Another pesticide surprise

Investigation of from declines uncovers new toxic hazards for us as well.

by Peter Montague, from Rachel's Environment & Health Weekly

The decline and disappearance of frog populations worldwide remains a mystery, despite efforts by hundreds of scientists to determine the causes. The other major problem facing frogs massive deformities observed since 1995 among frog populations in California, Iowa, Minnesota, Missouri, Ontario, Quebec, South Dakota, Texas, Vermont, and Wisconsin1 is now better understood.

During the past six months, press interviews with research scientists, and published studies, have shed a bit of light on both problems though true consensus has not yet emerged on either one. No one is even sure whether the two problems are connected, though new evidence indicates they are.

Some scientists still doubt that frogs are actually disappearing worldwide. They prefer to believe that the simultaneous declines and disappearances of frog populations in North and South America, Europe, and Australia reported since 1980 are nothing more than the normal ups and downs of any wild population. However, Scientific American said in August that the "majority viewpoint" among scientists now is that the widespread declines and disappearances are "highly abnormal."[2] "I think we're close to consensus now," says David Wake, a well-known frog researcher at the University of California at Berkeley.[3]

There are roughly 5,000 species of amphibians worldwide. Of these, 242 inhabit the United States. A recent study by the Nature Conservancy and the Natural Heritage Network identified 92 of these 242 (or 38 percent) as endangered, imperiled, or vulnerable[2] (meaning they are likely to become extinct within 5, 20, or 100 years if present trends continue.)

James La Clair at the Scripps Research Institute in La Jolla, California, says, "Although amphibians have lived on this planet for over 300 million years, nearly 120 times [as long as] modern man, reports within the last three decades have shown that numerous amphibian species are either suffering from serious population loss or have disappeared altogether."[1] La Clair says there are very likely "a collection of causes," but one way or another they can all be traced back to "the expansion of humankind." Loss of frog habitat chiefly wetlands is probably the biggest single cause. Global warming and accompanying droughts may contribute because frogs develop from eggs that thrive in water. The artificial stocking of streams with trout and bass plays a role, too. Pesticides and other chemicals certainly exacerbate the problem (more on this below). Laboratory experiments have shown beyond doubt that ultraviolet light from the sun can interfere with the development of frogs' eggs.[4] Acid rain may contribute to the problem as well. Humans eating frogs' legs in large quantities are not helping. And there are other causes, such as infectious agents.

A group of Australian researchers reported this summer that they have identified one particular fungus that is killing frogs in locations as far apart as Queensland, Australia and Panama in Central America.[5] The fungus which has never before been reported to harm any vertebrate species causes changes in the skin of frogs, somehow contributing to their deaths. The mechanism is not understood, but frogs breathe oxygen through their skin and the fungus may cause suffocation.

No one knows why an ancient fungus would suddenly start killing frogs in places as far apart as Australia and Panama. It is conceivable that the fungus was transported to these places only recently on the boots or equipment of researchers studying the disappearance of frogs. Another possibility is that the fungus has been present in these locations for a long time but frogs are now succumbing to it because their immune systems have been impaired by recent changes in the environment. One candidate would be increased ultraviolet light, which is well-known to damage the immune systems of many animals, including frogs. In recent years, chlorinated chemicals released by humans have thinned the protective layer of ozone in the upper atmosphere, thus allowing about 10 percent more ultraviolet light from the sun to reach the surface of the Earth.[6]

Certain industrial chemicals released into the environment may also be damaging the immune systems of frogs. One particular class of chemicals called retinoids has come under strong suspicion because retinoids can cause severe birth defects in many animals, including frogs and humans. The medicine Accutane, prescribed for treating acne, is a retinoid known to cause major birth defects in humans.

The deformities now being found in large numbers of frogs at many locations in the United States and Canada are grotesque. Herpetologists (scientists who study amphibians and reptiles) have reported finding frogs with missing legs, extra legs, misshapen legs, paralyzed legs that stick out from the body at odd places, legs that are webbed together with extra skin, legs that are fused to the body, and legs that split into two halfway down. They have also found frogs with missing eyes and extra eyes. One one-eyed frog in Minnesota had a second eye growing inside its throat.

Dr. David Gardiner, a research biologist at the University of California at Irvine, has been studying retinoids for at least a decade, and in recent years he has probed frog deformities.[7] To him, retinoids are the obvious culprit in the mystery of the misshapen frogs because of the peculiar kind of limb deformities being observed. "There is no other known mechanism for this [besides retinoids]," Gardiner says. "Much of early development is controlled by retinoids," he says. "Our body [and the body of a frog] is completely dependent on them," he told a reporter.[8]

Exposure to retinoids could also make frogs more susceptible to infectious diseases, Gardiner says: "The kinds of chemicals that would target development of limbs would target all organ systems," including the immune system. Frogs with abnormal legs would also very likely have abnormal immune systems. This could explain why some frogs are now suddenly falling victim to infectious agents that they resisted for millions of years.

James La Clair and his associates at the Scripps Research Institute in La Jolla, California, recently showed that a popular antimosquito insecticide, called S-methoprene, breaks down in the environment to several different kinds of retinoids.1 Under laboratory conditions, La Clair was able to show that the ultraviolet light in sunlight causes S-methoprene to break down into half a dozen retinoids, and that these retinoids in turn can cause frog deformities of the kind being seen now in the United States and Canada.

S-methoprene was introduced in the 1970s to control mosquitoes, which breed in water. It is sold under trade names like Altosid, APEX, Diacon, Dianex, Kabat, Manta, Minex, Pharoid, Precor, Yuvemon, and ZR 515.

It is also widely sold in flea powders. La Clair calculates that the amount of flea power used to treat a ten-pound pet one time contains enough S-methoprene to contaminate 110,000 liters of water to a level that would cause deformities in frogs.[1]

S-methoprene is also widely used in agriculture to treat cattle gazing areas, tobacco, and certain grain crops. It is also sometimes added to cattle feed.

S-methoprene mimics a hormone that inhibits developing pupae from molting; thus it is known as an "insect growth regulator." Because vertebrate species do not have a pupal stage of growth, scientists assumed S-methoprene could not harm amphibians or mammals. When fed to mammals, S-methoprene is about as toxic as sugar.

Now La Clair's work has shown that this seemingly-harmless chemical can be transformed into a potent teratogen by exposure to sunlight for just a few hours. The implications of this research, which was reported in Environmental Science & Technology, a journal of the American Chemical Society, are profound. For one thing, it means that once again the pesticide regulators at U.S. Environmental Protection Agency (EPA) have missed a key feature of a chemical whose safety they regulate. Secondly, it shows once again that relying on risk assessment leads to bad public health decisions. EPA's risk assessments have routinely failed to evaluate the breakdown byproducts of the pesticidal chemicals that the agency has deemed safe enough to allow as residues on our dinner plates. Third, it means that thousands of pesticides now in common use need to be retested to see if their breakdown byproducts are dangerous to humans or other species. However, this additional testing is unlikely to occur any time soon because EPA currently estimates that it is at least 15 years behind schedule in safety-testing the pesticides to which we and the frogs are currently being exposed.[9]

Indeed, the situation is worse than the agency makes it out to be. Congress ordered EPA to reevaluate and modernize all pesticide safety tests in 1972, and it demanded that the agency complete the job by 1977. Since 1972 the Agency has been doing its best to comply, but each year new revelations have come to light, new evidence showing that pesticides can harm humans and the environment in ways that no one imagined, so additional tests have been required. Thus La Clair's work is just the latest surprise in a long chain of unpleasant surprises. EPA officials in 1996 estimated that they will complete their pesticide safety reevaluations (which they were ordered by Congress to complete in 1977) in the year 2011 34 years late IF they can keep the work on schedule.9 Meanwhile, the frogs and we continue to be exposed to thousands of poorly-understood government-approved industrial poisons.

In sum, Dr. La Clair's research into the deformed frogs of North America serves to remind us that pesticides are now too dangerous to be safely regulated, even by the most powerful government the world has ever known.

Or is it that pesticide manufacturing corporations are now too dangerous to be safely regulated, even by the most powerful government the world has ever known? It's a fair question.

  Reprinted from Rachel's Environment & Health Weekly, with permission. Subscription information may be obtained by writing to: PO Box 5036, Annapolis, MD 21403-7036.
  1. James J. La Clair and others, "Photoproducts and Metabolites of a Common Insect Growth Regulator Produce Developmental Deformities in XENOPUS," Environmental Science & Technology Vol. 32, No. 10 (1998), pgs. 1453-1461.
  2. Rodger Doyle, "Amphibians at Risk," Scientific American, August, 1998, pg. 27.
  3. William Souder, "Evidence Grows, Suspects Elusive in Frogs' Disappearance," Washington Post July 6, 1998, pg. A3.
  4. Andrew R. Blaustein and others, "UV repair and resistance to solar UV-B in amphibian eggs: A link to population declines?" Proceedings Of The National Academy Of Sciences Vol. 91 (March 1994), pgs. 1791-1795.
  5. Lee Berger and others, "Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America," Proceedings Of The National Academy Of Sciences Vol. 95 (July 1998), pgs. 9031-9036. See also Jocelyn Kaiser, "Fungus May Drive Frog Genocide," Science Vol. 281, No. 5373 (July 3, 1998), pg. 23; and see Carol Kaesuk Moon, "Newly Found Fungus Tied to Vanishing Frog Species," New York Times June 28, 1998, pg. unknown. This is not the first fungus linked to frog deaths; see Andrew R. Blaustein and others, "Pathogenic Fungus Contributes to Amphibian Losses in the Pacific Northwest," Biological Conservation Vol. 67 (1994), pgs. 251-254.
  6. J.B Kerr and C.T. McElroy, "Evidence for Large Upward Trends of Ultraviolet-B radiation Linked to Ozone Depletion," Science Vol. 262 (November 12, 1993), pgs. 1032-1034. See also Mario Blum-thaler and Walter Ambach, "Indication of Increasing Solar Ultraviolet-B Radiation Flux in Alpine Regions," SCIENCE Vol. 248 (April 13, 1990), pgs. 206-208.
  7. See
  8. Maggie Fox, "Common chemical may be to blame for dead frogs," Reuters wire service August 5, 1998.
  9. John Wargo, Our Children's Toxic Legacy; How Science And Law Fail To Protect Us From Pesticides (New Haven, Conn.: Yale University Press, 1996), chapter 5