MONDAY, Sept. 27, 2004 (HealthDayNews) -- The vast majority of West Nile infections this year have occurred in West Coast and Rocky Mountain states, with almost no human infections recorded in the densely populated Northeast, where the virus first took hold five years ago.
Overall, there have been just 10 cases of human infection with West Nile virus recorded this season from Maine to Washington, D.C. Some states -- Delaware, Maine, Massachusetts, New Hampshire, New Jersey, Rhode Island and Vermont -- have failed to record one case, according to new statistics from the U.S. Centers for Disease Control and Prevention.
That compares to 507 cases recorded in California so far this year, 355 in Arizona, and 225 in Colorado.
Why this geographic shift west? The answer may lie in a growing immunity to the virus within a key reservoir -- birds, experts suggest.
"Remember, this is an organism that maintains it ability to occasionally infect humans by regularly infecting other animals," explained Dr. Theresa Smith, a medical epidemiologist at the CDC. Those animal "reservoirs" include large mammals such as horses, dogs and deer, but most commonly birds.
Infection with West Nile virus can kill crows and other birds, and those that survive become immune to further infection. Increasing immunity among bird populations in the Northeast may be protecting humans, too, Smith said.
"Essentially, as the virus finds a new place to live, it at first causes a lot of infection. We've seen that all across the United States -- wherever the virus is new, that's where we've always seen the most numbers," she said. That would explain high rates of infection on the West coast, which has only recently been introduced to the virus.
Back in the Northeast, however, bird and animal populations appear to have steadily gained immunity against the pathogen, Smith said.
That pattern seems to be emerging in the Midwest as well, where states such as Indiana (three cases), Michigan (five cases) and Ohio (three cases) have gotten off relatively lightly this year.
Southern states have seen moderate caseloads -- according to the CDC; Florida has identified 31 cases this year, Louisiana 34, and Texas 52.
Overall, CDC officials say there have been 1,604 human cases of West Nile virus infection recorded so far in 2004 across the country. In the contiguous United States, only Washington state has remained free of either animal or human infection.
Other factors may be keeping infection rates to near zero in the Midwest and Northeast, experts say.
The virus lurking inside mosquitoes is most active in hot weather, "and we've had a relatively cool summer here," said Dr. Annie Fine, an epidemiologist with the New York City Department of Health and Mental Hygiene. The first case of West Nile virus infection ever recorded in the United States occurred in New York City in 1999.
Fine credits some of her city's success against the virus to its targeted mosquito-control program, which uses mosquito and bird surveillance to prioritize specific neighborhoods for larvacide and pesticide-spraying efforts.
"Here in New York City we have a very aggressive program," Fine said. "We've also done a lot of work to educate people to remove standing water from their properties."
However, just because the city recorded only one case of human West Nile virus infection this year doesn't mean mosquito-control efforts can be relaxed, Fine said.
"Although this summer we have had what seems to be a low number of cases, we just don't know -- next year could be different, it's difficult to predict what this virus will do," she said. In terms of bird immunity, she pointed out "there are always [new] birds migrating in from other places, and of course young birds being born, that aren't immune."
As birds, other animals and even humans become more immune to the virus, a similar drop in cases might occur later in states such as California or Colorado that were hit so hard this year, Smith said. "It could happen even next year. Especially if they happen to have a very cool, dry year next year --- they may not have many infections."
Approximately 80 percent of people who are infected with West Nile virus will not show any symptoms. Up to 20 percent of people will display symptoms that can include fever, headache, body aches, nausea and vomiting. Those most vulnerable are people over 50 years old, according to the CDC.
For more on how you can protect yourself from the West Nile virus, visit the Centers for Disease Control and Prevention.