Malformed Frogs - What are Some of the Possible Causes?
 No one knows for sure yet what's causing
the frog abnormalities. It is likely that there is no "one" cause that will be responsible for all of the abnormalities observed. It is more
likely that several factors may be involved. Frog deformities have many causes, some natural and some quite unnatural. There are three leading
theories: ultraviolet radiation, parasites and "something in the water."
Theory #1: Ultraviolet Radiation
One theory is that ultraviolet (UV) radiation from the sun plays a role, either through direct toxicity or by
breaking down chemicals in the environment into other, unknown compounds. Ambient UV-B radiation will kill some amphibian eggs under field and
lab conditions. Ozone depletion of the atmosphere leads to more UV-B radiation striking the earth. In experiments with UV, the EPA lab in Duluth
has produced deformities in lab frogs that resemble some of those seen in the field. And a researcher in Oregon is accumulating evidence that
UV radiation deforms Pacific tree frogs there.
Additional Information:
Theory #2: Parasites
At least 41 species of trematodes (flatworm) have amphibian larvae as intermediate host (i.e. borrow into their tissues to form metacercarial cysts). A California researcher made news in 1999 with a study that established the role of parasitic flatworms (Ribeiroia) in malformations of tree frogs in California. Results from these studies reveal that 30-50% of the abnormalities observed are extra limbs. In Vermont, after examining over 7000 northern leopard frogs, only one frog had an extra limb. Researchers that have examined normal and abnormal leopard frogs from Vermont have not found a consistent association between abnormal frogs and trematode parasite cysts. Minnesota researchers have studied hundreds of frogs, and have found that both normal and abnormal frogs can be heavily, lightly or not at all infected with trematodes.
Additional Information:
 Theory
#3: Something in the Water - Contaminants
Numerous laboratory studies of contaminants amply demonstrate the susceptibility of amphibians to compounds such as metals, petroleum products, herbicides and pesticides. Researchers from the National Institute of Environmental Health (NIEHS) found that water from study sites in Minnesota and Vermont causes malformed frogs in the laboratory, and that water from "normal" sites (no malformed frogs) does not. Some of the chemicals in the water have been identified and tested for their ability to cause frog malformations. Moreover, filtering the water in these experiments through activated carbon removes the effect, indicating probable organic compounds. Also, adding thyroid hormone to the water moderates some of the effects. Thyroid hormone is essential for normal frog development, so this finding could point to chemicals in the water acting as endocrine disrupters. Several papers have been published on the results of this ongoing work.
Additional Information:
Link Between Pesticides and Abnormal Frogs?
Dr. Martin Ouellet, a researcher with McGill University in Montreal,
is convinced that a major factor in frog malformities is pesticides. His findings are based on years of research involving the evaluation
of over 30,000 frogs along a 150-mile stretch of the St. Lawrence River Valley. His studies show that the incidence of limb malformities averages
20% in agricultural regions subject to a variety of pesticides and other chemicals and only 1.5% in non-agricultural areas. On one farm in
the St. Lawrence Valley of Quebec, every frog he found was malformed. Dr. Ouellet's autopsy reports reveled three conditions: frogs that resemble
males externally but are actually females; frogs poisoned by clogged and yellowed livers; and, frogs with altered DNA.
Additional Information:
Updated: February 2001
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Water Quality Division 
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