Tests have confirmed West Nile virus in several birds in Douglas County, the first to be reported there, and in five more horses in Churchill County, officials said Tuesday.
The disease already had been confirmed in birds in Carson City and Lyon County.
Douglas County mosquito-control expert Ron Lynch said he has found several dead birds in Carson Valley and at Kingsbury Grade and Lake Tahoe - two of which have tested positive for the virus.
Lynch found a Steller jay on Kingsbury Grade and a crow on County Road in Minden on Friday. It was confirmed Tuesday that both birds had West Nile virus.
Since then, Lynch has found two more dead crows, another on County Road and one on Circle Drive in Gardnerville.
"People saw them before they died and say they acted strange," Lynch said.
In Lahontan Valley in Churchill County, the mosquito-borne virus had already been confirmed in two other horses and several other equines are exhibiting symptoms of the sometimes-deadly illness.
A researcher with the Nevada Department of Agriculture said he received blood samples Tuesday from four more Churchill County horses suspected of suffering from the virus. Results from those samples should be in later this week, he said.
The symptoms in horses can vary widely, but often include massive disorientation and loss of muscle control.
While more humans die of the flu every year than from West Nile virus, horses have proven more susceptible to the ravages of the disease.
An estimated 30 percent of unvaccinated horses that show symptoms of the virus die. There is a vaccine for horses, but none for humans.
Since the virus was first confirmed by tests on a dead crow in Carson City a week and a half ago, about a dozen horses in the Lahontan Valley have either tested positive for, or are showing symptoms of, West Nile.
No other equine cases of the virus have been reported in Nevada so far.
While the virus is currently showing up more near Fallon than in other areas in Nevada, Churchill County Mosquito and Weed Abatement Manager Nancy Upham said, "Fallon's not going to be a hot spot."
Equine cases will almost certainly start showing up in other counties too, she said.
Since ill horses confirmed the presence of West Nile in Churchill County, the local mosquito abatement district isn't waiting for the test results on its "sentinel chickens." District officials are trapping mosquitos from around the valley in carbon monoxide traps borrowed from the state. Captured mosquitos will be tested to determine which mosquito inhabited areas contain the most West Nile-infected insects.
"Then we'll operationally prioritize those areas," Upham said.
The district is now switching its focus from killing mosquito larvae to killing adult mosquitos, starting in the places most infected with the virus.