ATLANTA (Reuters) - The West Nile virus is spreading faster and into new areas of the United States this year, prompting fears that the nation could be facing another record outbreak of the deadly mosquito-borne disease.

The Centers for Disease Control and Prevention, which has tracked the virus since it first surfaced in the Americas in 1999, reported on Thursday that the virus had killed four people and infected about 150 others in 16 states this summer.

At this same time last year, the CDC had confirmed slightly more than 100 cases in four states. The 2002 outbreak killed 284 people and infected about 4,000 others before becoming dormant in the winter.

The bulk of the record number of cases last year occurred in the six weeks after the first week of August, primarily in southern and midwestern states. This year's outbreak so far is centered in Colorado, which reported no human cases in 2002.

There have been 111 cases and four deaths due to West Nile in the state, according to Colorado health officials. The CDC, however, has positively linked only 72 cases and one death in the state to the virus.

"We are starting the epidemic with more cases and more areas affected than last year," Dr. Julie Gerberding, director of the CDC, said in a news conference in Atlanta. "If the same pattern proves to hold true, we could be seeing an even greater number of affected people (this year)."

Gerberding said it was not clear why Colorado had been so hard hit by West Nile this summer. Some have attributed the outbreak to the combination of a wet, cool spring and hot July as well as the tendency for the virus to emerge stronger in the year following its spread into an area.

West Nile was detected in some mosquitoes and animals in Colorado last year.

Most people who are bitten by a mosquito carrying the virus suffer nothing more than headaches and flu-like symptoms, but the elderly, chronically ill and those with weak immune systems can develop fatal encephalitis and meningitis when infected.

U.S. health officials have been particularly concerned with the virus' ability to spread through blood transfusions and organ transplants.

It is estimated that up to 200,000 Americans may have been exposed to West Nile since 1999, when it killed seven people in the New York City borough of Queens. An estimated 4.5 million Americans receive blood or blood products annually.

The U.S. Food and Drug Administration in cooperation with blood banks, laboratories and drug manufacturers has spearheaded development of new tests designed to screen blood for the virus.

The FDA also has issued tougher guidelines on blood donation since last year. In addition to new tools designed to find the virus, U.S. researchers are working on a vaccine that they hope would provide immunity to West Nile.

The CDC recommends that people take steps to drain mosquito-breeding areas in their yards and wear protective clothing and insect repellents containing DEET when outside. (Additional reporting by Keith Coffman in Colorado).

Source: Morbidity and Mortality Weekly Report, August 8, 2003.