extended to intrahost diversity, increases in parasite richness reduced overall infection success, including infections by the most virulent parasite. However, the effects of parasite richness on host growth and survival were context dependent; pathology increased when parasites were administered additively, even when the presence of the most pathogenic species was held constant, but decreased when added species replaced or reduced virulent species, emphasizing the importance of community composition and assembly. These results were similar or stronger when community structures were weighted by their observed frequencies in nature. The field data also revealed the highly nested structure of parasite assemblages, with virulent species generally occupying basal positions, suggesting that increases in parasite richness and antagonism in nature will decrease virulent infections. Our findings emphasize the importance of parasite biodiversity and coinfection in affecting epidemiological responses and highlight the value of integrating research on biodiversity and community ecology for understanding infectious diseases.

Background Amphibians are a prime example of the global biodiversity crisis, as they represent the most threatened group of vertebrates. Yet, amphibian macroparasites remain one of the most poorly described groups of parasites, with the majority of species lacking comprehensive morphological, molecular, or ecological data. Among these, digenean trematodes constitute a dominant group and feature multi-host life cycles. This study examines the first intermediate hosts of trematodes, aquatic gastropods, to assess the occurrence, prevalence, and seasonality of amphibian trematodes. Methods A total of 5362 snails from five families (Bithyniidae, Hydrobiidae, Lymnaeidae, Physidae, and Planorbidae) were collected in three stream systems in North Rhine-Westphalia, Germany and investigated for trematode infections. Detailed information on amphibian-infecting trematode cercarial morphology and measurements were provided via light microscopy and scanning electron microscopy (SEM). Comprehensive molecular analyses were conducted and novel sequences generated for multiple genetic markers (28S rDNA, ITS1-5.8S-ITS2, ITS2, cox 1, and nad 1). Results Three trematode taxa infecting amphibians as second intermediate and/or definitive hosts ( Lecithopyge , Cephalogonimus , and Opisthioglyphe ) were identified exclusively from lymnaeid snail hosts. A total of 79 novel sequences were generated for 21 trematode isolates. Phylogenetic analyses based on 28S and ITS1-5.8S-ITS2 sequences resulted in concordant taxonomies. Distinct seasonal infection patterns allowed for insights into the species’ life cycles. Conclusions Our findings highlight significant gaps in the knowledge of amphibian macroparasites and underline the value of studying cercariae occurrence in snail intermediate hosts as a method for monitoring amphibian trematode biodiversity without affecting amphibian populations. Graphical Abstract

Infectious disease dynamics depend on the speed, number and fitness of parasites transmitting from infected hosts (‘donors’) to parasite-naive ‘recipients’. Donor heterogeneity likely affects these three parameters, and may arise from variation between donors in traits including: (i) infection load, (ii) resistance, (iii) stage of infection, and (iv) previous experience of transmission. We used the Trinidadian guppy, Poecilia reticulata, and a directly transmitted monogenean ectoparasite, Gyrodactylus turnbulli, to experimentally explore how these sources of donor heterogeneity affect the three transmission parameters. We exposed parasite-naive recipients to donors (infected with a single parasite strain) differing in their infection traits, and found that donor infection traits had diverse and sometimes interactive effects on transmission. First, although transmission speed increased with donor infection load, the relationship was nonlinear. Second, while the number of parasites transmitted generally increased with donor infection load, more resistant donors transmitted more parasites, as did those with previous transmission experience. Finally, parasites transmitting from experienced donors exhibited lower population growth rates on recipients than those from inexperienced donors. Stage of infection had little effect on transmission parameters. These results suggest that a more holistic consideration of within-host processes will improve our understanding of between-host transmission and hence disease dynamics. This article is part of the themed issue ‘Opening the black box: re-examining the ecology and evolution of parasite transmission’.

Parasitism is one of the most common consumer strategies and contributes a large portion to biological diversity. Trematodes in the family Diplostomidae are common in freshwater ecosystems worldwide, often residing in the eyes or brain of fish and then infecting fish-eating birds as adults. As a result, some species have broad geographic distributions due to the bird host's motility. In contrast to the cosmopolitan nature of diplostomids, only a single species, Tylodelphys darbyi, has been identified in New Zealand to date, and only from the South Island. Tylodelphys darbyi has a 3-host life cycle consisting of an unidentified snail, a freshwater fish (Gobiomorphus cotidianus), and the Australasian crested grebe (Podiceps cristatus australis). To date, T. darbyi has been found in 2 locations, Lake Hayes, in the eyes of G. cotidianus, and Lake Wanaka, adults recovered from grebes. Considering the near ubiquity of the fish host in New Zealand, it is likely the bird, listed as nationally vulnerable, is the limiting factor in the range of T. darbyi. Up to 10 G. cotidianus were sampled from 10 mountain lakes known to have populations of grebe in the Otago and Canterbury regions of New Zealand's South Island. The eyes of all fish were examined and any metacercariae present were set aside for genetic analysis. In addition to expanding the known range of T. darbyi to at least 4 water bodies across the South Island, 2 new taxa of diplostomid were identified. A lens-infecting metacercariae clustered with Diplostomum spathaceum, while the metacercariae from the humor clustered with Diplostomum baeri.

The red-eared slider, Trachemys scripta elegans (Wied, 1938), has been introduced worldwide, partly because of the exotic pet trade in the 1980s and 1990s. When T. s. elegans is released or escapes into natural environments, it often establishes new feral populations due to its tolerance for a variety of aquatic ecosystems. Therefore, it is now considered one of the most invasive species in the world because it can compete with native turtle species. In the present study, our objectives were to identify the potential for polystome spillover and spillback resulting from the introduction of the red-eared slider into new environments in North America. Fieldwork investigations were thus conducted mainly in aquatic habitats in Florida and North Carolina, United States, but also in Connecticut, Indiana, Kansas, Maine, Nebraska and New York. Using DNA barcoding based on cytochrome c oxidase I (COI) sequences, we surveyed the species diversity of polystome within American freshwater turtles. These included T. s. elegans but also Apalone ferox , Apalone spinifera , Chelydra serpentina , Chrysemys picta , Kinosternon baurii , Pseudemys spp., Sternotherus minor and Sternotherus odoratus . Genetic evidence confirmed that invasive populations of T. s. elegans in southern Europe have transmitted their own polystomes to native host species following spillover effects, and revealed here that T. s. elegans in non-indigenous habitats in the United States acts as a new reservoir of infection for native polystomes following spillback effects, thus increasing indigenous parasite transmission in the wild. Together, these findings raise further concern about the spread of non-native turtles and their impact on parasite transmission. La tortue à tempes rouges, Trachemys scripta elegans (Wied, 1938), a été introduite dans le monde entier, en partie grâce au commerce d’animaux de compagnie exotiques dans les années 1980 et 1990. Lorsque T. s. elegans est relâchée ou s’échappe dans des milieux naturels, elle établit souvent de nouvelles populations sauvages suite à sa tolérance à une variété d’écosystèmes aquatiques. De ce fait, elle est aujourd’hui considérée comme l’une des espèces les plus invasives au monde, car elle peut concurrencer les espèces de tortues indigènes. Dans cette étude, nos objectifs étaient d’identifier le potentiel de propagation et de transmission des polystomes résultant de l’introduction de la tortue de Floride à tempes rouges dans de nouveaux environnements en Amérique du Nord. Des campagnes de terrain ont donc été menées principalement dans les habitats aquatiques de Floride et de Caroline du Nord aux États-Unis, mais aussi dans le Connecticut, l’Indiana, le Kansas, le Maine, le Nebraska et l’État de New York. En utilisant le code-barre ADN basé sur les séquences de la cytochrome c oxydase I (COI), nous avons étudié la diversité des espèces de polystomes chez les tortues d’eau douce américaines. Celles-ci comprenaient T. s. elegans mais aussi Apalone ferox , Apalone spinifera , Chelydra serpentina , Chrysemys picta , Kinosternon baurii , Pseudemys spp., Sternotherus minor et Sternotherus odoratus . Les preuves génétiques ont confirmé que les populations invasives de T. s. elegans en Europe du Sud ont transmis leurs propres polystomes aux espèces hôtes indigènes suite à des effets de débordement, et ont révélé ici que T. s. elegans dans les habitats non indigènes des États-Unis agit comme un nouveau réservoir d’infection pour les polystomes indigènes suite à des effets boule de neige, augmentant ainsi la transmission des parasites indigènes dans la nature. Ces résultats suscitent de nouvelles inquiétudes quant à la propagation des tortues non indigènes et à leur impact sur la transmission du parasite.