DENVER -- West Nile virus has not reached Colorado yet, but officials say it is only a matter of time.

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The virus has spread to 34 states since it first appeared in New York in 1999. Five Louisiana residents have died, and dozens more have been sickened this summer in the worst U.S. outbreak to date.

What's delaying its arrival in the state, now expected in late August or September, is this summer's drought, experts say.

"Mosquitos that transmit this virus spend three of their four life stages in water, and so if there's a lack of water, generally the mosquito populations are down also," said John Pape, of the Colorado Department of Public Health.

Health officials are using sentinel chicken flocks, mosquito traps and dead-bird surveys to track the virus. So far, all the Colorado bird and mosquito test results have been negative.

But state health workers expect to perform more bird necropsies in coming weeks as residents become aware that dead crows and magpies may signal the arrival of West Nile virus.

Officials say a solitary dead crow or magpie is probably no cause for concern, but anyone who notices a dozen or more dead birds should contact health or animal-control officials.

Six Coloradans with encephalitis have been tested for West Nile virus, but there was no sign of antibodies to the virus, said James Beebe, chief microbiologist for the Colorado Department of Public Health and Environment.

In Colorado, the primary focus isn't about how the virus will affect humans but how it will impact livestock, 7NEWS reported.

Dr. John Young, a veterinarian, this summer has inoculated between 500 and 600 horses with a West Nile vaccine, 7NEWS reported.

"I'm trying to get them all vaccinated and have at least four weeks before we see the first case," said Young.

The virus is spread by mosquitoes and can infect animals and people.

"In the East, the cases that they have seen, about a third of the animals may die that get it, so it's a fairly significant disease," said Young.

Health officials say there is little chance of eliminating the virus, which causes flulike symptoms and sometimes deadly swelling of the brain.

But less than 1 percent of the people who get bitten and become infected will get severely ill, health officials say,

"So I guess that's the biggest message is that this is not a reason people should shut themselves in the doors, or be worried," said Pape.

But you can take steps to protect yourself against becoming infected by using insect repellent, wearing protective clothing and eliminate standing water in yards.