Ribeiroia

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Ribeiroia
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Platyhelminthes
Class:	Trematoda
Order:	Plagiorchiida
Family:	Psilostomatidae
Genus:	Ribeiroia Travassos, 1939
Type species
Ribeiroia insignis Travassos, 1939[1] = Ribeiroia ondatrae (Price, 1931)
Species
Ribeiroia congolensis Ribeiroia marini Ribeiroia ondatrae

Ribeiroia (

Laboratory studies

Experimental exposure to Ribeiroia ondatrae cercariae has been shown to cause limb malformations in various frog, toad and

Ribeiroia infection has been linked to malformations in naturally occurring amphibian populations, especially in the western and midwestern US.^[8] In a large-scale study in the western US, both the presence and abundance of Ribeiroia infection predicted higher-than-baseline (e.g. >5%) frequencies of abnormalities in one or more amphibian species.^[9] Limb malformations were observed in nine species and ranged in frequency from <5% to nearly 90%. The role of Ribeiroia in explaining accounts of amphibian malformations in other regions is varied; Ribeiroia has been associated with malformation "hotspots" in the midwestern and northeastern US,^[10] but it was not detected in malformation surveys from Vermont, Alaska, Bermuda, and Michigan.^[11]

Geographic distribution

Ribeiroia ondatrae is widely distributed across the US, including records from 22 species in 37 states (Johnson and McKenzie 2008). Most records of Ribeiroia-associated malformations have been reported from California, Oregon, Washington, Montana, Minnesota, Wisconsin, New Jersey and Pennsylvania. Less is known about the distribution of Ribeiroia in Canada and Central America. In amphibians, both the abundance of the parasite and the number of observations appear to correlate positively with major bird migratory flyways, with concentrations along the Pacific, Mississippi, and Atlantic flyways (Johnson et al. 2010). This observation suggests definitive (bird) host activity is an important determinant of parasite distribution at the continental scale, but this hypothesis has yet to be rigorously tested. Ribeiroia ondatrae is most often recovered from amphibians in lentic habitats, such as ponds, lakes, and wetlands, which support planorbid snails. Other species of Ribeiroia occur in the Caribbean and South America (R. marini and R. marini guadeloupensis) and in Africa (R. congolensis). However, whether these species infect amphibians or cause limb malformations has not been explored.

Evidence for emergence

Dating back over 200 years, there have been reports of amphibian malformations suggesting that the general phenomenon is not a new occurrence in amphibian populations (Ouellet 2000). Despite the historical occurrence of amphibian malformations, the real question is whether such malformations are increasing in prevalence and severity in wild amphibian populations. Limited historical data on malformations and the absence of baseline malformation data has complicated attempts to assess whether malformations are emerging. However, research over the past decade has shown that malformations are emerging in some amphibian populations. For example, the background rate of malformations of northern leopard frogs in Minnesota increased from 0.4% in 1958–1963 to 2.5% in 1996–1997 (Hoppe 2000), with recently observed malformations more severe and more diverse. Similarly, Johnson and Lunde (2005), in a comparison of recent (1990–2000) and historical (1899–1989) publications, found that recent reports document:

1. a wider range of more severe malformations,
2. a greater number of affected amphibian species,
3. a larger number of affected sites, and
4. a higher frequency range of affected individuals at reported sites.

For both of these studies, however, the mechanisms driving the apparent increase in malformations were not identified (see Factors influencing emergence). In a more rigorous study, Johnson et al. (2003) confirmed that historical malformations at six of eight sites (1947–1990) were associated with Ribeiroia infection. Moreover, three of these sites continued to support limb malformations at frequencies of 7-50% in one or more species. One pond in particular (Jette Pond in western Montana) showed increased prevalence of malformations in Pseudacris regila from 20% in 1960 to 46% in 2000, which was correlated with an increased intensity of Ribeiroia infection. Although more research is necessary, these studies provide evidence that malformations, especially those linked to Ribeiroia infection, are emerging in some amphibian populations.

Factors influencing infection

Nutrients - Nitrogen and phosphorus enrichment are widespread forms of anthropogenic environmental change that may influence parasite infection (Johnson and Carpenter 2008). Nutrient enrichment leads to eutrophication, which has been linked with emergence of Ribeiroia ondatrae through direct and indirect effects on aquatic food webs (Johnson and Chase 2004). The underlying mechanism through which eutrophication can increase infection involves increasing the density of infected first-intermediate snail hosts and by increasing the production of parasites by infected snails (Johnson et al. 2007).

Pesticides - Pesticides are still considered a potential factor influencing amphibian malformations (Johnson et al. 2010). However, recent investigations have focused on the interaction of pesticides with parasite infection (Kiesecker 2002; Rohr et al. 2008). Field experiments indicated a link between agricultural run-off and increased infection (Kiesecker 2002). Laboratory studies further demonstrated that pesticide exposure reduced host immunocompetence against parasites as the biological mechanism (Kiesecker 2002). Furthermore, there may be a link between nutrient addition and pesticide contamination jointly leading to increased infections by simultaneously increasing exposure and suppressing host immune systems (Rohr et al. 2008a, b).

Biodiversity - Host species

Parasite-induced malformations and amphibian conservation

Perhaps the most unexplored, complex and vital aspects of trematode-caused amphibian malformations are the population-level consequences. Do these parasites and the malformations they induce pose a conservation risk? While no direct evidence linking trematode deformities and amphibian population declines exists, there are legitimate reasons for concern (Johnson and McKenzie 2008). For instance, in multiple species of frogs and toads, laboratory studies show that even low levels of Ribeiroia infection can induce 30-95% mortality (see Johnson et al. 2010). Correspondingly, multi-year field studies show that in localities of high infection and malformations in metamorphosing frogs, <2% of amphibians returning to breed exhibit malformations, suggesting Ribeiroia infection and malformations have deleterious consequences for individual survival and fecundity (Johnson et al. 2001). In specific wetlands that have historically exhibited a high prevalence of infection and malformations, several amphibian species have notably declined or disappeared (see Johnson and McKenzie 2008 for review). In light of these data, and the increasing evidence that Ribeiroia infections are on the rise (Johnson and McKenzie 2008), it is prudent to treat Ribeiroia ondatrae as a threat to amphibian populations and diversity, particularly in combination with other stressors.

See also

• Ribeiroia ondatrae

References

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Notes