DENVER -- West Nile virus, which has spread west since it surfaced in New York City in 1999, is expected to reach Colorado by late summer, a state health expert warns.

Mosquitoes spread the virus by feeding on infected birds. In the most severe cases, the virus can cause deadly encephalitis, or inflammation of the brain.

So far, seven people have died from the virus in New York. Doctors said that less than 1 percent of those infected develop severe illness.

The emergence of the virus in New York was the first time it had been found in North America. It spread south -- along the Atlantic seaboard -- and west, along the Gulf of Mexico since then (pictured, left).

"It's pretty hard to predict what this virus actually is going to do," John Pape, an epidemiologist with the Colorado Department of Public Health and Environment told the Colorado Springs Gazette.. "But, given the patterns of movement we've seen over the last couple of years, I think it's very possible it could reach here, especially our eastern plains, by sometime late this summer."

West Nile virus is a concern for horse owners. Hundreds of horses have been infected in Florida, which has been hit especially hard, Pape said. About a quarter of infected horses die.

There's no doubt West Nile virus will stretch into Colorado, Jack Galt, a veterinarian with Rocky Mountain Equine Clinic in Monument, Colo. It's just a question of when. "If it doesn't make it this year, it will be here next year for sure."

Additional Resources:

• CDC: West Nile Virus
• West Nile Virus FAQ