Pity the frog.
These beloved amphibians have endured so much stress in the last two decades that it’s a wonder they’re still singing. They’ve lost huge amounts of habitat to development. Whole wetlands are gone, and the vernal ponds where they breed and escape predators are disappearing so fast that many frogs must make do with concrete stormwater-retention areas instead of leafy natural habitats. Because they easily absorb chemicals through their skin, everything hurts more: toxins from air pollution, pathogens from waterways, and even pesticides in the environment that have been known to alter hormones and turn males into females.
Now a virus that’s relatively new to the Chesapeake Bay region is putting an additional stressor on these amphibians. Ranavirus is a nasty and often fatal infection that can kill 90 percent of the tadpoles in a particular pond. The annihilation takes place within 72 hours, and the animals are often decomposed by the time anyone discovers them.
“It kills almost 100 percent of the juveniles,” said Scott Smith, a Maryland Department of Natural Resources wildlife ecologist who is part of a five-state research project to learn more about the virus.
According to Smith’s work, which the U.S. Fish and Wildlife Service is funding, ranavirus was present in 50 percent of the 62 sites examined in Maryland, Delaware and New Jersey. (The study is also looking at sites in Pennsylvania and Virginia.) Though juvenile wood frogs appear to be the most susceptible, ranavirus has also been found in salamanders, turtles and other amphibians. The sick turtles and salamanders contract the disease when they eat the larvae of infected frogs or swim in tainted ponds.
Death from ranavirus is swift for tadpoles. They will begin swimming erratically. Their bellies will become red with hemorrhages and bloated. And then slicks of tadpoles will form on the top of ponds, decomposing.
“The die-offs are very, very rapid,” Smith said. “It’s easy to miss them, because they occur unseen, deep in the woods, and then they decompose.”
Some residents have been calling the Maryland Department of Natural resources to question why they haven’t heard as many frogs this year, or why the peeping hasn’t happened in the usual places. Smith said he couldn’t conclude that whole populations are being wiped out from the ranavirus, because where frogs go depends on the weather and an ever-changing habitat. But the virus could play a role, as could another stressor, the chytrid fungus, which has been around for close to a century.
“We are seeing more ranavirus, but is it because we’re looking for it, and more aware of it? I’m not sure,” Scott said. “But I would not be surprised if, when we’re done with our study, we’ll find ranavirus in every county in Maryland, and almost every county in the region.”
Scientists confirmed Maryland’s first case of ranavirus in 2001, in Prince George’s County. A few years later, they confirmed a major die-off in box turtles from the disease in neighboring Montgomery County when biologists moved large populations to build the Inter-County Connector. Since then, it’s turned up in several state forests, parks and nature centers.
At Oregon Ridge Nature Center in Cockeysville, executive director Winnie Tan said Smith has tested several tadpoles and confirmed cases of ranavirus.
Tan said she and her staff are taking precautions to contain the spread. They sanitize their nets and their shoes. Because many visitors tromp through the woods to look for frogs, Tan said she is planning to put up signs to remind visitors to take precautions.
At Anita C. Leight Estuary Center on Otter Point Creek, which is 33 miles east of Oregon Ridge, park manager Kriste Garman said she hasn’t seen evidence of the virus yet. But with a confirmed case of the virus in a box turtle nearby, the staff is concerned.
“It certainly is something we are very, very aware of. We have all sorts of protocols to prevent the spread. It’s a pretty serious issue that we’re trying to deal with,” she said.
Evidence suggests that bullfrogs, which carry the virus but don’t show symptoms of illness from it, may be spreading it as they travel to mate.
Mike Clifford, who chairs the educational committee of the Virginia Herpetological Society, said he’s most worried about susceptible populations of rare animals, like the tiger salamander. If 90 percent of those juveniles die, the species could go extinct — there simply aren’t that many left.
“Right now, as far as native amphibians of Virginia and Maryland, there are none of them that have gone extinct. That would be the worst thing we would worry about,” Clifford said. He would hope that the species become resistant to ranavirus and that future generations will be able to live with it.
Clifford’s own yard, in a rural area outside of Richmond, has proved a decent lab for assessing the health of amphibians. This year, he’s seen two emaciated toads, but the pickerel frogs look as healthy as ever. Could ranavirus have infected the toads? Clifford wonders, but he can’t say for sure.
“It’s not like chickenpox where you can look at the animal and say, ‘that’s ranavirus,’” Clifford said. “It takes a lot of testing. We really just don’t know.”