West Nile virus spreads, evolves in Midwest, South

With West Nile virus' rapid colonization of the Midwest and South, researchers are discovering that infections among new species of birds and mosquitoes, varying geography and changing weather patterns are in essence granting one disease multiple personalities.

Although the basic tenets of disease surveillance and control remain intact, many states have been forced to reconsider past strategies, as an evolving disease that likely persists in hibernating mosquitoes and hitches rides with migrating birds has dared public health officials to keep pace.

Last year, West Nile virus was documented in 27 states and the District of Columbia. This year, it has already appeared in 14, including a new foray into eastern Texas.

Dr. Richard Berry, chief of Ohio's vector-borne disease program, said state officials there had been expecting West Nile's return after the virus began popping up in Illinois and Michigan earlier this spring.

"Nevertheless, it is surprising to see it for the first time," he said of West Nile's regional debut in 2001. "It's a different type of virus and a different type of epidemiology than we're used to."

In hibernating mosquitoes

Although scientists believe West Nile is slightly less lethal to humans than St. Louis encephalitis, West Nile has displayed much more persistence than its North American cousin. Most health officials agree that the early re-emergence of West Nile throughout the Midwest and South this spring indicates the virus' ability to survive in hibernating mosquitoes, a feat first documented in New York City after the 1999 outbreak and likely aided by the succession of relatively mild winters since then.

"We expected it to expand, but it really has gone more rapidly than I had anticipated," said Bob McLean, director of arboviruses at the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis. "The alarming thing is that so many new sites have become sources of the virus. The other big worry is that in places like Florida it can become really ingrained."

So far, the virus seems to be fulfilling the grim forecast. Although West Nile activity in Florida peaked in late September, the state reported infections among horses, birds and sentinel chickens (posted to detect the virus) through December. Florida officials waited until Dec. 31 to remove 53 counties from a medical alert.

Ten days later they collected the first West Nile-infected bird of 2002, a wild turkey in the state's panhandle. CDC grants to the state for increasing lab capacity and surveillance have echoed the concern, with a jump in funding from $200,000 last year to $1.5 million this year.

Sentinel chickens

Among this year's plans, Florida officials hope to beef up the state's sentinel chicken surveillance system, which has met with considerable success. Although used less heavily in the hard-hit northern part of the state last year, sentinels tested positive in 20 counties and provided the first warning of the virus in four.

Florida health officials have been considerably less impressed with the utility of the dead-crow-density system deemed successful in New York.

"So far in our analysis, we didn't see the correlation between density and human risk," said Dr. Lisa Conti, a public health veterinarian and epidemiologist for the Florida Department of Health.