West Nile virus was suspected when sick and dead eagles started to show up in early December, but wildlife officials were reluctant to identify the mosquito-borne virus as the culprit because it has never been reported so late in the year.
"This is a major development. This is not typically a time we would expect to see West Nile virus. We monitor West Nile every year, but during mosquito season. This is an unusual event," said Leslie McFarlane, wildlife disease specialist for the Utah Division of Wildlife Resources (DWR). "It is concerning and something we will have to watch for going into the future."
As the number of eagle deaths grew, the investigation turned to the ice-free waters of the Great Salt Lake. Migrating eared grebes gather on the lake in massive numbers approximately 2 million each fall and linger until their food sources diminish before continuing south.
It is common for large numbers of the grebes to die each fall, usually from avian cholera. McFarlane said it appears that up to 20,000 grebes died this year and the bald eagles quickly honed in on the easy meal.
"We became more suspicious when the eagles were showing up pretty much centered around the Great Salt Lake," McFarlane said. "We are not sure West Nile is killing the grebes, but they are definitely a carrier of it."
If it ends up that the eared grebes died from West Nile virus, it would mark the first time such deaths have been recorded in that species. The National Wildlife Health Center is performing tests on the grebes.
"People become infected with West Nile virus after being bitten by a mosquito that carries the virus," said JoDee Baker, an epidemiologist with the Utah Department of Health, in a release. "Although there are other very rare ways you can get the virus, such as receiving contaminated blood or organs from an infected person, 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's health."
McFarlane said an estimated 10,000 grebes remain on the Great Salt Lake, but they are likely close to flying south.
"As long as we have live grebes that are sick out there and infected dead birds that eagles will eat, we may continue to lose bald eagles," she said.
Efforts to clear the Great Salt Lake of the dead grebes would be expensive and a tremendous undertaking. Wildlife biologists believe the West Nile virus does not remain in the carcasses of dead animals very long.
"It doesn't do well in the environment without a host," McFarlane said.
The infected grebes could have contracted the virus before arriving at the Great Salt Lake, or could have been infected while staging for the migration. McFarlane said there is no way to tell.
Bald eagles also recently arrived in Utah. The state has a small population of resident bald eagles and the numbers are bolstered by up to 1,200 during some winters.
Wildlife officials say the loss of raptors from this event will not affect the overall health of wintering bald eagles in Utah or the overall population in the United States.
The migrating eagles tend to congregate on food sources like the grebes and an annual feast of carp at Farmington Bay Waterfowl Management Area in February. McFarlane said the chance of eagles passing West Nile virus to each other is possible, but not likely.
"It is possible they can get it through saliva and feces, but it has not been documented a lot in the wild," she said. "It does happen in captive care."
DaLyn Erickson, with the Wildlife Rehabilitation Center of Northern Utah in Ogden, has treated nine of the bald eagles. Five of them died or had to be euthanized. All were suffering from head tremors, lower body paralysis and an inability to digest food.
Erickson started treating four eagles in her care with anti-inflammatories last week and it appears to be working.
While she is glad to finally have a name for what is impacting the eagles, Erickson is not happy with the prognosis.
"I wish there was a cure for West Nile, but there is not," she said. "We are doing all we can to help them, but it is basically up to the bird's immune system if they can fight it off. All we can do is help them with that fight."
Three of the birds being treated at the Wildlife Rehabilitation Center of Northern Utah are now able to stand and are eating. A juvenile male that arrived at the center in a more compromised state is not faring as well, but has shown signs of improvement.
"West Nile does damage to the brain, the heart and other organs," Erickson said. "We need to see if that damage is permanent or if it will heal in each individual case."
Baker and McFarlane encourage people not to touch sick or dead birds, including eagles. Instead, call the nearest DWR offices in Ogden, Vernal, Springville, Cedar City or Price with the animal's location. The Help Stop Poaching Hotline, 1-800-662-3337, is another option on weekends and holidays and after hours.
State officials say this late case of West Nile virus should not concern people with livestock.
"Because mosquitoes aren't active in the winter we see no eminent danger to domestic livestock in Utah, including backyard chickens, horses or other small or large farm operations," said Bruce King, state veterinarian with the Utah Department of Agriculture and Food.
West Nile virus
West Nile is a mosquito-borne virus that can cause disease in humans, birds, horses, and some other mammals. The virus was first found in the U.S. in 1999 and in Utah in August 2003.
There is no proof that West Nile virus can spread from human to human or from animal to human. Infection occurs from being bitten by a mosquito; not all mosquitoes carry the virus.
Source: Utah Department of Health; Office of Epidemiology