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Deformities have been lurking in frogs for years, but it was the discovery of these frogs by a group of students in Minnesota that brought these strange frogs into the limelight. That Minnesota finding in August 1995 was two years ago, and the reports are still running rampant. Unusually high numbers of deformed frogs were found in Minnesota, Vermont, Wisconsin, and Quebec; clusters of deformed frogs were also found in California, Colorado, Idaho, Mississippi, Montana and Ohio. Most of the deformed frogs found were of the mink and leopard species, but the green and cricket frog have also been noted for a high percentage of abnormalities. Scientists speculate that those species were most vulnerable because they spend most of their lives in the water (Montague, 1996). The types of external deformities found in the frogs range from the absence of limbs, extra limbs where they shouldn't be, fused feet and legs, excessive skin, and missing or misplaced eyes, "including an eye that was found growing in a frog's throat"(Channel , 1997).

Unfortunately the frogs deformities did not end at external mutations. The frogs had distended bladders and digestive systems that didn't work properly and testis that were unusually thin, said Robert McKinnell, a professor of genetics and cell biology at the University of Minnesota (Montague, 1996). Even more alarming is the fact that there are also internal deformities found in frogs that appear to be normal. In some sites, normal looking frogs were tested and were found to have smaller than average jawbones, and unusual formations on their spines.


The discovery of deformed frogs are causing international alarm. Frogs are seen as the pH paper of the environment, and their deformities may reflect a poisoned environment. Which brings up the scary possibility that whatever is poisoning the frogs may be poisoning humans too. As Hillary Carpenter, a toxicologist for Minnesota Department of Health said, "What's driving this whole issue is not deformed frogs, it's the potential for effects on human health". (Meersman, 1997). There seems to be many theories to on what is causing the frog deformities. Some scientist believe man-made chemicals that were inadvertently released into the environment mimic the frogs' hormones and cause deformities, others blame natural parasites, increased ultraviolet radiation due to the hole in the ozone layer, viruses, algae blooms, predators, and pesticides. Until there is solid evidence each theory has equal merit. In this review I will evaluate the theories of pesticides, UV radiation, natural parasites, as causes of the deformities.


The effects of sunlight and development have been ignored by scientists, until recently. There are indications everywhere that it may be a factor in a lot of things, says Dave DeFoe, aquatic biologists at the Duluth lab (Meyers, 1997). Some scientists think the increases in ultraviolet light might be a link to the recent observations of developmental abnormalities (primarily limb deformities) in amphibian species. Andrew Blaustein, an ecology professor wonders if the thinning of the ozone layer might also have to do the eye problems found in some frogs. It has been documented that the recent increases in the intensity of UV light at many places around the world has corresponded with the incidence of deformities(Kerr, 1993). More importantly, some of the largest relative increases in UV light have been shown to occur in late spring and early summer (Herman et al. 1996), a period which coincides with reproduction and critical windows of development of amphibians species in northern latitudes.

Ultraviolet light can cause direct biological damage of structures ranging from oxidative stress to the direct damage of DNA through the formation of thymidine timers. There are a number of naturally-occurring and anthropogenic compounds that can greatly increase UV-induced damage by acting as photosensitizes; foremost among these are number of polycyclic aromatic hydrocarbons (PAH) (Ankley 1997). Unlike concentrations of most contaminants in the environment the levels of PAH have remained constant. Scientists think that this is due to a relatively constant input from sources like the combustion of fossil fuels.


Scientists have also looked at the idea that large doses of UV radiation combined with the pesticide methoprene might be the cause of the deformities. Methoprene is a pesticide universally used to control mosquitoes and is often used in water! The habitat in which, amphibians spend most of their lives or the most important stage in life, reproduction. Because the most important stages in a frog's life occurs in water, and their skin permeable to water, scientists are now focusing on what's in the water. This leads me to the second theory.


In nature frog mutations occur in about 1% of the population. In ponds where pesticides are used on the surrounding lands, 69% of the frogs were deformed (Reed, 1997). If pesticides are causing deformities then it has to be a new type of pesticide, because the deformities would have shown up earlier. This new theory rules out DDT or PCB, pesticides that have been used for decades. Hormones control the major changes in a frog's maturation, by controlling bodily processes. The deformities could be linked to chemicals that mimic growth hormones. Minnesota researches believe inherited genetic mutations are not the answer. So we can presume that whatever is causing the deformities occurs early in the developmental stage.

Scientists have been able to mimic the deformities found in nature in their labs by using retinoic acid. Retinoic acid is a by product of a chemical called retinoid, it is found in acne medications and skin-rejuvenating creams. Pesticides can mimic retinoic acid so it is able to cause defects in frog development. "We should start screening chemicals, and we should start with pesticides, to see if they mimic naturally occurring retinoid in the body," says David Gardiner, a molecular biologist from the University of California at Irvine (Reed, 1997).

Researchers have started to test the waters where deformed frogs were found. They took water from three northern Minnesota wetlands, where the frogs where found, and used mixed that water with tap water in different ratios. They then used mixtures to grow frogs' eggs. They also tested plain tap water, and 100% pond water. When researchers used mixtures with more than 50 percent, most of the African clawed frog embryos showed a wide range of deformities. The African clawed frogs are of a different species but also helps in lab researching. Some samples of plain tap water produced deformed embryos. Minnesota has distributed bottled water at four northern Minnesota households. These households were where tap water samples produced deformed embryos. Scientists are starting a chemical analysis of the water to try to figure out what specific contaminant is the cause. They will also try to determine what effect it has on humans.


There has been little testing to see if environmental chemicals cause birth defects in frogs, but recently a large study has linked birth defects in humans to pesticide use in Minnesota (Garry 1996). Researchers from the University of Minnesota and the EPA linked the Minnesota birth registry from 1989-1992. The results of the study showed two specific points.

Births in the general population of western Minnesota, where pesticides are used the most, showed an increase in birth defects, compared to the rest of the state. This was most evident in children that were conceived during the spring.

The birth defect rate for all birth defects was increased in children born to those who applied pesticides themselves, opposed to general population.

They also found that families who lived in predominantly agricultural regions of Minnesota were more likely to have children with birth "abnormalities". Researchers then divided the agricultural part of the state into smaller areas, based on the type of crops that are grown in each region. They then examined 12 specific herbicides. The most obvious results were obtained for the herbicides 2, 4-D and MCPA [4-chloro-2-methylphenoxyacetic acid]. Areas designated by the Minnesota Department of Agriculture as "high use" areas for these herbicides, were compared against "low use" areas. High-use areas showed an increased in birth defects (about an 86% increase) that ranged from central nervous system defects, circulatory, respiratory, urogenital, and musculoskeletal system. High-use areas also showed a 51 percent increase for all birth defects combined. The deformities found in frogs may be an alerting signal to us from the environment.


The main spokesman in this theory seems to be Stan Sessions, a developmental biologist from Hartwick College. He questions whether a chemical could be a cause of deformities because there are no deformed fish or animals found in the ponds where the deformed frogs were found. To prove his theory Session showed a three-legged frog to a groups of his colleagues. In the dissected frog leg, tiny parasitic flatworms were packed into a joint where the frog was missing its leg. Session then said that he could mimic the deformity by placing tiny plastic balls, in a frogs' joint. This caused it to sprout extra legs. Session says that cysts form around the parasite after it enters a tadpole. The cysts in turn disrupts the development of emerging limbs, causing them to sprout one or two legs where they shouldn't be (Sessions).

The flukes may be preying on the frog population to complete its reproductive cycle . The flukes, which live in intestine of garter snakes are excreted as eggs into ponds and eaten by snails. In the snail, the worm will multiply; one egg can hatch hundreds of thousands of larvae. "But almost no one else thinks it's that simple" Tietge said. " Parasites simply can't explain all of the problems. A few of the deformed frogs have parasites, but most haven't. And most frogs with parasites don't have deformities" ( Meyers, 1997).


At this point in time scientist are filled with speculation but no answers. It will take months maybe years before anyone will unlock the mystery behind the cause of deformed frogs. And in the time it takes for the mystery to be solved, new theories will be hatched. In the mean time one can only wonder what mother nature has in store for us, or what the consequences of our careless actions will be. The direction of our future lies in the fate of our little amphibian friends, and as Kermit the Frog said "It's not easy being green!


Ankely, Gerald T. "Are the in Ultraviolet Light a Plausible Factor Contributing to Amphibian Deformities?" (online research journal). NAAMP III- deformed frogs. 20 Nov. 1997. Available WWW:

Herman, J.R., P.K. Bhartia, J. Ziemk, Z. Ahmad and D. larko. 1996. UV-B increases (1979-1992) from decreases in total ozone. Geophys. Res. Lett. 23:2117-2120.

Kerr, J.B. and C.T. McElroy. 1993. Evidence for large upwards trends of ultraviolet-B radiation linked to ozone depletion. Science 262:1032-1034.

Meersman, Tom. "Confounded by Frogs:Deformed-'Frog is Short on Answers, Long on Speculation.' " Minneapolis Star Tribune. 17 Apr. 1997.

Montague, Peter. "Frogs Give Warning." PlanetENN (Oct. 21, 1996): 1-5. Online. Internet. 22 Nov. 1997. Available WWW:

Reed, David. "Mutant Frogs 'Scientists Link Deformities to Parasites, Pesticides' " The Salt Lake Tribune 1 May 1997. Online. Yahoo. 10 Nov. 1997. Available WWW:

Sessions, Stanley. "Evidence that Trematodes Cause Deformities, including Extra Limbs, n Amphibians" (online research journal. NAAMP III-deformed frogs. 20 Nov. 1997. Available WWW:

Vincent F. Garry and others, "Pesticide Appliers, Biocides and Birth Defects in Rural Minnesota," ENVIRONMENTAL HEALTH PERSPECTIVES Vol. 104, No. 4 (April, 1996), pg. 394-399.

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