Amphibian abnormalities: understanding relationships between multiple environmental factors and nation-wide patterns of disease

Background/Question/Methods

A central principle in disease ecology is that environmental conditions often moderate the likelihood and severity of disease occurrence. The relationship between environment and amphibian abnormalities—including missing, extra, or deformed limbs—is poorly understood at a broad spatial scale. Recent work has revealed abnormalities often cluster in space, but vary substantially through time and within spatial clusters. This pattern suggests that broad-scale underlying risk factors combine with dynamic local phenomena to drive abnormalities. Regional-scale studies support several hypotheses for causes of amphibian abnormalities, including infection by the trematode parasite Ribeiroia ondatrae, eutrophication, contaminants, and sub-lethal predation (as main effects or interactions). We combined multiple modeling techniques to evaluate the relative roles of broad-scale, local, and temporally variable environmental factors on amphibian abnormalities using a 10-yr, nationwide dataset of amphibians (48,081 frogs) collected on U.S. National Wildlife Refuges. We evaluated two types of abnormalities to distinguish between potential causes: malformations (developmental limb anomalies) and surficial abnormalities (e.g. cysts, bifurcated tails). 

Results/Conclusions

Preliminary analyses in the lower 48 states revealed a strong relationship between the presence of R. ondatrae parasites and skeletal malformations. This relationship was mediated by longitude, suggesting different factors may dominate across space. Overall, skeletal malformation prevalence was higher on mudstone substrates, and was positively associated with spring temperature and precipitation. Breeding sites on low-permeability mudstone with high spring rainfall likely have longer hydroperiods than sites on permeable substrates with little rainfall, increasing frogs’ exposure to detrimental environmental factors and the likelihood that multiple factors coincide locally. In contrast, surficial abnormalities were primarily related to spring rainfall and the proximity of breeding sites to roads, suggesting a combined effect of contamination from road runoff and hydroperiod. Together, these results suggest that exposure time combined with additional factors (e.g. parasites, contaminants) that are coincident in space and time may drive the dynamics of amphibian abnormalities, and lend insight into factors that may assist in disease mitigation.