West Nile virus spreads west, as predicted: peak disease season.(News)

West Nile virus case counts rose exponentially during August as the arbovirus made its predicted shift to the western United States, and health officials braced for an onslaught of new cases throughout the warm, mosquito-friendly months of early autumn.

Colorado remained the hardest hit state, reporting 263 of the nation's 883 confirmed cases as of Aug. 25. South Dakota, with 157 cases, and Nebraska, with 130, were also stricken disproportionately with the arbovirus, largely spread through mosquitoes. At least 33 other states reported human cases, including 19 deaths.

Newly instituted screening tests identified 163 infected blood donors, many of them asymptomatic and therefore not included in case counts, in states throughout the South and Great. Plains. In all cases, infected blood was identified before it entered the blood bank distribution system.

Meanwhile, viral activity in mammals, birds, or mosquitoes was reported in nearly every state in the continental United States, sparing only Rhode Island and a cluster of states in the far West.

On Aug. 22, California's Department of Health Services reported its first West Nile virus-positive mosquito pool, heralding the feared spread of the virus to the West Coast.

A few states, including Oregon, Washington, Nevada, and Idaho, remained free of the virus, but health officials there were doubtful they would escape the virus in 2003.

"We can only hope," said Leslie Tengelsen, D.V.M., deputy state epidemiologist of Idaho. "This virus picks up speed as summer moves into fall. But the closer we get to hard-kill ...