The West Nile virus's westward march has apparently reached Colorado, where health officials reported four animal cases Thursday.

If the cases are confirmed by the Centers for Disease Control and Prevention, Colorado would become the 38th and westernmost state to report the mosquito-borne virus this year.

CDC Director Dr. Julie Gerberding has said birds and mosquitoes could spread the virus to the Pacific Coast. The CDC lists 160 confirmed or probable human cases of the disease and nine deaths so far this year.

New York, which had the first U.S. cases of West Nile three years ago, reported its first human case of 2002 on Thursday in New York City. Illinois health officials reported three new human cases of the virus, all in the Chicago area.

Colorado health officials said a crow and two horses tested positive for West Nile in Weld County, just south of Cheyenne, Wyo. A third horse was found with the disease in southeastern Colorado.

"This has been something we have known is coming," said Dr. Ned Calonge, the state's acting chief medical officer. "Even with the presence of the virus in our state, the chances of any one person becoming seriously ill is remote."

The virus was first detected in the United States in 1999. From that year through 2001, the CDC confirmed 149 human cases and 18 deaths.

Most people who get it see mild flulike symptoms, or no symptoms at all. But it can cause encephalitis, a potentially fatal swelling of the brain, especially in older people and people with weakened immune systems.