Utah wildlife experts believe they have solved the mystery of what killed at least 29 bald eagles over the last month: West Nile virus.
The majestic birds, the national symbol of the United States, apparently became infected after eating smaller birds with the disease, according to the Utah Division of Wildlife Resources.
In December, hunters and farmers across five counties in northern and central Utah began finding the normally skittish raptors lying, listless, on the ground. Many suffered from seizures, head tremors and paralysis in the legs, feet and wings.
Several ailing birds were taken to the Wildlife Rehabilitation Center of Northern Utah, where most died within 48 hours. In a release, wildlife officials said a recent die-off of eared grebes that stop at Utah's Great Salt Lake was the most probable culprit.
Each year, 2 million grebes visit the region. Most years, a small percentage that visit the Great Salt Lake die from avian cholera, Leslie McFarlane, wildlife disease coordinator for the Utah Division of Wildlife Resources, said in a news release.
"Every time grebes die, 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. That led us to believe that something else might have killed the grebes this year," McFarlane said.
Between 750 and 1,200 bald eagles visit Utah in the winter, when the predators eat mostly dead animals. Since all of the eagles that have died have been within flying distance of the lake, McFarlane said she believed the eagles might have contracted West Nile virus after eating grebes in the area.
West Nile virus usually affects birds, including eagles, during warmer months, when mosquitoes that carry the disease are active. Officials say the sick birds do not pose a risk to either humans or livestock.
Testing at the Utah Veterinary Diagnostic Laboratory in Logan and the National Wildlife Health Center in Madison, Wis., ruled out such possible causes as toxic chemicals, poisons, bacterial infections and several other viruses.
West Nile virus can live for a few days in the carcass of a bird, so there's still a chance that additional eagles will get sick and die, even after the grebes leave. But the risk to eagles should decrease quickly.
"Even though it's difficult to watch eagles die," McFarlane 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."
Utah rehabilitation centers are still treating five sick eagles, which appear to be responding well to treatment, the news release said.