Environmentally induced limb malformations in mink frogs (Rana septentrionalis)

Hind limb malformations in free-living northern leopard frogs (Rana pipiens) from Maine, Minnesota, and Vermont suggest multiple etiologies

BACKGROUND

Reports of malformed frogs have increased throughout the North American continent in recent years. Most of the observed malformations have involved the hind limbs. The goal of this study was to accurately characterize the hind limb malformations in wild frogs as an important step toward understanding the possible etiologies.

METHODS

During 1997 and 1998, 182 recently metamorphosed northern leopard frogs (Rana pipiens) were collected from Minnesota, Vermont, and Maine. Malformed hind limbs were present in 157 (86%) of these frogs, which underwent necropsy and radiographic evaluation at the National Wildlife Health Center. These malformations are described in detail and classified into four major categories: (1) no limb (amelia); (2) multiple limbs or limb elements (polymelia, polydactyly, polyphalangy); (3) reduced limb segments or elements (phocomelia, ectromelia, ectrodactyly, and brachydactyly; and (4) distally complete but malformed limb (bone rotations, bridging, skin webbing, and micromelia).

RESULTS

Amelia and reduced segments and/or elements were the most common finding. Frogs with bilateral hind limb malformations were not common, and in only eight of these 22 frogs were the malformations symmetrical. Malformations of a given type tended to occur in frogs collected from the same site, but the types of malformations varied widely among all three states, and between study sites within Minnesota.

CONCLUSIONS

Clustering of malformation type suggests that developmental events may produce a variety of phenotypes depending on the timing, sequence, and severity of the environmental insult. Hind limb malformations in free-living frogs transcend current mechanistic explanations of tetrapod limb development.

Effects of laboratory ultraviolet radiation and natural sunlight on survival and development of Rana pipiens

Changes in solar ultraviolet (UV) radiation have been proposed as a possible factor contributing to seeming increases in hind-limb malformations in anuran amphibians in North America. One purpose of this study was to reproduce results from an earlier experiment (G.T. Ankley, J.E. Tietge, D.L. DeFoe, K.M. Jensen, G.W. Holcombe, E.J. Durhan, and S.A. Diamond. 1998. Environ. Toxicol. Chem. 17: 2530–2542) in which Rana pipiens (northern leopard frog) developed hind-limb ectromelia and ectrodactyly when exposed to an artificial source of UV radiation in the laboratory. Additionally, we sought to assess the effects of natural sunlight on survival and development in this species. Animals exposed to UV radiation in the laboratory from stage 25–26 (just prior to hind-limb development) through forelimb emergence exhibited a dose-dependent elevation of hind-limb dysmorphogenesis. The observed malformations usually were bilateral and typically symmetrical, and consisted primarily of missing and reduced digits. Exposure of animals at this life stage to full intensity natural sunlight caused significant mortality, while exposure to screened sunlight (ca. 50–60% of ambient) resulted in an elevated incidence of abnormal hind limb pathology (ectromelia, ectrodactyly) identical to that observed in organisms exposed to UV radiation in the laboratory. Owing to uncertainties in dose extrapolation, for example, in the context of animal behavior, the potential significance of our results in terms of explaining adverse effects in amphibians from the field is as yet unclear.

Strategies for assessing the implications of malformed frogs for environmental health

The recent increase in the incidence of deformities among natural frog populations has raised concern about the state of the environment and the possible impact of unidentified causative agents on the health of wildlife and human populations. An open workshop on Strategies for Assessing the Implications of Malformed Frogs for Environmental Health was convened on 4-5 December 1997 at the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina. The purpose of the workshop was to share information among a multidisciplinary group with scientific interest and responsibility for human and environmental health at the federal and state level. Discussions highlighted possible causes and recent findings directly related to frog deformities and provided insight into problems and strategies applicable to continuing investigation in several areas. Possible causes of the deformities were evaluated in terms of diagnostics performed on field amphibians, biologic mechanisms that can lead to the types of malformations observed, and parallel laboratory and field studies. Hydrogeochemistry must be more integrated into environmental toxicology because of the pivotal role of the aquatic environment and the importance of fates and transport relative to any potential exposure. There is no indication of whether there may be a human health factor associated with the deformities. However, the possibility that causal agents may be waterborne indicates a need to identify the relevant factors and establish the relationship between environmental and human health in terms of hazard assessment.