West Nile’s Spread Slows in State

The spread of the West Nile virus in California has slowed significantly in recent weeks, prompting experts to believe that the disease will take a far smaller toll here than in other states.

Confirmed human cases of West Nile -- now at 586 -- appeared to have peaked in late July and early August and have been declining steadily since then, according to state statistics. Between Aug. 7 and Aug. 20, officials confirmed that 132 people contracted the virus. Thirty cases were confirmed between Aug. 21 and Sept. 11.

Despite fears that West Nile would sweep across California, the virus appears to have been confined mostly to Southern California -- notably San Bernardino County and eastern Los Angeles County. As a result, total human cases have been less than some expected.

“We feel it could have been far worse,” said Dr. Carol Glaser, acting chief of the viral disease section of the state health department. “We think it’s settling down, but we’re keeping our fingers crossed.”

Although 16 Californians have died from complications of the virus, that is far fewer deaths than in other, less populous states hit by West Nile. For example, Colorado recorded nearly 3,000 human cases and 63 deaths last year. In Texas, 37 people died, while Nebraska recorded 29 deaths.

Officials believe California’s relatively cool August and September, combined with aggressive efforts to spray wetlands and remove standing water, helped contain the virus. Standing water and wetlands attract mosquitoes, which spread the virus to humans.

West Nile spread this spring and summer from the desert areas of San Bernardino County into other parts of Southern California and then to Northern California.

Health experts believe West Nile will begin to recede by mid-October as the mosquito season ends. But they stressed that it should still be taken seriously.

“People should not relax,” said Dr. Jonathan Fielding, Los Angeles County’s public health director. “There’s been a diminution of West Nile cases, but we really don’t know whether it’s permanent or temporary.”

Most infected people show no symptoms. About 20% experience flu-like symptoms, including headache, fever, rash and weakness. Less than 1% of those infected die from the disease.

Blood banks, which turned to more rigorous testing in late July as increasing numbers of donors tested positive for the virus, have seen rapidly decreasing rates of infected donors.

In August, about one in 200 donors tested positive for West Nile in San Bernardino County, once the epicenter for the virus, said Dr. Frederick Axelrod, chief executive of the Blood Bank of San Bernardino and Riverside Counties. The rate is now about one in 1,800, he said.

“We actually slowed down considerably in September,” Axelrod said. “What this tells us is the epidemic is subsiding, and we really expect that by the end of October, it should be gone.”

But health experts urged continued caution.

Experts believe California’s aggressive efforts to kill mosquitoes made a major difference.

Unlike California, harder-hit states such as Colorado, Nebraska and South Dakota had virtually no organized mosquito-control programs.

Since West Nile first appeared in the United States in New York in 1999, California vector-control agencies have been spraying larvicide in anticipation of the mosquito- and bird-borne virus. Rice farmers in the Sacramento Valley have mosquito-eating fish in their rice paddies.

“Look at the transmission rates of California compared to Arizona and Colorado. They’re much lower,” said Ted Toppin, a spokesman for the Mosquito and Vector Control Assn. of California in Sacramento. “We’ve had far fewer deaths per population and fewer serious illnesses per population, and that’s a testament to effective mosquito control.”

There were more than 4,800 human cases of West Nile reported nationwide by September 2003 as the virus spread through states with less stringent abatement programs, according to government statistics. During the same period this year, there have been 1,600 cases, with the bulk in California and Arizona.

“From our perspective, it’s too early to declare victory,” Toppin said.

Another reason West Nile has not infected more humans is that the virus appears not to have gained a foothold in San Diego, as well as the Bay Area and other parts of Northern California.

Rather, the virus seems to have hopscotched up into the wetlands of the northern Sacramento Valley.

The region is also in the path of the Pacific Flyway, a migration route stretching from Central America to the Arctic used by birds, which carry the virus. The first West Nile death outside of Southern California was reported last week in Tehama County, a sparsely populated county at the northern tip of the Central Valley’s greenbelt.

Health officials said Northern California, which includes large expanses of land flooded for agriculture and recreation, might be hit harder in 2005.

“For sure Northern California will have more cases next year, no doubt about it,” Glaser, of the state health department, said.

But Southern California has likely seen the worst of West Nile, many officials say. Based on patterns elsewhere, West Nile is detected the first year, hits humans hardest in the second year and tends to recede in the third year. West Nile was first discovered in California in 2003.

“There seems to be a second-year phenomenon with West Nile,” Glaser said. “The first year, you’re hit lightly. The second year, everything is geared up, then the third year things slow down.”

But health experts agree there’s no way of knowing for sure.

“I don’t think we can make the statement with a high level of confidence that it will be better next year. But that is our hope,” said Fielding, the Los Angeles County health official. “In the meantime, we should prepare for next season with the same care and precautions and careful planning we did this year.”