Apr. 19, 2002 In recent years, the frequency of malformed frogs, toads, salamanders and other amphibians found with missing limbs, extra limbs, and skin webbings has increased. The shrinking populations of many North American amphibian populations underscore the need to understand the causes and implications of this phenomenon. Now a new study suggests that a parasite may be to blame for many of the abnormalities found in amphibians of the western United States. In the research article “Parasite (Ribeiroia Ondatrae) infection linked to amphibian malformations in the western United States,” appearing in the May issue of the Ecological Society of America’s journal Ecological Monographs, Pieter Johnson of the University of Wisconsin-Madison and colleagues describe the results of their broad-scale field survey. Covering parts of California, Oregon, Washington, Idaho, and Montana, the team of researchers looked for malformations in over 12,000 amphibians representing 11 species of amphibians. The group looked at the relationships between the frequency and severity of abnormalities and a variety of factors in a particular aquatic site, including the abundance of a parasite (Ribeiroia) and pesticide contamination.
The collaborative and interdisciplinary effort, which included academic researchers, as well as federal scientists from the U.S. Geological Survey and the U.S. Fish and Wildlife Survey, found malformed amphibians at a wide variety of aquatic sites, ranging from montane lakes and ephemeral pools to irrigation canals and impoundments. While the researchers did not find a relationship between pesticides and the frequency of malformed amphibians, they did find a striking connection between malformed amphibians and the presence of Ribeiroia.
“The presence of this parasite was a powerful predictor of the presence and frequency of malformed amphibians in an aquatic system. The greater an amphibian population’s infection with Ribeiroia, the more frequent and severe the population’s limb malformations,” said Johnson.
Amphibians at sites supporting the parasite exhibited 6 times as many abnormalities as the average number of malformations recorded at sites without the parasite. The researchers found the parasite embedded around the base of amphibians’ limbs and tails, where they form cysts beneath the skin. The frequency of abnormalities varied substantially among sites and species, ranging from 0 to nearly 90 percent. Pacific treefrogs (Hyla regilla) exhibited the greatest number of abnormalities, with more than 1000 abnormal tadpoles and young frogs found at 55 sites. When they co-occurred at an aquatic site, Pacific treefrogs exhibited more abnormalities than did western toads, marginally more than bullfrogs, and less than California newts.
In order to understand if and why the parasite may have become more common in recent years, the research team also gathered information on the ecology and life history of Ribeiroia, which has a multi-host life cycle.
The paper notes that the final stage of the parasite’s life cycle is dependent upon predation; in order to complete its final developmental stage, the parasite depends upon a bird or mammal to eat an infected amphibian or fish. The parasite then sexually matures and releases eggs via bird or mammal feces. When the eggs hatch they invade the tissue of Planorbella, an aquatic snail.
The study revealed that this aquatic snail is a significant indicator of both the presence and abundance of the parasitic infection. The presence and abundance of this snail were the only two factors related to the presence or abundance of Ribeiroia.
The researchers note that both Planorbella and increased parasitic diseases are associated with artificially created wetlands, which have been on the increase, often replacing natural wetlands.
“People assume that parasites are “natural” and therefore of no conservation concern,” says Johnson. “However, we suspect that nutrient pollution from fertilizers and cattle may be increasing the numbers of snails, parasites, and therefore malformed amphibians.”
Johnson notes that the parasite may be of particular concern to declining amphibian populations, such as Western Toads and Columbia Spotted Frogs, which were often found infected and malformed.
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