Amphibian Deformities *7 April 2004
A middle school class in rural Le Sueur, Minnesota, stumbled upon a large population of deformed leopard frogs in the summer of 1995 while on a biology field trip (Souder 2000). Their teacher contacted the Minnesota Pollution Control Agency, who later determined that 30 to 40 percent of the frogs found by the middle schoolers, in Ney Pond, were deformed. This episode shocked the public and news of the deformed frogs in Minnesota spread all across the country. At the same time, amphibian biologists were discovering that amphibian deformities were not just occurring in Minnesota, but that reports of mass deformities were coming in from around the world (Kiesecker et al. 2004).
Figure 1. (Right) Northern leopard frog (Rana pipiens) with dual bony triangles in the tibiofibula of the right hindlimb. Note also the mirror image duplication of the right foot (©Pieter Johnson). (Left) R. pipiens with two extra hind limbs extending ventrally from pelvic girdle (© Pieter Johnson).
The widespread occurrence of deformities* (or malformations, or abnormalities) in natural populations of amphibians has recently been perceived as a major environmental problem (Ouellet et al. 1997). There has been quite a lot of debate over what causes amphibian deformities (an educational group has used the controversy as a subject of a critical thinking project) and it has proven difficult to determine what is actually causing deformities in wild populations. For example, it is very easy to induce deformities in laboratory-raised embryos; but these deformities do not always match up to the types of deformities found in the wild (Sessions 2003). It is also difficult to determine if deformity rates have truly increased in recent years or if the increased number of reported deformities are a result of increased media attention (Johnson et al. 2003).
Despite these uncertainties, one thing has become very clear, deformities, like the overall amphibian decline problem, are likely to be a result of multiple causes, all related to human-induced environmental damage (Blaustein and Johnson 2003a). The original hypotheses put forward to explain the increased rate of amphibian deformities are increased levels of ultraviolet (UV) radiation, chemical contamination and parasitic infection (Sessions 2003, links direct you to the website of Stanley K. Sessions). Research by Johnson et al. (1999) linked a trematode, now known to be Ribeiroia ondatrae, to amphibian limb deformities. This and other research has made parasitic infection the likeliest explanation for most out breaks of amphibian deformities (for a good review see Blaustein and Johnson 2003b) but it is also possible that the observed malformations are caused by several different factors, acting alone or in combination (for an example see Kiesecker 2002). Further research has also shown a direct relationship between human habitat alterations and sites where Ribeiroia parasites are especially abundant, thus linking amphibian deformities to human induced environmental damage (Blaustein and Johnson 2003b).
Web Resources: We recommend several websites with excellent summaries of the deformity problem: See Stanley K. Sessions at Hartwick College. A report on the malformed frogs in Minnesota can be found on the U.S. Geological Survey's web page.
Beyond the internet: A Plague of Frogs: The Horrifying True Story, is a book written for the general public by William Souder, the journalist who first broke the frog deformity story for The Washington Post. In his book, Souder follows the mystery of the amphibian deformity and the search for answers.
Have you seen malformed frogs in your area? If so, The North American Reporting Center for Amphibian Malformations (NARCAM) is a web site designed as an avenue for people to report on the health of their local amphibian populations. We highly encourage people to submit reports on the general health of their local amphibians. Reports provide important baseline data on the health and fitness of existing amphibian populations. These reports help identify patterns in amphibian malformation occurrences and can help direct future research. While NARCAM was originally set up to report malformed amphibians, they are also interested in observations of normal amphibian populations. Obtaining both kinds of reports provides a more accurate picture of the current state of amphibian populations in a particular area.
* The terminology used to describe the observed abnormalities has not been completely standardized and there is currently a disagreement over whether these abnormalities should be considered deformities, malformations or simply morphological abnormalities. On this page, we use the term "deformities" for simplicity and because it is the most widely recognized by the general public.
Blaustein, A. R. and Johnson, P. T. J. 2003a. The complexity of deformed amphibians. Frontiers in Ecology and the Environment 1: 87-94.
Blaustein, A. R. and Johnson, P. T. J. 2003b. Explaining frog deformities. Scientific American 288: 60-65.
Johnson, P. T. J., Lunde, K. B., Ritchie, E. G., and Launer, A. E. 1999. The effect of trematode infection on amphibian limb development and survivorship. Science (Washington D. C.) 284: 802-804.
Johnson, P. T. J., Lunde, K. B., Zelmer, D. A., and Werner, J. K. 2003. Limb deformities as an emerging parasitic disease in amphibians: Evidence from museum specimens and resurvey data. Conservation Biology 17: 1724-1737.
Kiesecker, J. M. 2002. Synergism between trematode infection and pesticide exposure: A link to amphibian limb deformities in nature? Proceedings of the National Academy of Sciences of the United States of America 99: 9900-9904.
Kiesecker, J. M., Belden, L. K., Shea, K., and Rubbo, M. J. 2004. Amphibian decline and emerging disease. American Scientist 92: 138-147.
Ouellet, M., Bonin, J., Rodrigue, J., DesGranges, J. L., and Lair, S. 1997. Hindlimb deformities (ectromelia, ectrodactyly) in free-living anurans from agricultural habitats. Journal of Wildlife Diseases 33: 95-104.
Sessions, S. K. 2003. What is causing deformed amphibians? Pages 168-186 in R. D. Semlitsch, editor. Amphibian Conservation. Smithsonian Institution, Washington.
Souder, W. 2000. A Plague of Frogs : The Horrifying True Story. Hyperion Press, New York, NY.