FORT COLLINS, Colo. -- Larimer and Weld counties declared health emergencies on Monday as the number of human cases of West Nile virus soared.

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"We felt that it was in our best interest to declare a health emergency to encourage the public to take this seriously," said Gaye Morrison of the Weld County Health Department.

Ann Watson of the Larimer County Health Department said many people infected with West Nile hadn't used mosquito repellant.

"Some of them even have outdoor jobs and say they have never used repellant," she said.

Fort Collins will start its first-ever mosquito-spraying effort on Wednesday.

"We're starting a targeting a ground-fogging program where we will spray pesticide into the air to kill adult mosquitoes in targeted parts of town," said Thomas Vosburg, Assistant City Mgr., Fort Collins.

Vosburb said the spraying would be carried out from 8 p.m. to midnight, starting in the southeast portions of Fort Collins.

Citizens who are concerned about the health effects of the mosquito-spraying program can call 9170-416-2242.

Larimer County reported 121 cases on Monday, more than double the number a week ago. Weld County reported 112, up from 83 on Friday.

Larimer, Weld and Pueblo counties each have reported two deaths as a result of West Nile virus. Boulder reported one death from the virus.

The latest figures from the Colorado Health Department show 338 people have been confirmed with West Nile virus this season. So far, the 78 percent of those with confirmed West Nile virus have come down with fevers. Only 13 percent of those with West Nile virus developed meningitis and 9 percent developed encephalitis, according to public health records.

The total cases are split almost evenly between women and men, and people infected with the virus range in age from 2 years old and 86 years old. The median age of all victims is 45 years old.

More Animals At Risk

West Nile virus has been found for the first time in alpacas in Colorado, raising concerns that more animals than thought are vulnerable to the disease.

The virus has infected hundreds of horses, donkeys, mules and birds.

Now, at least two alpacas in the Loveland area have been infected. Several more have died, possibly from the virus, said Wayne Cunningham, a state veterinarian.

"This is our first go at recognizing it in alpacas," Cunningham said last week.

Alpacas, native to South America and related to llamas, have long, fleecy wool. Experts said they likely haven't developed resistance to the virus.

House pets and pigs might be next -- or might already be infected.

Dogs, cats and pigs, which aren't tracked by the state, are probably infected with West Nile at high rates, said Richard Bowen, a professor of biomedical sciences at Colorado State University. Those animals usually don't show symptoms of the illness and rarely die.

The impact on such animals as alpacas is probably small, said Nicholas Komar, a research biologist with the Centers for Disease Control and Prevention in Fort Collins.