OGDEN -- The mysterious illness that has been afflicting bald eagles in Utah for the past month is no longer a mystery, but the fact that West Nile virus is lingering into the winter is unusual.
Laboratory results have confirmed what officials with the Utah Division of Wildlife Resources have been suspecting: West Nile virus killed dozens of bald eagles in Utah.
As of Tuesday morning, the number of dead eagles had reached 27. Twenty-one of those birds were found dead in the wild. Six additional birds died while being treated at rehabilitation centers.
Testing at the Utah Veterinary Diagnostic Laboratory in Logan and the National Wildlife Health Center in Madison, Wisc., has definitively ruled out many other possible causes of death, including toxic chemicals or poisons, lead poisoning, bacterial infections and several other viruses.
The possibility of West Nile virus as the cause of illness had crossed the minds of staff members at the Wildlife Rehabilitation Center of Northern Utah in Ogden, executive director Dalyn Erickson-Marthaler said.
"It didn't take us too off-guard," Erickson-Marthaler said of Tuesday's released findings, "but we had not put a lot of weight into it because it is December."
The rehabilitation center has received nine affected bald eagles to date, she said. Of those, four are still alive and receiving treatment at the center as of Tuesday.
"They appear to be responding well," she said.
Officials aren't certain how the eagles got West Nile virus, as the disease typically affects birds during warmer months, when mosquitoes that carry the disease are active. They think the birds might have contracted the virus after eating infected eared grebes that died recently on Great Salt Lake.
Leslie McFarlane, wildlife disease coordinator for the Utah Division of Wildlife Resources, said more than 2 million eared grebes stop at Great Salt Lake during their winter migration. Almost every year, about 1 percent of the population that visits the lake dies from a bacterial disease called avian cholera.
"Every time grebes die," she said, "we send some of the dead birds to a laboratory for testing. Usually, avian cholera jumps out as the cause of death. This year, though, the initial laboratory results were not as conclusive."
In the winter, bald eagles obtain most of their food by eating dead animals. Since all of the eagles that have died have been within flying distance of the lake, McFarlane thinks the eagles might have contracted West Nile virus after eating grebes that died at the lake from the disease.
However, there are no human health concerns.
JoDee Baker, epidemiologist with the Utah Department of Health, said people do not need to be concerned -- dead grebes and dead eagles do not pose a risk to people.
"People become infected with West Nile virus after being bitten by a mosquito that carries the virus," Baker said. "Although there are other very rare ways you can get the virus, mosquitoes are by far the most common method of transmission."
Since the mosquitoes that transmit West Nile virus aren't active in the winter, there's no risk to the public, Baker said.
Still, Baker and McFarlane encourage people not to touch sick or dead birds, including eagles. Instead, they say people should call the nearest DWR office. A wildlife officer or a biologist will be dispatched to retrieve the bird.
Dr. Bruce King, state veterinarian with the Utah Department of Agriculture and Food, said domestic livestock are safe, too.
"Because mosquitoes aren't active in the winter," he said, "we see no imminent danger to domestic livestock in Utah, including backyard chickens, horses, or other small or large farm operations."
McFarlane said the migration of eared grebes through Utah is almost over for the winter. However, West Nile virus can live for a few days in the carcass of a bird that has just died, so there's still a chance that additional eagles will get sick and die, even after the grebes leave.
She said between 750 and 1,200 bald eagles visit Utah during winter.
"Even though it's difficult to watch eagles die," she said, "the deaths that have and still might occur won't affect the overall health of the bald eagle population that winters in Utah or the overall population in the United States."
Contact reporter Bryon Saxton at 801-625-4244 or firstname.lastname@example.org, or follow him on Twitter at @BryonSaxton.