What Causes Deformed Frogs?
Extra Legs on a Frog are Usually Caused by a Parasite
When people find frogs with too many legs, they often suspect pollution. In many cases, this type of frog deformity results from parasitic disease caused by a fluke.
Almost everyone has heard a news report about someone finding a frog with three, four, or even nine legs. Deformed frogs turn up now and then, sometimes occurring in high numbers in a particular area and sparking fears that some kind of pollution has occurred that may cause birth defects in people too. As usual, it’s not that simple - habitat destruction and chemical pollution may contribute indirectly, but the usual cause of extra legs on a frog is a parasite: Ribeiroia ondatrae.
How does Ribeiroia ondatrae cause deformities in frogs?
Ribeiroia ondatrae is a fluke, a flat worm with a complicated life cycle that disrupts the normal development of legs in frog tadpoles—the adult parasite lives in birds and mammals, probably in the intestine or liver:
- Eggs of Ribeiroia are passed into water with feces.
- Larvae hatching out of the eggs penetrate the body of an aquatic snail and begin to reproduce inside the snail.
- Eventually, parasitic forms called cercariae leave the snail, re-enter the water, and penetrate the body of a tadpole.
- The larvae travel to the part of the tadpole where legs are developing and encyst there. They disrupt the normal growth of legs, resulting in extra legs, abnormal legs and, ultimately, deformed frogs.
- Having deformities interferes with the frog’s normal movement and makes it more likely to be caught and eaten by a bird or other predator, thus probably increasing the chances that the parasite will reach its final host and complete its lifecycle.
Why are there more deformed frogs now?
People started to notice lots of frogs with extra legs in the mid-nineteen-nineties and initially suspected that this was the result of some environmental change. Chemical fertilizers and other pollutants, as well as increased exposure to ultraviolet light were suspected, as well as parasitic disease. It turns out that all of these things can cause frog deformities, but Ribeiroia ondatrae is usually the culprit when extra legs are present. The parasite has been around for a long time but has caused an epidemic in frogs since 1995. It looks as though environmental factors are responsible to some degree:
- Human interference with wetlands and the creation of ponds to provide water for crops and livestock has increased the quiet weedy aquatic habitat where the snail host, Planorbella, thrives. An increase in snails means higher numbers of parasitic larvae in the water to penetrate tadpoles.
- Pesticides and increased levels of ultraviolet light may make frogs and other amphibians more susceptible to infection with Ribeiroia ondatrae.
- Fish and other predators, often introduced by humans, can influence the behavior of amphibians, placing them at increased risk of becoming infected with the parasite.
Scientists are still trying to understand the complex interactions of habitat, organisms, and human activities that have led to an astounding increase in the number of frog deformities. For now, we can at least be thankful that Ribeiroia ondatrae doesn’t affect humans in the same way.
Articles about other parasitic flukes:
Sources:
“Amphibian Deformities and Ribeiroia infection: an Emerging Helminthiasis.” Johnson, Pieter T. J., and Sutherland, Daniel R. Trends in Parasitology: 19(8), Aug 2003; 332 – 335
“Explaining Frog Deformities.” Blaustein, Andrew R., and Pieter T. J. Johnston. ScientificAmerican.com: Jan 14, 2003
“Parasite (Ribeiroia ondatrae) Infection Linked to Amphibian Malformations in the Western United States.” Johnson, Pieter T. J., Kevin B. Lunde, E. Michael Thurman et al. Ecological Society of America: Ecological Monographs: 72(2), May 2002; 151-168
