VOA常速英语:Reptiles, Amphibians in US Succumbing to Deadly Ranavirus

Since the mid 1990s, a type of virus known as a ranavirus has been taking a devastating toll on reptiles and amphibians -- especially turtles, frogs, toads and salamanders -- in more than 20 states across the U.S. Hundreds of thousands of these animals have died from the lethal virus and the disease continues to spread. Scientists are stepping up their efforts to better understand and combat the pathogen.

Tracking the virus

A few years ago, Scott Farnsworth, a graduate student at Towson University in Maryland, was sent to a wooded park in Maryland to relocate box turtles safely away from a new highway.

Farnsworth and his team tagged 100 turtles with radio transmitters. But then the reptiles started turning up dead. And not just turtles. They began seeing massive die-offs of toads, young frogs called tadpoles and salamanders. Lab analyses showed the culprit was the ranavirus, a class of viruses that mostly infect cold blooded animals.

"It's pretty quick. We can go from seeing no outer signs," he explained. "To having complete mortality for all of the ones in the pond within a few days."

While amphibians die within hours of infection, box turtles can survive as long as a month. A lab test showed the animal died struggling to breathe. Ranavirus often infects amphibians during their egg and juvenile stages, leaving them unable to swim. But it affects only adult turtles. "It could send them on a glide path towards extinction," said Farnsworth.

Farnsworth carefully checks for signs of life at a pond where all the animals died last year.

A device records water temperature in the pond every hour so scientists can correlate the temperature to evidence of the virus. Nearby, Farnsworth finds salamanders developing their legs.

For now, they look fine, but in a few weeks the virus could show up again. Scientists don't know much about ranavirus, or how to eliminate it. "If this continues the way we've seen it, this could be devastating to several species," Farnsworth noted.

When a group of animals dies in large numbers, the animals that feed on them also decline. And the species that were food for them - such as mosquitoes and insects - flourish.

Ranavirus spreading

"Ranavirus infection in wild and captive amphibians is a very serious disease," said David Green is an animal disease specialist at the National Wildlife Health Center of the U.S. Geological Survey. "When the infection shows up in a wetland, we expect nearly 100 percent of the animals to be killed in that wetland."

Green says scientists have reported ranavirus in 25 US states and in several other countries. He predicts it will take years before scientists are able to control it.

Meanwhile, after a long day of study, Scott Farnsworth heads home. On his way, he checks on the red back salamanders. For now, they look okay.