environment grow where one should be. In some cases, parasites can also lead to the absence of a limb. Increased numbers of parasites are probably coming from increases in snail populations, which act as a host for parasites, he says. But Michael J. Lannoo of Ball State University in Muncie, Ind., says the range erious deformities in frogs have 1750, "it's just not normal to see such of limb deformities seen in the Minnebeen reported in 56 out of 87 high percentages of deformed animals,'' sota frogs is beyond that reported for counties in Minnesota. They also says Tietge. David M. Hoppe, a herpetol- parasites. have been found in Wisconsin, South ogist from the University of Minnesota, Martin Ouellet, a veterinarian and Dakota, Vermont, Japan, and the Cana- Morris, who along with another wildlife graduate student at McGill University's dian provinces of Ontario and Quebec. biologist has been sampling frogs in Min- Redpath Museum in Montreal, thinks agSo far, deformities have been seen in nesota for the past 20 years, says it ricultural chemicals could be the cause. six frog species and appear to be more wasn't until 1994 that they found a sig- In studies in Quebec, he says, he found common in those that spend more time nificant number of deformities. that in ponds contaminated with pestiin water. There are two layers of concern about cides, about 12% of frogs have deformiIn late September, the Environmental frog deformities, says Tietge. "If it is ties compared with only 1 or 2% in Protection Agency held a meeting of fed- demonstrated to have a cause that would ponds in relatively pristine areas. eral, state, and Canadian officials and sci- pose a risk to human health, then obviLannoo also speculates that the cause entists to discuss possible causes and ously, it's going to create more concern must be some kind of chemical contamibegin design of a research program to and attention," he says. Even "if it's not nation. He points to methoprene as a address the problem. The attendees de- a health issue, we should not ignore it," possible candidate. Methoprene, a syncided that parasites or some kind of he adds, pointing out that frogs are an thetic analog of a natural insect juvenile chemical contamination is the most like- important part of ecosystems and frog hormone, came into widespread use in ly cause, says Joseph E. Tietge, a research populations with high levels of deformi- the mid-1980s on ponds and wetlands to biologist at EPA's Midcontinent Ecology ties will likely decline over time. keep juvenile mosquitoes from maturing, Division, Duluth, Minn., who organized Genetic causes of the deformities have and in dogs and cats to control fleas. It the meeting. been more or less ruled out. Debra L. Carl- does not have any known adverse effects The deformities, which usually occur son of Augustana College in South Dakota on mammals. in the frogs' hindquarters, include extra has been analyzing the chromosomes in The structure of methoprene is very legs, missing legs, misshapen legs, and legs tissues of deformed frogs and so far has similar to that of retinoic acid. "If retinosticking out from the body at odd places. found no genetic mutations that correlate ic acid is injected with a micropipet into Some frogs also have a missing eye or an with deformities. This means that the de- the limbs of developing frogs in the laboeye in an unusual place such as the back velopmental abnormalities must have their ratory, it produces a majority of the limb of the head. Surveys this spring found the origin in the egg or tadpole stage. deformities we're seeing in nature," Lanpercentage of deformed frogs in many Stanley K. Sessions, a professor of vet- noo says. "But it has not been shown Minnesota ponds ranging from a low of erinary medicine at Hartwick College, that you can repeat the retinoic acid ex10% to a high of 95%. The surveyed ponds Oneonta, N.Y., believes the problem is periment with methoprene and obtain are located in both agricultural regions and parasites. In a study on frog deformities the same results, nor has it been shown fairly pristine vacation areas. in Northern California, he found that that methoprene has been applied to the While isolated occurrences of de- cysts form around parasites after they en- wetlands where the frog deformities are formed frogs have been observed since ter a tadpole and cause multiple legs to showing up." Methoprene caused no problems when tested on frogs in the lab, says John P. Hopley, toxicologist at Sandoz Agro, Des Plaines, 111., which is one of two manufacturers of the chemical. As is common U.S. practice, these tests employed an African species in which 30% of the DNA differs from that in U.S. species. However, Sandoz is concerned, Hopley says, and is "following up on a big study done in Wisconsin," which looked at the effects of methoprene on many different species. Obviously, much work remains to be done before the frog mystery is solved. Frogs have not yet been examined for parasites, nor have frogs or the spring pond water where they spawn been analyzed for chemical contaminants. Bette Hileman Frog found In Minnesota pond has extra limbs.
Frog Deformities Pose A Mystery
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24 NOVEMBER 18, 1996 C&EN
