A Maine biologist says a virus-caused die-off of some 200,000 tadpoles in his backyard pond is the largest documented mass natural death of amphibians ever recorded in academic literature.

Bowdoin College professor Nathaniel Wheelwright documented the tadpoles’ deaths in June of last year and recently published a paper about it in academic journal Herpetological Review. The article says the deaths were likely due to ranavirus, a disease that can cause swift mass deaths of amphibians.

Wheelwright’s work is increasing interest in ranavirus among scientists around the country. Scientists have documented the disease in more than 20 states, and some believe it is everywhere in the continental United States.

Amphibians are the most imperiled class of vertebrate animals in the world, with about a third of them considered threatened.