West Nile virus, a mosquito-borne disease that can cause brain infections in humans, is expected to move into Nevada this year.
But that's no reason to panic, said Environmental Health Specialist Michael Faisy.
"The disease is unique in the way it's spread, but most who acquire the virus won't show any symptoms," he said. "Of those who do, only a small percentage will develop encephalitis."
The Centers for Disease Control lists a total of 4,156 laboratory positives among humans in the United States last year, the bulk of the cases in Illinois, Michigan, Ohio, Louisiana and Indiana. Of those, 284 died.
"The states hardest hit are densely populated and located primarily in the East and South," Faisy said. "This area is still pretty rural so there's no need for panic."
He recommends minimizing exposure to mosquitoes. Patch any loose screens to keep them out of the house. When outdoors, use insect repellent, wear long-sleeve shirts and full-length pants, and eliminate standing water in eaves, troughs or any outside container. The incubation period for mosquito larvae is between five and seven days.
"Thousands of mosquitoes can breed in a Mason jar with 2 to 3 inches of water in it," Faisy said.
To monitor the disease, Carson City officials are working with Washoe, Douglas, Lyon and Churchill counties to trap mosquitoes.
Faisy said the insects are taken to the Animal Disease Laboratory in Reno and checked for the virus. Sentinel chicken flocks placed throughout the area will also be monitored. The closest flock to Carson City is in south Washoe Valley.
"If we find them there, it's a good bet the disease has spread throughout the area," he said. "We'll be keeping in touch throughout the season and notify the public if there are any positives."
The virus is transmitted primarily between mosquitoes and birds, which harbor it in the bloodstream for one to four days. During that period, the virus can be reintroduced to mosquitoes, but birds normally acquire a lifelong immunity.
Officials will spray standing water in the area, the largest around Silver Saddle and Anderson ranches.
A nontraditional pesticide called Altosid that affects only mosquito larvae is used to alter mosquitoes' hormonal balance so they can't develop.
A surfactant is also used to destroy the surface tension of the water, making it difficult for mosquitoes to lay eggs. Surviving larvae will not be able to come to the surface of water to breathe.
"These are nontraditional pesticides and aren't considered poisonous," Faisy said. " We don't use poisons much anymore."
The virus does not usually reproduce adequately in mammals and doesn't cause extensive illness in dogs or cats. Studies in an epidemic area in New York in 1999 showed that pets were frequently affected, but none actually acquired the disease.
Horses can be more seriously affected, the disease occurring in 5 percent to 10 percent of an uninoculated population. Studies from the Centers for Disease Control show that about 40 percent of the affected animals die.
"We expect a higher mortality rate in the wild-horse population," said Dr. David Thain, veterinarian for the Nevada Department of Agriculture. "Domestic horses get some kind of care, but that won't happen with the wild horses."
"Horses should be vaccinated now," he said. "The inoculation requires two shots, three to six weeks apart, and immunity is slow to develop. Usually, we don't see titers (a test to determine the level of antibodies formed) until three weeks after the second shot. The titer peaks in about eight weeks."
