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Wildlife Rehabilitation Center of Northern Utah

Mystery solved: West Nile virus killed Utah bald eagles

By Amy Joi O'Donoghue and Alex Cabrero  |  Posted Dec 31st, 2013 @ 10:58pm


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SALT LAKE CITY — State wildlife officials hope the West Nile virus that has felled 27 American bald eagles in Utah will leave with the infected grebes that brought it here during their winter migration.

"The problem will leave with the grebes," said Mark Hadley, spokesman with the Utah Division of Wildlife Resources, adding that the eared grebes are slated to leave the state by mid-January.

Hadley said wildlife biologists suspect the eagles contracted the virus after eating infected eared grebes that died recently on Great Salt Lake.

Laboratory results announced Tuesday from testing performed at the Utah Veterinary Diagnostic Laboratory in Logan and the National Wildlife Health Center in Madison, Wis., ruled out many other possible causes of death, including avian influenza.

West Nile virus activity this time of year in Utah is a likely a first, according to JoDee Baker, an epidemiologist with the Utah Department of Health.

"In fact, we haven't ever in Utah seen something like this before," she said.

West Nile virus is spread to humans through the bite of infected mosquitoes, which are gone with the first hard freeze, Baker said.

Both Baker and Hadley stress there is no threat to public health, but people who come across any more infected eagles should contact the state wildlife agency as a precaution against other infections.

Dr. Bruce King, state veterinarian with the Utah Department of Agriculture and Food, said domestic livestock are safe, too, as well as horses and barnyard chickens.

West Nile virus and birds

"West Nile virus has been detected in variety of bird species. Some infected birds, especially crows and jays, are known to get sick and die from the infection."

The virus "is transmitted to birds through the bite of infected mosquitoes," and "mosquitoes become infected by biting infected birds. Some birds that are predators ... or scavengers ... may become infected after eating sick or dead birds that carry the virus."

"Since West Nile virus was discovered in the U.S. in 1999, it has been detected in over 300 species of dead birds. Although some infected birds, especially crows and jays, frequently die of infection, most birds survive."

Source: CDC.gov

Officials say they remain uncertain how the eagles got West Nile virus, as the disease typically affects birds during warmer months, when mosquitoes that carry the disease are active.

"(The grebes) might have gotten it farther north and flown in with it," Hadley said. "They can carry the virus within them for a long time before they develop symptoms and die."

Leslie McFarlane, the division's wildlife disease coordinator, said more than 2 million eared grebes stop at the 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, we send some of the dead birds to a laboratory for testing," McFarlane said. "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.”

Although the mystery behind the 27 bald eagle deaths appears to have been solved, more deaths could continue, Hadley warned, until the grebes leave.

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