Parasites and Disease
Disease due to parasitic infection (including worms, bacteria, fungi and viruses) is one of the most studied potential causes of amphibian malformations. Trematode parasite infection is probably the most well-studied cause of malformations in frogs and toads to date. Trematodes are parasitic flatworms (a worm-like animal) that live inside snails and vertebrate hosts, including amphibians. In the late 1990s, scientists identified the parasite responsible for malformed amphibians as Ribeiroia ondatrae (Johnson et al. 1999; Session and Ruth 1990). During the parasite’s life cycle, Ribeiroia moves first from freshwater snails, then to larval amphibians (tadpoles), and finally to birds, or less frequently mammals. During this process, infected snails release large numbers of cercariae (the free-swimming form of trematodes), which seek out and infect larval amphibians. These cercariae burrow into the developing limbs of amphibians, sometimes causing severe disruptions to limb growth leading to malformations. Despite strong links between Ribeiroia infection and amphibian malformations in some geographic regions, the parasite does not explain all accounts of malformations in amphibians. For example, some wetlands with a high occurrence of abnormal frogs, such as those in Alaska, have never been found to harbor the Ribeiroia parasite (Reeves et al. 2008).
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UVB Radiation
Declines in the earth’s ozone layer have been connected with seasonal increases in the level of UV-B radiation, which is thought to have harmful effects on wildlife. Because many amphibian species lay their eggs in shallow water, they may be especially susceptible to increases in UV-B radiation (Blaustein et al. 1994). However, previous studies have shown that the levels of UV-B exposure to which amphibians are exposed to in natural wetlands will often not be enough to trigger the development of malformations (Diamond et al. 2002). Thus, while UV-B may pose some degree of threat to amphibians (particularly to eggs laid in shallow water), most evidence suggests that UV-B is not a major cause of limb abnormalities.
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Unknown Factors/Interactions
Given the variability in the types and severity of amphibian malformations across North America (and even at the same site across years), most scientists agree that multiple interacting factors are almost certainly responsible for driving amphibian abnormalities. For example, Johnson et al. (2007) found that eutrophication resulting from nutrient runoff can increase the levels of Ribeiroia parasite infection (see “Parasites and Disease” box) through effects on the intermediate snail hosts. Specifically, excess nutrients stemming from agricultural fertilizers, livestock manure, or urbanization can enhance algal growth in aquatic systems, ultimately promoting the growth and reproduction of the snail hosts for the infectious parasite. Yet, the Ribeiroia parasite has not been found to occur in all parts of the US where malformed amphibians have been observed (e.g., Texas, Florida, Bahamas) so there must be other factors out there in addition to disease that are responsible for causing malformed amphibians. This is just one example that highlights the complexity of investigating amphibian malformations in nature.
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