Testing has been completed on several of the 27 bald eagles that have died in Utah, and West Nile virus has been found to be the result of those deaths. Biologists believe the eagles contracted the disease after eating eared grebes that had been infected with West Nile.
After a flurry of bald eagle deaths, some were sent for testing to the Utah Veterinary Diagnostic Laboratory in Logan and the National Wildlife Health Center in Madison, Wis. Those labs ruled out other possible causes of death such as lead poisoning, bacterial infections, avian influenza and other illnesses.
Mark Hadley, public information officer with the Utah Division of Wildlife Resources, said biologists think it is likely the bald eagles ate eared grebes that also tested positive for West Nile virus that had died in the Great Salt Lake. Because of migration patterns, however, Hadley said it is at least good news that the outbreak occurred when it did.
"The grebes should be migrating out within a week or so," Hadley said. "The timing is good because, for the bulk of bald eagles, their migration peaks around mid-February. The vast majority that is going to come [to Utah] are still on their way here."
West Nile virus can live on in the carcass of a dead bird for several days, so eagles can still contract the virus by eating grebes even after the rest of the flock has migrated.
Hadley said the West Nile virus arrived in Utah around 10 years ago and is "probably not going to go away." However, biologists still do not know whether the eared grebes contracted West Nile in Utah or carried it in from out of state.
"We haven't had anything happen like this in Utah, and there's been nothing nationally that's happened [like this]," Baker said.
The Dept. of Health conducts West Nile monitoring and Baker said only seven human cases were reported in 2013. The current outbreak poses no risks to humans, however, as mosquitoes that carry the virus are not active during the winter, he said.
Hadley said waterfowl can carry the disease for a while and can transmit it either orally or while in a large group. Biologists don't expect a die-off like this every year, however, and this outbreak should not affect the overall bald eagle population.
"The winter population of [bald] eagles is between 750 and 1,200," Hadley said. "It's sad and hard to see eagles die, but when you look at the overall number, it won't affect the overall health of the population."
Biologists still expect additional bald eagles to contract West Nile and Hadley said it will take some time for the risk to be gone.
The DWR still urges people to contact one of their regional offices if a dead or sick bald eagle is found. Once contacted, a biologist will be dispatched to remove the bird.