Deformed frogs leap into spotlight at health workshop

Continental-extent patterns in amphibian malformations linked to parasites, chemical contaminants, and their interactions

Widespread observations of malformed amphibians across North America have generated both concern and controversy. Debates over the causes of such malformations-which can affect >50% of animals in a population-have continued, likely due to involvement of multiple causal factors. Here, we used a 13-year dataset encompassing 53,880 frogs and toads from 422 wetlands and 42 states in the conterminous USA to test hypotheses relating abnormalities and four categories of potential drivers: (i) chemical contaminants, (ii) land use practices, (iii) parasite infection, and (iv) targeted interactions between parasites and pesticides. Using a hierarchically nested, competing-model approach, we further examined how these associations varied spatially among geographic regions. Although malformations were rare overall (average = 1.6%), we identified 96 hotspot sites with 5%-25% abnormal individuals. Using the full dataset of 934 collections (without data on parasite infection), malformation frequency was best predicted by the presence of oil and gas wells within the watershed. Among collections also examined for parasite infection (n = 154), average parasite load and its interaction with pesticide application positively predicted malformations: wetlands with a greater abundance of the trematode Ribeiroia ondatrae were more likely to have malformed amphibians, but these effects were strongest when pesticide application was also high, consistent with prior experimental research. Importantly, however, the influence of these factors also varied regionally, helping explain divergent results from previous studies at local scales; parasite infection was more influential in the West and Northeast, whereas pesticide application and oil/gas wells correlated with abnormalities in the Northeast, Southeast, and western regions of the USA. These results, based on the largest systematic sampling of amphibian malformations, suggest that increased observations of abnormal amphibians are associated with both parasite infection and chemical contaminants, but that their relative importance and interaction strength varied with the spatial extent of the analysis.

Vitellogenin of the northern leopard frog (Rana pipiens): development of an ELISA assay and evaluation of induction after immersion in xenobiotic estrogens

An immunoassay for leopard frog (Rana pipiens) vitellogenin was developed for studying endocrine disruption. Male frogs were injected with estradiol-17β to stimulate vitellogenin for purification. SDS-PAGE revealed high amounts of a 170-180 kDa protein, which was confirmed to be vitellogenin by Western blotting. Vitellogenin was purified by DEAE chromatography and used to generate a polyclonal antibody. A competitive ELISA was developed for leopard frog vitellogenin with a detection limit of 6.0 ng mL(-1) and a working range of 20-1000 ng mL(-1). The intra-assay coefficient of variation averaged 5.47% for control sera and 9.71% for estrogen-treated sera. The inter-assay coefficient of variation averaged 8.21% for control sera and 9.93% for estrogen-treated sera. Recovery of purified vitellogenin averaged 95.2%. Vitellogenin was measured in male frogs immersed in the estrogenic compound diethylstilbestrol (DES) for various times and doses. Serum vitellogenin was detected within five days after immersion in 1.0 mg L(-1) DES and levels continued to increase through 20 d. In a 20-day dose-response experiment, serum vitellogenin was detected in frogs immersed in 0.01 mg L(-1) DES and vitellogenin concentration increased with dose. Immersion of frogs in one of several xenobiotic estrogens (nonylphenol, octylphenol, bisphenol-A) for 20 d did not increase vitellogenin for any treatment, suggesting that this frog may be less sensitive than fish to endocrine disruptors. Vitellogenin induction in R.pipiens may be a useful amphibian model system for field studies of endocrine disruption, due to its broad geographic range.

Localized hotspots drive continental geography of abnormal amphibians on U.S. wildlife refuges

Amphibians with missing, misshapen, and extra limbs have garnered public and scientific attention for two decades, yet the extent of the phenomenon remains poorly understood. Despite progress in identifying the causes of abnormalities in some regions, a lack of knowledge about their broader spatial distribution and temporal dynamics has hindered efforts to understand their implications for amphibian population declines and environmental quality. To address this data gap, we conducted a nationwide, 10-year assessment of 62,947 amphibians on U.S. National Wildlife Refuges. Analysis of a core dataset of 48,081 individuals revealed that consistent with expected background frequencies, an average of 2% were abnormal, but abnormalities exhibited marked spatial variation with a maximum prevalence of 40%. Variance partitioning analysis demonstrated that factors associated with space (rather than species or year sampled) captured 97% of the variation in abnormalities, and the amount of partitioned variance decreased with increasing spatial scale (from site to refuge to region). Consistent with this, abnormalities occurred in local to regional hotspots, clustering at scales of tens to hundreds of kilometers. We detected such hotspot clusters of high-abnormality sites in the Mississippi River Valley, California, and Alaska. Abnormality frequency was more variable within than outside of hotspot clusters. This is consistent with dynamic phenomena such as disturbance or natural enemies (pathogens or predators), whereas similarity of abnormality frequencies at scales of tens to hundreds of kilometers suggests involvement of factors that are spatially consistent at a regional scale. Our characterization of the spatial and temporal variation inherent in continent-wide amphibian abnormalities demonstrates the disproportionate contribution of local factors in predicting hotspots, and the episodic nature of their occurrence.

DDTs in rice frogs (Rana limnocharis) from an agricultural site, South China: tissue distribution, biomagnification, and potential toxic effects assessment

Contamination with agricultural pesticides such as dichlorodiphenyltrichloroethane (DDT) and its metabolites, dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD), is among several proposed stressors contributing to the global declines in amphibian populations and species biodiversity. These chemicals were examined in insects and in the muscle, liver, and eggs of rice frogs (Rana limnocharis) from the paddy fields of an agricultural site in South China. The ΣDDT (sum of DDT, DDE, and DDD) concentrations ranged from 154 to 915, 195 to 1,400, and 165 to 1,930 ng/g lipid weight in the muscle, liver, and eggs, respectively. All the DDTs (DDT, DDE, and DDD) showed higher affinity for the liver relative to muscle tissue and can be maternally transferred to eggs in female frogs. The average biomagnification factors for DDTs ranged from 1.6 to 1.9 and 1.5 to 2.9 in female and male frogs, respectively, providing clear evidence of their biomagnification from insects to frogs. Compared with the reported DDT levels demonstrated to have toxic effects on frogs, DDTs in the present frogs are unlikely to constitute an immediate health risk. However, the adverse impacts of high DDT residues in eggs on the hatching success and their potential toxicity to the newly metamorphosed larval frogs should be assessed further.

Frog decline, frog malformations, and a comparison of frog and human health

The decline in frog populations and the increase in the frequency of frog malformations are discussed. Topics considered for analysis include chytridiomycosis, retinoids, UV-B radiation, chemical contaminants, environmental threats, introduced invasive species and predation, unsustainable use, and enigmatic decline. Care must be taken to distinguish between hypotheses, laboratory experiments, and the findings in feral frog populations. Clearly, the causes of population decline and malformations are heterogeneous. The subject of frogs and humans is addressed under three subheadings: the importance of frogs to human societies, medical implications of frog studies, and a comparison of frog and human disease factors.

Organochlorine insecticides, polychlorinated biphenyls, and metals in water, sediment, and green frogs from southwestern Michigan

In an attempt to explain the etiology of frog deformities and population declines, many possible causative factors have been examined, including the input of synthetic chemicals into aquatic systems, where frogs spend much of their lives, including their entire developmental stages. Deformities in populations of green frogs in wetlands of southwestern Michigan that are influenced by agricultural, urban, or industrial inputs were assessed in this study. Of the 1445 green frogs (Rana clamitans) examined, only four (0.3%) exhibited morphological deformities. This deformity rate is less than the recognized background level of deformities for this species, which is approximately 1%. Concentrations of organochlorine insecticides, polychlorinated biphenyls (PCBs), and metals were determined in water, sediment, frog eggs, tadpoles, and adult green frog tissues. Concentrations of all individual organochlorine insecticides in tissue were less than 6 ng/g, wet wt. Concentrations of sigmaPCBs in tissue did not exceed 100 ng/g, wet wt. Concentrations of toxic metals were less than the limits of detection. Because no significant numbers of green frog deformities were observed in this region, it can be assumed that at these low concentrations, physical malformations in green frogs should not be observed. Significance of study. This study provides information on the incidence of deformities in green frog populations in southwestern Michigan and offers background data on chemical residues in green frogs and their environment.

Ontogeny of B cells expressing IgM in embryonic and larval tissues of the American grass frog, Rana pipiens

Affinity-purified, fluorochrome-tagged F(ab')(2) antibody fragments specific for heavy (mu) chains of Rana pipiens IgM were prepared from hyperimmune rabbit sera. By using two-color immunofluorescent procedures we observed that (1) the first cells expressing IgM, termed pre-B cells, lack detectable quantities of membrane or surface IgM but contain detectable quantities of cytoplasmic IgM (smu(-)/cmu(+)), (2) sIgM(+) B cells were the second type of IgM containing cell to appear in development, and (3) plasma cells, which contain copious quantities of cIgM, were the final phenotype to appear in the development of B cells expressing IgM. These cells were first observed in the pronephros of the developing urogenital system. Shortly after their appearance in the pronephros, cells in B lineages were observed in the liver. These observations (1) are consistent with recent studies of B lymphopoiesis in the aorta-gonad-mesonephros (AGM) region in endothermic vertebrates, including mice, (2) suggest that there are fundamental ontogenetic and phylogenetic similarities between cells and tissues of developing vertebrate immune systems, and (3) evoke questions concerning the possible function(s) of lymphocytes in developing anurans up to metamorphosis and beyond.

The effect of trematode infection on amphibian limb development and survivorship

The causes of amphibian deformities and their role in widespread amphibian declines remain conjectural. Severe limb abnormalities were induced at high frequencies in Pacific treefrogs (Hyla regilla) exposed to cercariae of a trematode parasite (Ribeiroia sp.). The abnormalities closely matched those observed at field sites, and an increase in parasite density caused an increase in abnormality frequency and a decline in tadpole survivorship. These findings call for further investigation of parasite infection as a cause of amphibian deformities in other sites and species.