Fragile frog populations, already in decline worldwide, face another threat in the form of a newly-identified and highly infectious tadpole disease, researchers say.

Researchers at the University of Exeter and the Natural History Museum in England say they've been conducting research on single-cell parasites called protists. Although only a single cell, they are complex, complete with DNA in their nucleus, the scientists say.

"Our work has revealed a previously unidentified microbial group that infects tadpole livers in frog populations across the globe, says Exeter researcher Thomas Richards.

The researchers tested tadpoles from six countries across three continents for the single-cell parasites.

A number of frog populations in both tropical and temperate habitats were found to be infected, which doesn't bode well for the world's frogs, Richards says.

"We now need to figure out if this novel microbe — a distant relative of oyster parasites — causes significant disease and could be contributing to the frog population declines," he explains.

The parasites are distantly related to Perkinsea sp., a marine parasite found in animals and algae, the researchers report in the journal Proceedings of the National Academy of Sciences.

The protists have been linked to mass mortality events in the United States, they say.

"There have been numerous outbreaks with this parasite, what we presume to be the same parasite, all over the eastern part of the United States," says University of Georgia wildlife ecology professor Michael Yabsley, a co-author of the study.

Amphibians, including frogs, toads, salamanders and newts, are among the most threatened species on Earth; in 2008, researchers classified 32 percent of the world's species as threatened or extinct, with an additional 42 percent considered in decline.

The new parasite is "just one more threat" to amphibians already decimated by climate change, habitat loss and fungal diseases, says Richards.

How big a problem the parasite might be is not clear, he says, noting that 99 percent of tadpoles don't survive to become frogs anyway because they fall victim to predators.

Still, Yabsley says, "It's certainly going to be one of the things we are worried about for the long-term health of amphibians."

Some scientists point to the massive decline in amphibian populations as proof the Earth may be going through the sixth "mass extinction event" in its history.