wsmd home
public notices
dec home > wsmd home > biomonitoring & aquatic studies > malformed frogs
Malformed Frogs
leopard frog image
Malformed Frogs in Vermont

In the summer of 1996 occurrences of abnormal frogs were reported by the public to the Vermont Department of Environmental Conservation from 12 sites. The twelve sites reported to have abnormal frogs were all located within the Lake Champlain Basin and spanned over 120 miles of Lake Champlain from the northenmost sites close to the Canadian border to the southernmost site near the Poultney River. VT DEC staff surveyed four of the reported locations in 1996 and found abnormal frogs at all four of the sites. All of the abnormalities encountered were with young of the year northern leopard frogs (Rana pipiens). A total of 290 northern leopard frogs were collected at the four sites, 13.1% of the frogs had observable external abnormalities. Abnormality rates ranged from 5 - 23%.

frog missing partial limb & frog missing entire limb

In 1997 survey work targeting the northern leopard frog was conducted by VT DEC, USEPA, USFWS, USGS-BRD and Middlebury College. This set the frame work for additional surveys conducted through 2000. During the 1997 survey over 2500 northern leopard frogs were examined from 19 sites representing 13 towns in 5 counties of Vermont. Roughly 8% of the frogs examined had some category of abnormality. Laboratory experiments conducted on normal and abnormal frog samples from the 1997 sites indicated that the abnormalities were not caused by viruses, bacteria, or parasites.

measuring frogs

Abnormal frogs were found at 17 of the 19 sites surveyed. Categories of abnormalities were primarily missing and partial hind limbs and shortened and missing digits.

Systematic field surveys have now been conducted by VT DEC for five consecutive summers (1997- 2002) at a total of 20 sites within the Lake Champlain Basin. Over 11,000 recently-metamorphosed northern leopard frogs have been recorded. External abnormalities were detected in 628/11,186 Rana pipiens examined yielding an abnormality rate of 5.6%.

several abnormal frogs Categories of abnormalities were primarily missing and incomplete hind limbs and shortened and missing digits. Based on five years of seasonal sampling records, observed abnormality rates can vary considerably both seasonally and annually at sites. The data suggest that several sampling events at a site seasonally and annually may be needed before adequately characterizing a site for presence or absence of abnormal frogs.

x-ray of abnormal frogThe USGS-BRD National Wildlife Health Center (NWHC) has evaluated R pipiens specimens collected from sites in the Lake Champlain Basin in 1997 (pdf, 957 KB), 1998, and 1999. Radiographic examinations performed by NWHC support current theories that many of the abnormalities are caused by primary errors in development (malformations) as opposed to deformations which occur later and are mechanical. "Of the Vermont frogs missing entire limbs, x-rays revealed that 73% were also missing bones in the hip, providing evidence that a predator had not removed the limbs, but that developmental errors were to blame." (USGS News Release, "X-Ray Studies Shed Light on Frog Deformities", March 29, 2000).

Malformed, Deformed, and Abnormal Frogs frog missing entire hind limb

The term "abnormality" is often used instead of "deformity" or "malformation" because it covers all aspects of frog disfigurement. It is very difficult to determine if a frog is "deformed" or "malformed" without conducting detailed examinations using x-rays or other laboratory tests. The term "malformed" refers to an "early developmental error," and the term "deformed" refers to an abnormality that occurs later in development, such as trauma.



Natural Deformity Rate

Nationally the prominence of "deformities" has only appeared during the last 5 years. While there are historical accounts of malformed frogs in the literature the number of locations and proportions appear to be unprecedented.

A major discussion point among researchers has been whether there is a natural rate of malformations that can be expected among wild populations of frogs. Most researchers believe a 1 percent malformity rate might be considered normal.

Amphibians are Environmental Sponges two frogs in water

Amphibians are very vulnerable to the environment around them. Their eggs and embryo's lack protective shells, are laid in water, and are sensitive to chemical, UVB and other environmental assaults during early cell division. The permeable skin of amphibians is designed to efficiently exchange gases and water; however they must remain in frequent contact with water or wet soil to prevent drying out. Thus they are vulnerable to any environmental factors that affect the skin. Amphibians are environmental sponges, soaking up whatever chemicals and toxins are present in the soil and water.

Citizens Reports of Abnormal Frogs in Vermont (1996-2000) Bull frog image

Citizens have reported abnormal frogs from over 100 towns representing all 14 counties in Vermont . Most of these reports have not been verified. Seven species have been reported with abnormalities including: Northern leopard frog, Green frog, Wood frog, Mink frog, Bull frog, Pickerel frog and American Toad. Most abnormal frog reports from citizens are from a very small sample size. The reports are valuable, but may not be accurately representing the incidence of abnormal frogs above the normal background level of 1%. Surveys that collect and examine dozens of frogs from one site will provide more useful data. Click here for help identifying amphibians.

Non-Profit Organizations Survey Hundreds of Sites in Vermont citizens sampling frogs

Thousands of frogs have been collected and examined through the efforts of non-profit organizations. The National Wildlife Federation NWF) surveyed dozens of sites during the 1998 and 1999 season. Click here to go to NWF's website. Vermont Public Interest Research Group (VPIRG) has organized Vermont citizens of all ages to help survey frogs in Vermont for the last several years.

Research Conducted in 2001

VT DEC conducted a US EPA funded study in the spring of 2001, "Investigations into the Causes of Amphibian Malformations in the Lake Champlain Basin of New England". The study included chemical characterization of water and sediment from sites, laboratory limb development assays using northern leopard frogs and field surveys characterizing northern leopard frogs from tadpole to metamorph. The final report can be downloaded by clicking on the links below:

Reporting Abnormal Frogs and other Amphibians

To report abnormal frogs and other amphibians that you have found in Vermont contact Rick Levey. Please include your name, phone number, location, description of species and abnormalities observed.


Additional Information



Links of Interest



Suggested Reading


  • Investigations of Abnormal Northern Leopard Frogs in Vermont - Abstract from 43rd Conference of the International Association for Great Lakes Research

  • Fort, D.J.,T.L. Propst, E.L.Stover, J.C. Helgen, R.B. Levey, K. Gallager, J.G. Burkhart. 1999a. Effects of pond water, sediment and sediment extracts from Minnesota and Vermont, USA on early development and metamorphosis of Xenopus. Environ Toxicol Chem 18 (10):2305-2315.

  • Meteyer, C.U., R.A. Cole, K.A. Converse, D.E. Docherty, M. Wolcott, J.C. Helgen, R.Levey, L. Eaton-Poole, and J.G. Burkhart. 2000. Defining anuran malformations in the context of a developmental problem. Journal Iowa Acad. Sci. 107(3)72-78.

  • Meteyer C.U., K. Loeffler, J. Fallon, K.A. Converse, E. Green, J.C. Helgen, S. Kersten, R. Levey. L. Eaton-Poole, J.G. Burkhart. 2000. Hind limb malformations in free-living Northern Leopard frogs (Rana pipiens) from Maine, Minnesota, and Vermont suggest multiple etiologies. Teratology 62: 151-171.

VT DEC Watershed Management Division 1 National Life Drive, Main 2  Montpelier, VT  05620-3522  Tele: 802-828-1535   Fax: 802-828-1544

State of Vermont Agencies & Depts.     Access Government 24/7     About Vermont.Gov     Privacy Policy

A Vermont Government Website Copyright 2003 State of Vermont - All rights reserved