Tadpole Slayer: Mystery epidemic imperils frogs

From Alaska to Florida, a novel and yet-unnamed protozoan is knocking off tadpoles. Species vulnerable to “the beast” belong to the genus Rana, which includes leopard frogs, green frogs, and bullfrogs, says ecologist John C. Maerz.

SICK TADPOLE. The organs of this river frog tadpole are riddled with tiny killer protozoa (inset). D. Stevenson/Univ. Georgia; (inset) K. Keel

His team at the University of Georgia in Athens stumbled across mass die-offs of southern leopard frog tadpoles in nearby ponds last year. Dissection showed the animals’ innards peppered with spherical, one-celled parasites. Genetic testing confirmed these are loosely related to Perkinsus, a disease-causing organism that affects marine shellfish.

Maerz’ group now offers the first published photos of the pathogen and descriptions of its effects in the September EcoHealth. Infected tadpoles become lethargic and developmentally stunted, the Georgia scientists report. Although the mystery parasite infects all organs, it clusters in the liver, sometimes tripling that organ’s size and giving the false impression that an animal is fat and robust. So many protozoa swamped and killed tissue in the liver of one sick tadpole, Maerz recalls, that throughout most of the organ “we could find no identifiable liver cells.”

He notes that his team did not discover the pathogen. It was first found by veterinary pathologist D. Earl Green of the National Wildlife Health Center in Madison, Wisc., part of the U.S. Geological Survey.

Since 1999, Green has quietly been recording a steady and growing incidence of the novel infection in frogs sent to his lab. All came from east of the Mississippi except for two outliers: frogs from Alaska’s Kenai Peninsula, several years ago, and one sample that he ran across 3 weeks ago from the West Coast.

Fueled by warm weather, “this infection kills steadily and slowly over the course of summer,” Green says. Although it targets tadpoles, there’s a chance that adults could also carry it and serve as amphibian Typhoid Marys.

When Green can steal a moment, he intends to publish his experiences with the pathogen—and name it. But that may require yet a bit more information on the shape of its mitochondria, explains Sanford H. Feldman of the University of Virginia in Charlottesville, a collaborator on Green’s studies. Feldman says his work indicates that “this wicked-looking organism is very primitive” and appears to “phylogenetically sit at the spot where animals and fungi diverged.”

It’s one of only three infectious agents capable of causing large die-offs of amphibians—almost all of which are in decline the world over. To date, the new protozoan has been reported only in the United States, Green says, where it has emerged as the “principal threat” that could lead to extinction of the Mississippi gopher frog. This amphibian’s sole wild population breeds in only one infected pond, where for at least 4 years virtually all tadpoles have died.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.