PORTLAND, Maine (AP) — A Maine biologist documented the die-off of some 200,000 tadpoles in a pond in his backyard, igniting new interest among scientists in ranavirus, a disease that can cause swift mass deaths of amphibians.

Bowdoin College professor Nathaniel Wheelwright recently published a paper about the die-off in the academic journal Herpetological Review, concluding the deaths in June of last year likely were due to ranavirus. It represents the largest documented mass natural death event of amphibians recorded in academic literature, he said.

"It was traumatic, and it was unexpected, and it was shocking," Wheelwright said of finding the pond full of thousands of tadpole corpses. "I had two thoughts — one, how sad to lose so many animals overnight, and two, what is the biology behind this strange event."

Wheelwright's work is emerging as scientists around the country are trying to learn more about ranavirus, which poses a threat to already struggling species of frogs, toads and salamanders. Amphibians are the most imperiled class of vertebrates in the world, with about a third of them considered threatened.

Scientists have documented the disease in more than 20 states, and some believe it is everywhere in the continental United States. The disease causes amphibians, especially larvae, to swell and hemorrhage. Whether cases of ranavirus are becoming more prevalent or if diagnosis is merely on the rise is a subject of current scientific inquiry, said Phillip de Maynadier, a biologist with the Maine Inland Fisheries & Wildlife Department.

De Maynadier said there have been a "flurry" of ranavirus die-offs in Acadia National Park over the last 15 years. However, he added that not enough data have been collected to determine if the disease is increasing in Maine or elsewhere. It's possible scientists are getting better at identifying it, he said.

"It's certainly well documented," he said. "We know where it's occurred. It's real. And it's very damaging that it does happen."

Ranavirus has existed in North America for at least 100 years, but it didn't come to the forefront of scientific research until about 15 years ago, said David Green, a veterinary pathologist with the U.S. Geological Survey based in Madison, Wisconsin. It has also been the subject of study in the mid-Atlantic states, where it has wiped out entire ecosystems of young amphibians. Many more mass die-offs of amphibians from ranavirus likely go unreported, Green said.

Some have suggested that warmer temperatures have stressed amphibians to the point where they are more susceptible to ranavirus. Green said additional stressors probably aren't necessary, as the disease is virulent enough to kill off amphibians without them. The big question is about the impact of ranavirus on species' populations declines, he said.

"We don't know how this is perhaps impacting populations biologically," Green said.

Wheelwright said he didn't see any young frogs this summer at his pond, suggesting the possibility of a "second summer where they got nailed."

Aram Calhoun, a biologist with the University of Maine who was studied ranavirus, said long-term data about ranavirus die-offs are needed to determine if the deaths in Wheelwright's pond are alarming. But amphibians are already imperiled by long-standing problems such as habitat loss, she added.

"What I don't think is we should be extrapolating what happened in his pool to all pools in New England and say that's a trend, because we really don't know that," she said.