Sterling Schaaf, manager of the Animas Mosquito Control District, said traps scattered throughout the district are yielding more of the pests than recent years.

The high water means more mosquito habitat. Schaaf said eggs laid decades ago can still hatch with sufficient moisture.

"They just lay there until the water's right," he said.

Other years that saw the mosquito population jump include 2005 and 1995.

In March, crews began treating standing water, such as ponds and stock tanks, with Altocid, a growth inhibitor that lasts months and prevents mosquitoes from reaching maturity. Later, crews spray for adult mosquitoes.

Schaaf said it has been several years since the district's traps yielded a mosquito that tested positive for West Nile.

The virus killed seven people in Colorado in 2007, the same number as the year before. Three cases were reported in La Plata County; none was fatal.

John Pape, an epidemiologist with the Colorado Department of Public Health and Environment, said factors that contribute to an increased presence of West Nile are moisture, warm temperatures and the population of birds, the species in which the virus originated.

"It's a fairly complex interaction," he said. "We know all the ingredients that go into the recipe."

Weld County, with 98 cases, was hardest hit by the virus last year. Boulder and Larimer counties were next, with 95 and 94 cases, respectively. State-wide, there were 576 cases, making it the second worst year since the 2002, when the virus first appeared in the state.

Pape said health officials have not registered any West Nile positive mosquitoes so far this year.

"The message is: There are definitely infected mosquitoes out there right now, but there aren't many of them, and the infection rates are pretty low," he said.

That won't be the case for long.

"They build up real quickly in July and August," he said.

Though last year the east side of the Continental Divide was hardest hit by West Nile, this year is anybody's guess.

"It varies from year to year," Pape said.

Only about 80 percent of people who contract West Nile get sick. Those who do can run a fever and experience fatigue while the sickness runs its course. In rare cases, the virus gets into the nervous system, causing meningitis or encephalitis.

There is no vaccine for West Nile, which was first detected in Uganda in the 1930s and now is found throughout the continental U.S. Here, the risk of infection begins to decline in early September, when mosquitoes are driven into hibernation by cooler temperatures.

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