Parasites or Pollution?

Something funny has been happening to frogs in the 1990s: their numbers have plummeted and they are more often found deformed. That much is not disputed. Nor is it argued that the recent declines among amphibians -- whose permeable skins make them more sensitive to ecological change -- may well be a harbinger of a more serious environmental problem. But scientists have been debating just why Kermit's kind is finding it less easy being green.

Explanations range from vanishing habitats, fungal infections, cannibalism and predation to the far more ominous. Some suggest that as the ozone hole widens, frog embryos are exposed to higher levels of ultraviolet radiation, leading to genetic mutations. Others blame pollution from pesticide runoff; among these culprits are chemicals called retinoids, which double as potent human teratogens. But recently a far less sinister cause has surfaced: parasites.

Indeed, two new papers in Science-- one by Stanley K. Sessions and his colleagues at Hartwick College and the other by Pieter Johnson, a recent graduate of Stanford University and his former colleagues at Stanford's Center for Conservation Biology -- demonstrate that flatworms called Ribeiroia are responsible for most of the observed limb deformities in a variety of frog species. The tiny trematodes burrow into the hindquarters of tadpoles, where they either physically or chemically scramble the cells that develop into legs. "It's about as close to using an egg beater on the limb bud cells as you can get," Sessions says. As a result, infected frogs sprout extra legs or no legs at all.

To obtain his latest findings, Sessions -- who first proposed that parasites were involved in 1987 -- took earlier experiments out into the field. Previous laboratory work showed that trematodes and retinoids caused distinct malformations: whereas parasitic infections led to additional but properly proportioned limbs, chemical exposure spawned limbs in which the parts appeared in the wrong order, the thigh-knee-thigh-knee pattern. Sessions's group set about looking for these telltale differences around the U.S.

They surveyed five species of frogs: 391 multilegged Pacific treefrogs (Hyla regilla) from eight sites in California and Oregon, as well as smaller samples of multilegged cascade frogs (Rana cascadae), wood frogs (Rana sylvatica), green frogs (Rana clamitans) and leopard frogs (Rana pipiens) from Oregon, Arizona and New York. In all, they found no thigh-knee-thigh-knee deformities--and every single specimen was infested with trematode cysts near their spare limbs. Moreover, deformities characteristic of parasitic infection were catalogued among an additional 1,686 long-toed salamanders (Ambystoma macrodactylum) found at one of the same sites.

In contrast, Johnson and his colleagues went from the field back indoors, after collecting treefrog eggs from a California site at which no deformities had been recorded. After the tadpoles hatched, they divided them into six groups. Four sets received light, intermediate, heavy or no exposure to Ribeiroia. Another set was subjected to a different trematode called Alaria mustelae, and the last set was subjected to both species. What they found was that Ribeiroia clearly caused missing or multiple legs, only Ribeiroia did so and that the severity of the deformity rose with the level of the exposure, whereas the percentage of tadpoles surviving to metamorphosis fell. "It worked much better than I had expected," Johnson notes.

Both Sessions and Johnson believe that the parasites take advantage of the frog's deformities to further their own reproductive success throughout their life cycle. Initially, immature trematodes infect aquatic snails and reproduce asexually, often reaching numbers that kill the snail. Once the larvae are in the water again, they attack tadpoles, with an eye to their third host, birds, where they reproduce sexually. The fact is that the birds find it easier to dine on the deformed frogs. And via the birds' feces, the parasites travel to the next pond.

That said, the involvement of natural parasites in producing misshapen or fewer frogs doesn't necessarily absolve human activities. The scientists speculate that fertilizer runoff, by adding extra nutrients to the water, may well help to increase the numbers of snails and so indirectly inflate the population of Ribeiroia. In addition, the worms may not be to blame for many of the other frog deformities recorded to date, including missing eyes and retained tails. Even so, it is an encouraging piece to fit into this troubling puzzle.