Scientists find virus is responsible for killing amphibians

The US Geological Survey (USGS) has announced that a little-understood iridovirus disease has caused a recent die-off of amphibians in North Dakota, part of a spate of amphibian population crashes causing concern around the world.

Research by the USGS and other scientists has identified deadly virus infections and chytrid fungus as being the causes of some recent dramatic amphibian population declines around the world, but are actively investigating other possible causes. These include increasing exposure to ultraviolet radiation due to ozone thinning, the spread of non-native predators, contamination from pesticides and other chemicals, and rising temperatures. Many experts suspect that the deaths may be due to a combination of factors.

The latest research, identifying infection from a group of viruses called iridoviruses, was carried out at as a result of the dramatic decline in US western tiger salamanders at the Fish and Wildlife Service’s Cottonwood Lake Study Area in North Dakota. In July, when the salamander’s numbers should have been at a peak, researchers caught a total of eight animals in their three traps, compared to last year, when they captured between 100 and 150 per trap.

Scientists at the USGS have only been studying the problem for three years, and are unable to say whether the virus is spreading. “It may have been there all along, recurring every two to three years when populations are dense or abundant, or when new species turn up every five years or so,” USGS pathologist David Green explained to edie.

“We’ve been studying amphibians in these 17 wetlands since 1992 and have other long-term data from the area since 1967, and have never seen or recorded any die-offs due to disease,” said David Muchet, a USGS researcher. Because the salamanders also exhibit unusual skin abnormalities, USGS is conducting additional testing to rule out a concurrent infection or toxin.

Iridovirus disease specifically infects cold blooded animals, with body temperatures below 30°C such as fish, amphibians and even insects, Green explained. Die-offs associated with iridovirus disease tend to last just a couple of weeks, and result in the demise of huge numbers of animals, such as in a lake in Maine, where one local human resident is reported to rake up dead amphibians every morning.

The die-offs involve multiple species of frogs, toads, salamanders and one species of newt, are occurring on private, state, and federal lands, and even in remote and pristine areas, says the USGS.

As far as scientists know, deaths caused by iridoviruses have been with abundant amphibian species, though Green admits that more work is required before researchers can be certain of this. Populations of more rare species, however, appear to be being killed off by a skin condition caused by the chytrid fungus. This disease has been implicated as a likely cause of major amphibian die-offs in pristine areas around the globe, including the mysterious disappearance of the Costa Rican golden toad.

“We’re not sure how it kills amphibians, but it is quite lethal,” said Green, explaining that it may either produce a, or could interfere with some skin functions unique to amphibians, such as respiration or water absorption. Outbreaks of chytrid fungus are far more subtle than those of the iridovirus, with lower death rates, but lasting for several months.

“In the state of Arizona there are three species of frog that are rather unique to that state,” said Green. “All of them are in serious decline and serious trouble. We don’t know if we can save them.”

“The US Geological Survey is leading the Government’s efforts to help determine why amphibians are disappearing,” said Interior Secretary Bruce Babbitt. “This is a crisis that has attracted worldwide concern. It requires timely, aggressive research. It is no exaggeration to say that USGS research on these die-offs has global implications.”

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