Discovery of a Novel Alveolate Pathogen Affecting Southern Leopard Frogs in Georgia: Description of the Disease and Host Effects

In April of 2006, we observed southern leopard frog (Rana sphenocephala) tadpoles in a pond in northeast Georgia that were dying from an unknown pathogen. Examination of affected specimens, as well as PCR characterization, revealed that all were infected with a novel alveolate pathogen closely related to freshwater and marine eukaryotic organisms and, to a lesser degree, to members of the genus Perkinsus. This pathogen has been documented in numerous mortality events in anuran tadpoles in the United States, although it has not yet been named nor clearly described. We subsequently conducted a systematic survey of this and four other ponds in the same area to document the extent of the pathogen and to describe the nature of infections in leopard frog tadpoles. Of 87 live tadpoles examined, 25% were infected with the alveolate pathogen, based on visual inspection of tadpole liver tissue. Affected tadpoles frequently had enlarged abdomens, swam erratically, and could be captured by hand. All organs of infected tadpoles were infiltrated but typically to a lesser extent than the liver and kidneys, which often had hundreds of thousands of the spherical, 6-μm organisms. Infected tadpoles tended to weigh more than noninfected ones, likely due to the massive organ swelling that coincided with infections. Infected tadpoles did not differ in developmental stage from noninfected tadpoles. Infection prevalence varied widely among ponds, and in one pond, we witnessed a rapid die-off of R. spenocephala tadpoles during our surveys, although we did find infected metamorphic frogs. The rapid mortality we observed as well as the vast number of organisms seen in specimens suggests that this pathogen has tremendous transmission potential, and therefore deserves further monitoring and study.