Shows Proximity to Ag and Lawns Doubles Risk of Frog Deformities
(Beyond Pesticides, December 2, 2005) A study published this month in Environmental Health Perspectives shows that proximity to agricultural land use alone or to ag land and lawns doubles the risk of limb deformities in amphibians. According to the authors, this study contributes to the study of human development and health hazards from pesticides and “demonstrate[s] the value of an epidemiologic approach to this problem.”
The study is based on the largest systematic sampling of limb deformities in wild amphibian populations to date with some 5,264 specimens collected from 42 Vermont wetlands. The authors, Brynn Taylor et al. of Yale University, analyzed the effects of multiple exposures while accounting for nontraumatic limb malformation and infection with the parasite Ribeiroia. A multiple logistic regression model that included human-associated land use as well as developmental stage, genus, and water-quality measures, then found an increased risk of deformity associated with a wetland’s proximity to agricultural land use and agricultural land use and lawns. The positive association is further maintained after adjusting for the effect of developmental stage and variation in water-quality measures such as nitrogen and pH.
The study highlights the leading notion that the increasing prevalence of amphibian limb deformities being discovered throughout the U.S. and Canada may likely have implications for human health. This study contributes significantly to the body of research, particularly as it is believed to be the first study to use a human epidemiologic methodology to assess an ecological problem, explain the authors. The use of multivariate techniques and large sample sizes to evaluate multiple stressors increase the dependability of the results.
The authors argue that exposure to sources of chemical pollution from runoff is a likely explanation for limb deformity. They also state that while the human health relevance remains to be determined, there is support for this information to shed light on human health risks. Numerous citations are provided of other studies that link pesticides (herbicides, fungicides, and insecticides) and their metabolites with effects on water quality, impaired hatching success, developmental problems, thyroid disruption, and impaired immune system defenses.
“The chemical teratogen hypothesis [that chemicals affect developmental malformations] has particular relevance to human health risk,” write the authors. “If waterborne chemical toxicants are involved in amphibian malformations, there is potential for shared exposure with human populations through dermal contact, ingestion, and inhalation routes.”
As a final disclaimer, the authors call for further study and warn against assigning a direct cause and effect based on the study since the malformations in ponds near agriculture could theoretically be due to other factors that were not assessed. “Water-quality measures,” state the authors, “are dependent on multiple factors and vary over a season and year to year depending on runoff sources, rainfall, temperature, and other factors” and the study relied on a limited number of measurements of water quality.
To review the study, see: Environmental Health Perspectives (Volume 113, Number 11, November 2005).