A horse in Fallon was destroyed after it became severely ill and tests confirmed it was infected with West Nile virus.
Dr. David Thain, state veterinarian, said the 5-year-old stallion had not been vaccinated against the mosquito-borne disease.
A veterinarian in Fallon euthanized the horse Wednesday, and test results received late Thursday confirmed West Nile virus, Thain said.
State agriculture officials began urging horse owners last year to vaccinate their animals when the likelihood of the disease arriving in Nevada seemed evident. Mules and donkeys also should be vaccinated.
Experts say about 30 percent of horses infected with the disease die. But Thain said the vaccine is 95 percent effective. Vaccinated animals must get an annual booster shot.
Tests conducted on dead crows found in Carson City and Lyon County and on mosquitoes trapped in Lyon, Mineral and White Pine counties confirmed the disease's presence in all of those areas this week.
So far, two human cases have been reported in Nevada - one in Reno and in Las Vegas.
Thain told KOH Radio in Reno on Friday that the state is expanding mosquito surveillance south into Hawthorne and Esmeralda County as a result of the Las Vegas case because it may have originated in northern Nye County.
He said officials in Washoe County are going to help with additional trapping and testing of mosquitoes at night in Churchill County where the horse was stricken.