How trematodes cause limb deformities in amphibians

Blood parasites of water frogs (Pelophylax esculentus complex) from the Danube Delta, Romania

Water frogs of the genus Pelophylax host a variety of parasites, from protozoa to helminths. Among the blood parasites, representatives of Apicomplexa, Trypanosoma and Nematoda show the highest prevalence. In this study, we focused on blood parasites of water frogs living in the Danube Delta, Romania. In total, 74 individuals of P. ridibundus and eight individuals of P. esculentus from six localities were examined. Blood parasites were detected microscopically and using a molecular marker (18S rDNA). 89.77% of frogs from all investigated localities were found to be infected with at least one parasitic group, specifically with haemogregarines (84.09%), nematodes (1.14%), and trypanosomes (63.64%). The parasitemia of haemogregarines and trypanosomes differed significantly among the studied locations. There was no statistically significant difference in parasitemia between male and female hosts. However, adults were found to have a significantly higher parasitemia in comparison with subadults infected with haemogregarines. Correlation between parasitemia and the body length of frogs infected with haemogregarines was also significant (r = 0.226). By comparing the 18S rDNA sequences with the corresponding GenBank sequences, Hepatozoon species identified in water frogs showed a close similarity (98.1-99.8%) to Hepatozoon magna. Trypanosomes showed the highest sequence similarity to Trypanosoma sp. isolate R10 clone L2-3, Trypanosoma ranarum, and Trypanosoma cobitis.

Case report: Disseminated larval trematodiasis caused by Clinostomum marginatum in a green tree frog (Hyla cinerea)

Clinostomum spp. are common parasites of piscivorous birds. Metacercaria are typically observed in the muscles or just under the skin of fish and rarely amphibians. We describe an unusually severe case of Clinostomum marginatum infection in an adult female green tree frog (Hyla cinerea) from Georgia (USA). The frog was found in November 2015 with a high number (>250) of widely disseminated, raised, subcutaneous nodules. The frog died in December. At necropsy, it was emaciated, and the skin was covered in raised uniform, tan-green, subcutaneous, ∼2-3 mm diameter nodules. Each nodule contained 1-3 C. marginatum metacercariae. Microscopically, high numbers of trematodes were within subcutaneous tissues and in coelomic and oral cavities, lung, liver, kidney, ovary, orbit and calvarium. Small to large numbers of lymphocytes and melanomacrophages were in connective tissues and epidermis. A 732 bp region of COI was 98.8-99.8% similar to numerous sequences of C. marginatum and, phylogenetically it grouped with these C. marginatum sequences. The ITS-1 region was 100% similar to a C. marginatum sample from a great egret (Ardea alba) from Mississippi. This report represents a novel finding of severe trematodiasis in a free-ranging amphibian with C. marginatum infection.

Acute mortality in California tiger salamander (Ambystoma californiense) and Santa Cruz long-toed salamander (Ambystoma macrodactylum croceum) caused by Ribeiroia ondatrae (Class: Trematoda)

In early September 2019, a morbidity and mortality event affecting California tiger salamanders (Ambystoma californiense) and Santa Cruz long-toed salamanders (Ambystoma macrodactylum croceum) in late stages of metamorphosis was reported at a National Wildlife Refuge in Santa Cruz County, California, U.S.A. During the postmortem disease investigation, severe integumentary metacercarial (Class: Trematoda) infection, associated with widespread skin lesions, was observed. Planorbid snails collected from the ponds of the refuge within seven days of the mortality event were infected with Ribeiroia ondatrae, a digenetic trematode that can cause malformation and death in some amphibians. We suggest sustained seasonal high-water levels due to active habitat management along with several years of increased rainfall led to increased bird visitation, increased over-wintering of snails, and prolonged salamander metamorphosis, resulting in a confluence of conditions and cascading of host-parasite dynamics to create a hyper-parasitized state.

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Mortality event in wild endangered salamanders in California.

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Infection with Ribeiroia ondatrae caused severe fatal skin lesions.

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Sustaining water levels may increase parasite transmission.

Infection sequence alters disease severity-Effects of the sequential exposure of two larval trematodes to Polypedates cruciger tadpoles

Multiple pathogens coexist in nature, and hence, host species often encounter several pathogens simultaneously. The sequence in which the host encounters the parasites influences interactions between parasites and host pathology. Here, the effects of infection by two cercaria (larvae of trematodes) types, pleurolophocercous cercaria of Acanthostomum burminis and a furcocercous cercaria, on the tadpoles of common hourglass tree frog (Polypedates cruciger) were examined. Ten days posthatch, tadpoles (Gosner stage 27/28) were used for infection exposures. First, in a single infection each cercaria type was introduced to the tadpoles separately. Second, coinfection of the two cercaria was carried out by alternating the sequences of exposure. For all the experiments, appropriate controls were instituted. Tadpoles of all groups exposed to parasites had lower survival levels compared to controls. Among the four groups exposed, the highest survival was observed in the coinfection when furcocercous was introduced first (82.5%). The lowest survival was observed in the coinfection when the A. burminis cercaria was introduced first (65.0%). In the coinfections, when A. burminis was introduced prior to furcocercous, survival of the tadpoles was reduced by 17.0% compared to the exposures of furcocercous prior to A. burminis. Prior infection with A. burminis induced negative effect on the host with an increased infection severity, while prior infection with furcocercous had reduced infection severity than lone exposures. These results suggest that furcocercous infections can be beneficial for hosts challenged with A. burminis provided that A. burminis exposure occurs second. None of the treatments had an effect on the growth of the tadpoles, but lengthening of developmental period was observed in some exposures. All exposed tadpoles developed malformations which were exclusively axial-kyphosis and scoliosis. However, there was no difference in the number of malformed individuals in the single infection (19.0%-25.0%) compared to coinfection (20.0%-22.5%) or between coinfections. The results suggest that the sequence of parasite exposure affects host-parasite interactions and hence the disease outcomes. Understanding the effects of coinfection on disease outcomes for hosts provides insight into disease dynamics.

Orthopedics in Reptiles and Amphibians

Musculoskeletal disorders are a common cause for presentation of reptiles and amphibians to the veterinarian. A clinical approach to orthopedic cases starts with a thorough history and review of husbandry, and identification of any underlying or concomitant disease. Medical management is indicated for pathologic fractures. Traumatic fractures may require surgical intervention. Stabilization options include external coaptation and/or external and internal fixation. Special considerations must be given to shell fractures in chelonians. Many techniques used in mammalian practice can be applied to reptiles and amphibians, although some species may require prolonged healing times by comparison.

How Temperature, Pond-Drying, and Nutrients Influence Parasite Infection and Pathology

The rapid pace of environmental change is driving multi-faceted shifts in abiotic factors that influence parasite transmission. However, cumulative effects of these factors on wildlife diseases remain poorly understood. Here we used an information-theoretic approach to compare the relative influence of abiotic factors (temperature, diurnal temperature range, nutrients and pond-drying), on infection of snail and amphibian hosts by two trematode parasites (Ribeiroia ondatrae and Echinostoma spp.). A temperature shift from 20 to 25 °C was associated with an increase in infected snail prevalence of 10-20%, while overall snail densities declined by a factor of 6. Trematode infection abundance in frogs was best predicted by infected snail density, while Ribeiroia infection specifically also declined by half for each 10% reduction in pond perimeter, despite no effect of perimeter on the per snail release rate of cercariae. Both nutrient concentrations and Ribeiroia infection positively predicted amphibian deformities, potentially owing to reduced host tolerance or increased parasite virulence in more productive environments. For both parasites, temperature, pond-drying, and nutrients were influential at different points in the transmission cycle, highlighting the importance of detailed seasonal field studies that capture the importance of multiple drivers of infection dynamics and the mechanisms through which they operate.

Continental-extent patterns in amphibian malformations linked to parasites, chemical contaminants, and their interactions

Widespread observations of malformed amphibians across North America have generated both concern and controversy. Debates over the causes of such malformations-which can affect >50% of animals in a population-have continued, likely due to involvement of multiple causal factors. Here, we used a 13-year dataset encompassing 53,880 frogs and toads from 422 wetlands and 42 states in the conterminous USA to test hypotheses relating abnormalities and four categories of potential drivers: (i) chemical contaminants, (ii) land use practices, (iii) parasite infection, and (iv) targeted interactions between parasites and pesticides. Using a hierarchically nested, competing-model approach, we further examined how these associations varied spatially among geographic regions. Although malformations were rare overall (average = 1.6%), we identified 96 hotspot sites with 5%-25% abnormal individuals. Using the full dataset of 934 collections (without data on parasite infection), malformation frequency was best predicted by the presence of oil and gas wells within the watershed. Among collections also examined for parasite infection (n = 154), average parasite load and its interaction with pesticide application positively predicted malformations: wetlands with a greater abundance of the trematode Ribeiroia ondatrae were more likely to have malformed amphibians, but these effects were strongest when pesticide application was also high, consistent with prior experimental research. Importantly, however, the influence of these factors also varied regionally, helping explain divergent results from previous studies at local scales; parasite infection was more influential in the West and Northeast, whereas pesticide application and oil/gas wells correlated with abnormalities in the Northeast, Southeast, and western regions of the USA. These results, based on the largest systematic sampling of amphibian malformations, suggest that increased observations of abnormal amphibians are associated with both parasite infection and chemical contaminants, but that their relative importance and interaction strength varied with the spatial extent of the analysis.

Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence

Novel fossil discoveries have contributed to our understanding of the evolutionary appearance of parasitism in flatworms. Furthermore, genetic analyses with greater coverage have shifted our views on the coevolution of parasitic flatworms and their hosts. The putative record of parasitic flatworms is consistent with extant host associations and so can be used to put constraints on the evolutionary origin of the parasites themselves. The future lies in new molecular clock analyses combined with additional discoveries of exceptionally preserved flatworms associated with hosts and coprolites. Besides direct evidence, the host fossil record and biogeography have the potential to constrain their evolutionary history, albeit with caution needed to avoid circularity, and a need for calibrations to be implemented in the most conservative way. This might result in imprecise, but accurate divergence estimates for the evolution of parasitic flatworms.

Malformations and mortality in the Asian Common Toad induced by exposure to pleurolophocercous cercariae (Trematoda: Cryptogonimidae)

Malformations and increased mortality due to infection by the digenetic trematode, Riberioa ondatrae have been reported for many species of amphibians. Severe malformations have also been reported in the Common Hourglass Tree Frog, Polypedates cruciger induced by pleurolophocercous cercariae in Sri Lanka in addition to the changes in the behaviour, development and survival of the host. We exposed pre-limb bud stage tadpoles (Gosner stages 25-26) of the Asian Common Toad, Duttaphrynus melanostictus to the same pleurolophocercous type cercariae under laboratory conditions. Molecular and morphological identification showed that these cercariae belonged Acanthostomum burminis infecting freshwater snakes as definitive hosts. These cercariae induced malformations (27.8%) and reduced survival to metamorphosis (53.8%). The magnitude of the effects increased with the dose of cercariae. Types of malformations were mainly axial, such as scoliosis and kyphosis. Severe limb malformations such as extra or missing limbs as reported for amphibians exposed to R. ondatrae were not observed in the D. melanostictus. Same authors reported a higher percentage of malformations previously when P. cruciger was exposed to the cercariae A. burminis compared to D. melanostictus. However, tadpoles of D. melanostictus, which are smaller compared to those of P. cruciger, experienced higher mortality than P. cruciger tadpoles. Trematode induced malformations and mortality in amphibians are highly variable and depend on multiple factors such as host species differences such as resistance to infection and tolerance, life-history characteristics such as size at metamorphosis and length of the metamorphosis period, and other factors such as size of the amphibian at the time of trematode exposure.

Ribeiroia ondatrae causes limb abnormalities in a Canadian amphibian community

A parasitic flatworm ( Ribeiroia ondatrae Price, 1931) is known to cause severe limb abnormalities and high mortality levels in American amphibian populations. The distributional pattern of this parasite—its main dispersal agent being birds—correlates with the boundaries of migratory flyways in the USA. Yet thus far, R. ondatrae have not been found in Canadian amphibians, which is surprising, considering that said flyways extend well into northern Canada. In this study, we report on a lake in British Columbia that is known to support amphibians with abnormalities similar to those induced by R. ondatrae. To determine if the parasite was present and if it was the cause of the abnormalities, we collected and necropsied metamorphs of the Columbia Spotted Frog ( Rana luteiventris Thompson, 1913) and the Pacific Chorus Frog ( Pseudacris regilla (Baird and Girard, 1852)), and we set up field enclosures to protect larvae from R. ondatrae. Abnormality levels were high in both species (>20%), with the vast majority being found in close proximity to the metacercariae of R. ondatrae. Moreover, the types of abnormalities closely matched those previously recorded in field and laboratory exposures of amphibians to R. ondatrae. Finally, larvae that developed in the same lake, but were protected from R. ondatrae by an enclosure, did not develop abnormalities. Collectively, these results demonstrate that R. ondatrae are both present in an amphibian community in Canada and responsible for causing limb abnormalities.

Presence of Ribeiroia ondatrae in the developing anuran limb disrupts retinoic acid levels

The widespread reports of malformed frogs have sparked interest worldwide to try and determine the causes of such malformations. Ribeiroia ondatrae is a digenetic trematode, which has been implicated as one such cause, as this parasite encysts within the developing tadpole hind limb bud and inguinal region causing dramatic limb malformations. Currently, the mechanisms involved in parasite-induced limb deformities remain unclear. We sought to investigate whether the level of retinoic acid (RA), a morphogenetic factor known to play a critical role in limb bud formation, is altered by the presence of R. ondatrae within the infected tadpole. Alteration of RA levels within the limb bud caused by the presence of the parasite may be achieved in three ways. First, metacercariae are actively secreting RA; second, cercariae, upon entering the limb/inguinal region, may release a large amount of RA; finally, the metacercariae may induce either an increase in the synthesis or a decrease in the degradation of the host's endogenous retinoic acid levels. Here, we show through high performance liquid chromatography and mass spectrometry that limb bud tissue of Lithobates sylvaticus, which has been parasitised, contains 70% more RA compared to the unparasitised control. Furthermore, parasites that have encysted within the limb buds appear to contain substantially less RA (56%) than the free swimming cercariae (defined as the infectious stage of the parasite). Taken together, these data illustrate for the first time that encystment of R. ondatrae leads to an increase in RA levels in the tadpole limb bud and may offer insight into the mechanisms involved in parasite-induced limb deformities.

Disease and the extended phenotype: parasites control host performance and survival through induced changes in body plan

BACKGROUND

By definition, parasites harm their hosts. However, some forms of parasite-induced alterations increase parasite transmission between hosts, such that manipulated hosts can be considered extensions of the parasite's phenotype. While well accepted in principle, surprisingly few studies have quantified how parasite manipulations alter host performance and survival under field and laboratory conditions.

METHODOLOGY/PRINCIPAL FINDINGS

By interfering with limb development, the trematode Ribeiroia ondatrae causes particularly severe morphological alterations within amphibian hosts that provide an ideal system to evaluate parasite-induced changes in phenotype. Here, we coupled laboratory performance trials with a capture-mark-recapture study of 1388 Pacific chorus frogs (Pseudacris regilla) to quantify the effects of parasite-induced malformations on host locomotion, foraging, and survival. Malformations, which affected ∼ 50% of metamorphosing frogs in nature, caused dramatic reductions in all measures of organismal function. Malformed frogs exhibited significantly shorter jumping distances (41% reduction), slower swimming speeds (37% reduction), reduced endurance (66% reduction), and lower foraging success relative to infected hosts without malformations. Furthermore, while normal and malformed individuals had comparable survival within predator-free exclosures, deformed frogs in natural populations had 22% lower biweekly survival than normal frogs and rarely recruited to the adult population over a two-year period.

CONCLUSIONS/SIGNIFICANCE

Our results highlight the ability of parasites to deeply alter multiple dimensions of host phenotype with important consequences for performance and survival. These patterns were best explained by malformation status, rather than infection per se, helping to decouple the direct and indirect effects of parasitism on host fitness.

Beyond immunity: quantifying the effects of host anti-parasite behavior on parasite transmission

A host's first line of defense in response to the threat of parasitic infection is behavior, yet the efficacy of anti-parasite behaviors in reducing infection are rarely quantified relative to immunological defense mechanisms. Larval amphibians developing in aquatic habitats are at risk of infection from a diverse assemblage of pathogens, some of which cause substantial morbidity and mortality, suggesting that behavioral avoidance and resistance could be significant defensive strategies. To quantify the importance of anti-parasite behaviors in reducing infection, we exposed larval Pacific chorus frogs (Pseudacris regilla) to pathogenic trematodes (Ribeiroia and Echinostoma) in one of two experimental conditions: behaviorally active (unmanipulated) or behaviorally impaired (anesthetized). By quantifying both the number of successful and unsuccessful parasites, we show that host behavior reduces infection prevalence and intensity for both parasites. Anesthetized hosts were 20-39% more likely to become infected and, when infected, supported 2.8-fold more parasitic cysts. Echinostoma had a 60% lower infection success relative to the more deadly Ribeiroia and was also more vulnerable to behaviorally mediated reductions in transmission. For Ribeiroia, increases in host mass enhanced infection success, consistent with epidemiological theory, but this relationship was eroded among active hosts. Our results underscore the importance of host behavior in mitigating disease risk and suggest that, in some systems, anti-parasite behaviors can be as or more effective than immune-mediated defenses in reducing infection. Considering the severe pathologies induced by these and other pathogens of amphibians, we emphasize the value of a broader understanding of anti-parasite behaviors and how co-occurring stressors affect them.

Mystery unsolved: missing limbs in deformed amphibians

Ballengee and Sessions (2009) claim that predatory attacks by small predators such as Sympetrum dragonfly larvae are sufficient to explain amphibian limb deformities in which the limb is partly or completely missing. This deformity type, the most common in nature, is not well explained by Ribeiroia infection which has also been nominated as a mechanism for limb deformities. We argue that the conclusions of the Ballanegee and Sessions study are not well founded. In part this is because the authors have provided no quantitative analysis of the association between limb deformities and predator densities. Our own data on frequencies of limb deformities suggest that missing hind limbs are often extremely rare even when Sympetrum and other small predators are common. While predatory attacks may contribute to observations of limb deformities, further study will be required to elucidate their role; other potential mechanisms deserve study as well. It is premature, and counterproductive, to draw any conclusions regarding the mechanisms behind the most common limb deformities recorded in natural populations.

Sublethal predators and their injured prey: linking aquatic predators and severe limb abnormalities in amphibians

While many predators completely consume their prey, others feed only on blood or tissue without killing the prey, sometimes causing ecologically significant levels of injury. We investigated the importance of sublethal predator attacks in driving an emerging issue of conservation importance: missing-limb deformities in amphibians. We combined long-term field data and manipulative experiments to evaluate the role of sublethal predation in causing abnormalities in two regions of central Oregon, U.S.A. Since 1988, western toads (Bufo boreas) in Lake Aspen have exhibited abnormalities dominated by partially missing limbs and digits at annual frequencies from <1% to 35%. On Broken Top volcano, we found comparable types and frequencies of abnormalities in Cascades frogs (Rana cascadae). Field sampling and observational data implicated two aquatic predators in these abnormality phenomena: introduced sticklebacks (Gasterosteus aculeatus) at Lake Aspen and corduliid dragonfly larvae (Somatochlora albicincta) at Broken Top. In experiments, these predators produced limb abnormalities identical to those observed in the respective regions. At Lake Aspen, in situ predator exclosures effectively eliminated abnormalities in toads, while comparisons among years with low and high stickleback abundance and between wetlands with and without sticklebacks reinforced the link between fish and amphibian abnormalities. Neither trematode parasite infection nor pesticide contamination could explain observed abnormalities. Our results suggest that predators are an important explanation for missing-limb abnormalities and highlight the ecological significance of sublethal predation in nature.

Amphibian ocular malformation associated with frog virus 3

During an on-going amphibian ecology study, a free-ranging American bullfrog (Rana catesbeiana) metamorph was captured in a pitfall trap adjacent to a constructed farm pond at the Plateau Research and Education Center (PREC) on the Cumberland Plateau near Crossville, Tennessee, USA. Grossly, the right eye was approximately 50% the size of the left. Stereo and light microscopic examination revealed two granulomas within the orbit. Electron microscopic examination revealed virus particles scattered throughout one structure but mostly aggregated toward the center. Subsequent PCR and sequencing (GenBank accession Number EF175670) confirmed frog virus 3 (FV3). This represents the first report of a malformation in an anuran associated with FV3.

All hosts are not equal: explaining differential patterns of malformations in an amphibian community

1. Within a community, different host species often exhibit broad variation in sensitivity to infection and disease. Because such differences can influence the strength and outcome of community interactions, it is essential to understand differential disease patterns and identify the mechanisms responsible. 2. In North American wetlands, amphibian species often exhibit extraordinary differences in the frequency of limb malformations induced by the digenetic trematode, Ribeiroia ondatrae. By coupling field studies with parasite exposure experiments, we evaluated whether such patterns were due to differences in (i) parasite encounter rate, (ii) infection establishment, or (iii) parasite persistence within hosts. 3. Field results underscored the broad variation in malformations and infection between host species; while nearly 60% (n = 618) of emerging American toads exhibited severe limb deformities such as bony triangles, skin webbings and missing limbs, fewer than 4% (n = 251) of Eastern gray treefrogs from the same pond were abnormal. Despite similarities in the phenology and larval development period of these species, they differed sharply in Ribeiroia infection. On average, toads supported 75x more metacercariae than did metamorphic treefrogs. 4. Experimental exposures of larval toads and treefrogs to a realistic range of Ribeiroia cercariae revealed strong differences in the sensitivity of these species to infection; exposed toads suffered elevated mortality (up to 95%), delayed metamorphosis, and severe limb malformations consistent with field observations. Treefrogs, in contrast, exhibited limited mortality and no malformations, regardless of exposure level. Ribeiroia cercariae were substantially less successful in locating and infecting Hyla versicolor larvae. 5. Our results indicate that the observed differences in infection and malformations owe to a lower ability of Ribeiroia cercariae to both find and establish within larval treefrogs, possibly stemming from a heightened immune response to infection. Because Ribeiroia is a highly pathogenic parasite with negative effects on larval and metamorphic amphibian survival, variation in infection resistance among species could have important implications for understanding patterns of species co-occurrence, competition, and community diversity.

Dietary retinoic acid induces hindlimb and eye deformities in Xenopus laevis

This study investigated the effects of dietary retinoic acid (RA) on frog hindlimb development. Xenopus laevis (African clawed frog) tadpoles were fed a diet supplemented with 0, 1, 10, or 100 microg of RA/g of food for 2 or 5 d at different stages of metamorphosis. Hindlimb deformities were induced in the group fed 100 microg of RA/g of diet for 5 d. Exposures beginning at mid-hindlimb bud development induced bilaterally bent tibiafibula (bony triangles), while exposures later in hindlimb development induced deformities of the feet, including fusion of the 1st and 2nd clawed digits and reduced length of the 4th and 5th digits (due to reduced, missing, or misplaced phalanges). There were also cases of extra phalanges in the 5th digit. The eye was another target of RA exposure. In one experiment, 58% of the tadpoles fed 10 microg of RA/g had a smaller or absent right eye. Additionally, 11% of the tadpoles fed 100 microg of RA/g of diet developed a smaller or absent left eye. Waterborne heavy metals (Zn or Cu) modified RA effects on the hindlimb and eye. Co-exposure to metals and RA resulted in cases of unilateral bony triangles and reduced rates of smaller eyes. There were also cases of extra hindlimb digits in Zn-exposed animals. Dietary RA exposure in tadpoles can cause some deformities that differ from waterborne RA exposures in previous studies. RA also induced deformities that resemble those in affected wild frog populations (bony triangles), although the patterns of other deformities and missing segments (phalanges and metatarsals) are not similar to those documented in the wild.

On the behavioural response ofRanaandBufotadpoles to echinostomatoid cercariae: implications to synergistic factors influencing trematode infections in anurans

We used high-speed videography of staged encounters between tadpoles of either Bufo americanus Holbrook, 1836 or Rana sylvatica LeConte, 1825 and Echinostoma Rudolphi, 1809 cercariae to understand why echinostomatoid trematodes, such as species from the genera Echinostoma and Ribeiroia Travassos, 1939 (implicated in anuran limb deformities), attack specific anatomical regions of tadpoles. Bufo and Rana tadpoles can shed cercariae on their skin from some parts of their body more easily than others. In particular, cercariae that enter the "dead-water zone" at the junction of a tadpole's body and tail appear particularly difficult for tadpoles to brush off. Cercariae that reach this recess can easily enter the inguinal region of tadpoles (as do Ribeiroia spp.) or ascend the tadpole's cloaca (as do Echinostoma spp.). When tadpoles sense cercariae contacting their skin they make explosive movements to shed those parasites. Factors that reduce tadpoles' activity, such as predator threat or certain pesticides, may increase a tadpole's susceptibility to echinostomatoid infection. Because Bufo tadpoles are unpalatable to many predators, they can afford to make more conspicuous evasive maneuvers than Rana tadpoles, and do so in the laboratory. Bufo tadpoles in the field also have a lower rate and different anatomical distribution pattern of Ribeiroia infection than Rana tadpoles. Factors that reduce tadpole activity in the field may act synergistically to increase parasite loads and subsequent deformities in anurans.

Assessment of environmental stressors potentially responsible for malformations in North American anuran amphibians

Several species of anuran amphibians from different regions across North America have recently exhibited an increased occurrence of malformations, predominantly of the hindlimb. Research concerning the potential causes of these malformations has focused extensively on three stressors: chemical contaminants, ultraviolet (UV) radiation, and parasitic trematodes. In this overview of recent work with each of these stressors, we assess their plausibility as contributors to the malformations observed in field-collected amphibians. There is as yet little evidence that chemical contaminants are responsible for the limb malformations. This includes chemicals, such as the pesticide methoprene, that could affect retinoid-signaling pathways that are critical to limb development. Exposure to UV radiation also seems to be an unlikely explanation for hindlimb malformations in amphibians. Although solar UV can cause hindlimb deficiencies in amphibians, a probabilistic assessment based on empirical dose-response and exposure data indicates that UV exposures sufficient to induce limb defects would be uncommon in most wetlands. Results of controlled studies conducted with some affected species and field-monitoring work suggest infection by digenetic trematodes as a promising explanation for the malformations observed in anurans collected from many field sites. Controlled experimentation with additional species and monitoring across a broader range of affected sites are required to assess fully the role of trematodes in relation to other stressors in causing limb malformations. If trematode infestations are indeed related to the recent increases in malformed amphibians, then the question remains as to what alterations in the environment might be causing changes in the distribution and abundance of the parasites.

Review of the Trematode Genus Ribeiroia (Psilostomidae): Ecology, Life History and Pathogenesis with Special Emphasis on the Amphibian Malformation Problem

Trematodes in the genus Ribeiroia have an indirect life cycle involving planorbid snails as first intermediate hosts, fishes or amphibians as second intermediate hosts and birds or mammals as definitive hosts. Although rarely pathogenic in definitive hosts, Ribeiroia infection can cause severe pathology and mortality in snails and amphibians. This group of parasites has gained notoriety for its prominent rol in the recent rash of amphibian deformities in North America. Under some circumstances, these malformations may enhance parasite transmission by rendering infected amphibian hosts more susceptible to definitive host predators. However, increasing reports of malformations in North American amphibian populations emphasize the importance of understanding infection patterns. Here we review important aspects of the biology, ecology, life cycle and pathogenesis of parasites in the genus Ribeiroia and identify priorities for future research. Based on available morphological descriptions and preliminary molecular data, three species of Ribeiroia are recognized: R. ondatrae in the Americas, R. marini in the Caribbean and R. congolensis/C. lileta in Africa. We further evaluate the influence of abiotic and biotic factors in determining the intensity and prevalence of Ribeiroia infection and malformations in amphibians, highlighting the importance of habitat alteration and secondary factors (e.g. aquatic eutrophication, contaminants) in promoting infection. Although not a "new" parasite, Ribeiroia may have increased in range, prevalence, or intensity in recent years, particularly within amphibian hosts. Nevertheless, while much is known about this intriguing group of parasites, there remains much that we do not know. Particular importance for future research is placed on the following areas: evaluating the phylogenetic position of the genus, establishing the molecular mechanism of parasite-induced malformations in amphibians, isolating the drivers of parasite transmission under field conditions and studying the consequences of malformations for parasite and host populations. Investigation of these questions will benefit enormously from a multidisciplinary approach that effectively integrates parasitology, developmental biology, immunology, herpetology and aquatic ecology.

Influence of Ribeiroia ondatrae (Trematoda: Digenea) infection on limb development and survival of northern leopard frogs (Rana pipiens): effects of host stage and parasite-exposure level

Recent evidence suggests that infection by larvae of the trematode Ribeiroia ondatrae accounts for a significant proportion of limb malformations currently observed in amphibian populations of North America. However, the effects of R. ondatrae infection on northern leopard frogs (Rana pipiens), one of the species most frequently reported with malformations, have not been adequately explored. Moreover, the risk factors associated with R. ondatrae-induced malformations have not been clearly identified. We examined the effects of timing of infection on tadpole survival and limb development. Rana pipiens tadpoles were individually exposed to R. ondatrae cercariae at the pre-limb-bud (Gosner stages 24 and 25), limb-bud (Gosner stages 27 and 28), or paddle (Gosner stages 31–33) stages of development and monitored through metamorphosis. The effects of infection were stage-specific. Infections acquired at the pre-limb-bud stage resulted in a high mortality rate (47.5–97.5%), whereas tadpoles infected at the limb-bud stage displayed a high malformation rate (16% overall), and the magnitude of effects increased with the level of exposure to cercariae. In contrast, infections acquired at the paddle stage had no effect on limb development or tadpole survival, which suggests that the timing of R. ondatrae infection in relation to the stage structure of tadpole populations in the wild is an important determinant of the degree to which populations are affected by R. ondatrae.