Epidemiologists expect West Nile Virus, the mosquito-borne disease that is seldom fatal to humans but often deadly for horses, to arrive in Garfield County this summer.

“We expect it to show up, but we don’t know the level of activity,” said John Pape, an epidemiologist with the Colorado Department of Public Health and Environment.

Garfield County officials are considering whether to use chemicals or naturally produced larvaecides to keep down mosquito populations, said Mary Meisner, public health director for the county.

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“We want something environmentally friendly,” Meisner said.

Veterinarians urge horse owners to get their horses vaccinated well in advance of the mosquitoes’ mid-May arrival. Dr. John Canning, a Carbondale veterinarian, said two vaccinations, three weeks apart, are required. Horses need one booster shot every year after that.

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Canning said mosquitoes should arrive earlier in the Rifle area than Carbondale due to the climate. But as a general rule, he recommends the vaccinations be completed by the first of April.

Last year, the vaccinations were $20 to $30 each, Canning said, but the drug’s manufacturer will increase prices on April 1.

“Just in time for mosquito season,” Canning said.

West Nile Virus was first discovered in the United States in 1999, and by 2002 made its way to California, said Pape (pronounced like poppy).

Last summer the virus spread into eastern Colorado and the Front Range, and Mesa and Moffat counties on the Western Slope.

Mosquitoes also infect birds with the virus, and birds in turn can spread the disease to other parts of the country. The specific mosquito is the culex tarsalis, Pape said. Not all mosquitoes carry the disease.

Pape said 80 percent of the people bitten by infected mosquitoes won’t even know they have West Nile Virus. Of the remaining 20 percent, 19 percent may develop a fever, headache and muscle pain. The remaining 1 percent is at risk for developing encephalitis, a dangerous inflammation of the nervous system, including the brain and spinal cord.

“Then it’s serious,” Pape said.

Older people are more likely to fall into that deadly 1 percent.

“The older you are, the less able you are to handle it,” Pape said. “Most of these people are over 50, so these folks need to pay more attention.”

Meisner and other health officials advise people to wear mosquito repellent when they go outside, and avoid going outside early in the morning or in the evening when mosquitoes are active.

In horses, West Nile Virus will kill about 30 percent of its victims. Other mammals, such as cattle, are not susceptible to the virus.

“A cow and a horse can be standing next to each other in a field. The horse will get the virus and the cow won’t. We don’t know why,” Pape said.

Horses cannot spread West Nile Virus to humans, or to other mammals. “It’s a dead-end disease,” Pape said.

Pape and Canning don’t expect infected mosquito populations to be spread evenly across Garfield County.

“There could be a hot spot in one area, and nothing 20 miles down the road,” Pape said.

Mosquitoes lay their eggs in standing water, where they turn into larvae before reaching adulthood, so irrigated fields are more likely to produce mosquitoes than drier areas.

Pape said epidemiologists don’t know how far into the high mountain valleys the mosquitoes will fly. “We have no idea what it’s going to do in those valleys,” he said.

Some mosquitoes are infected with West Nile Virus at birth if the mother was infected, while others catch it after feeding on birds or horses.

Meisner said West Nile Virus will be the focus of this year’s National Public Health Week, which takes place April 7-12.

Contact Lynn Burton: 945-8515, ext. 534

lburton@postindependent.com