By Laura Zuckerman

Dec 31 (Reuters) - An unprecedented wintertime outbreak of West Nile virus has killed more than two dozen bald eagles in Utah and thousands of water birds around the Great Salt Lake, state wildlife officials said on Tuesday.

At least 27 bald eagles have died this month in the northern and central parts of Utah from the blood-borne virus, and state biologists reported that five more ailing eagles were responding to treatment at rehabilitation centers.

The eagles, whose symptoms included leg paralysis and tremors, are believed to have contracted the disease by preying on sick or dead water birds called eared grebes that were infected by the West Nile virus, said Leslie McFarlane, Utah wildlife disease coordinator.

Some 20,000 of the water birds have died in and around the Great Salt Lake since November in an outbreak that may be a record in North America, McFarlane said. Initial testing suggested an infectious bacterial disease such as avian cholera caused the deaths, but findings released on Tuesday showed West Nile virus was the culprit, McFarlane said.

The dead birds do not pose a risk to people, Utah Health Department epidemiologist JoDee Baker said in a statement. Yet Baker urged those who find sick or dead birds to avoid handling them.

Utah wildlife specialists said bird deaths tied to West Nile virus were unusual in wintertime in Utah since mosquitoes - the primary vector - are not usually active during colder months.

'NOT BEEN SEEN BEFORE'

McFarlane said Utah had an unusually warm fall that extended the breeding season for mosquitoes to late October. But scientists may ultimately be unable to determine if grebes infected by West Nile virus migrated to Utah or if they contracted it there, she said.

West Nile virus, which was first detected in North America in 1999 and has since spread across the continental United States and Canada, can live for a few days in carcasses of infected birds. It can be transmitted to birds of prey and scavengers that feed on them, according to the federal Centers for Disease Control and Prevention.

But the epidemic in Utah may be unprecedented in North America for the masses of birds killed over a broad geographic area and for the number of bald eagles affected, said McFarlane.

"This is really kind of undocumented. Eagles have been known to feed on birds infected with West Nile virus but the transmission hasn't happened on this large of a scale. And the total number of birds we're talking about is on a grand scale that may not have been seen before," she said.

Additional testing of grebes and eagles is underway to shed more light on the mix of factors that converged to trigger the extensive die-off and to determine how much of a risk it might pose to other types of birds.

More than 2 million eared grebes stage at the Great Salt Lake amid a yearly winter migration from Canada and U.S. states west of the Mississippi River, according to the Cornell Lab of Ornithology in Ithaca, New York.

The water birds are expected to end their stopover in Utah and fly to the Southwestern United States and Mexico the second week of January, reducing the disease risk to bald eagles, McFarlane said.

From 750 to 1,200 bald eagles migrate to wintering grounds in Utah each year, she said.

Bald eagles, the national symbol of the United States, were removed from the federal threatened and endangered species list in 2007 after they soared back from near extinction. (Reporting by Laura Zuckerman; Editing by Steve Gorman, Dan Whitcomb and Sandra Maler)