KIMBALL, Neb. -- Three horses and a crow infected with West Nile virus have been found in Colorado, marking the virus' westernmost spread since it was first detected in the United States three years ago in New York.

INTERACTIVE All About West Nile Larvicides May Help Control Virus Discuss: Virus Overblown? Worried About West Nile Virus? Preventive Measures? RESOURCES Colorado Hotline: CO-HELP or 877-462-2911 Fight the Bite Colorado Symptoms CDC Cases By County General Info Insecticide Tips Horse Info

Health officials confirmed Thursday that samples taken from one dead crow and two horses in Weld County and one horse in Pueblo County tested positive for the mosquito-borne virus.

One horse in Weld County and the Pueblo County horse have died, while the third horse is recovering. The crow was found within the Greeley city limits.

Preliminary testing for all four animals was completed Wednesday, and further testing to confirm the results was completed Thursday, said Dr. Ned Calonge, the state's acting chief medical officer.

"This has been something we have known is coming," he said. "Even with the presence of the virus in our state, the chances of any one person becoming seriously ill is remote."

At least 12 humans have been tested for the virus and found to be free of it. Samples from other birds are still being tested.

The virus most likely was carried into Colorado by migratory birds moving along the Platte and Arkansas rivers, Calonge said.

Colorado is the 38th state including the District of Columbia where the virus has been found this year. It has killed seven people in Louisiana and two in Mississippi. A total of 156 people have been infected in eight states and the District of Columbia, the CDC said.

The virus has been moving west. It was confirmed this month in animals in Kansas and Nebraska, including Kimball County, Neb., 16 miles from the Colorado state line.

The elderly, the very young and those with weakened immune systems have the greatest risk of infection. Symptoms include a fever, headaches and body aches. Severe infections can result in encephalitis, an inflammation of the brain, or meningitis, an inflammation of the brain's lining and the spinal cord. There is no treatment or cure for the disease caused by the virus, Calonge said.

Calonge said at least 80 percent of people bitten by an infected mosquito never exhibit symptoms. Most of those who get ill recover within seven to nine days, he said.

He said chances are low that any humans would be infected in Colorado. Epidemiologist John Pape said the type of mosquitoes that carry the virus generally stop feeding on blood by early to mid September. The drought has reduced mosquito populations.

The CDC said there is no evidence to suggest the virus can be spread from animal to animal, person to person or animal to person. State veterinarian Dr. Wayne Cunningham said protective steps have been taken at the site of next week's Colorado State Fair in Pueblo, and no scheduled activities have been changed.

Calonge urged preventive steps including use of mosquito repellent, removing any standing water where mosquitoes can breed and wearing protective clothing. Mosquitoes that carry the virus are most active at dawn and dusk, he said.

He also asked that people who find dead crows, jays or similar birds report them to local health officials for possible testing.

Vaccinations for horses are available, but the two-shot series must be completed over at least three weeks to be fully effective, said state veterinarian Wayne Cunningham. A human vaccine is being developed, Calonge said.

The state Agriculture Department has warned veterinarians to watch for telltale signs of the West Nile illness in horses, and eastern Colorado residents have been urged to contact health officials if they find dead birds.

The state has been testing 23 chicken flocks kept in strategic locations to determine whether certain disease pathogens including West Nile virus enters the state.

Earlier this month, the CDC announced an additional $10 million will be sent to states to fight infected mosquitoes. The agency already handed out $17 million a year, but as more states requested help, the fund didn't cover requests. About $3.7 million will be earmarked for the hardest-hit states, Louisiana and Mississippi.

WNV History

West Nile virus was first detected in North America in New York in 1999. It has since been detected in at least 35 states. An infected horse died Wednesday in southeastern Kansas.

Louisiana officials have confirmed 85 human cases in the state, 59 of them involving encephalitis, a potentially deadly inflammation of the brain. They are waiting for test results on 90 other blood samples.

A Centers for Disease Control and Prevention report released Thursday said there had been at least 112 human cases of the virus this year. After the report was written, human cases were reported in Indiana, Alabama and Washington, D.C.

Most people bitten by an infected mosquito will suffer no more than flulike symptoms, but the weak and the elderly can get encephalitis. Officials in a number of states have boosted mosquito-spraying and urged people to protect themselves by using bug repellent and wearing long sleeves.