The name evokes images of African deserts and rain forests, but West Nile virus quickly has made itself at home across the United States.
Since the first U.S. cases appeared in New York in 1999, the virus has spread from coast to coast, and public health experts agree that this mosquito-borne infection won't go away.
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"It's here to stay," says Richard Pollack, Ph.D., an instructor of tropical public health at the Harvard School of Public Health.
The number of reported cases more than doubled in 2003 compared with 2002, for a total of 9,858 cases and 262 deaths, according to figures from the U.S. Centers for Disease Control and Prevention (CDC). Locally acquired human cases appeared everywhere in the continental United States except Oregon and Washington.
Compared with 2002, when the outbreak centered on the Midwest and the Deep South, in 2003 West Nile moved toward the sunset. The largest numbers of cases were in Rocky Mountain and Great Plains states. Five states alone — Colorado, South Dakota, Nebraska, North Dakota and Texas — accounted for three-quarters of the cases and more than half of the deaths.
California reported its first human cases of West Nile virus as the 2003 season waned, and many more are expected in 2004.
Bigger, Or Better Reporting?
It's actually unclear whether more people got sick from West Nile in 2003 than in 2002, says Jennifer Brown, D.V.M., M.P.H., a veterinarian in CDC’s Epidemic Intelligence Service.
"As time passes, the surveillance [reporting system] might be getting better, and if the surveillance is better it looks like there are more cases," she says.
But it is clear, nonetheless, that West Nile has moved rapidly across the country, infecting people, mosquitoes, birds and other animals.
Most of the increase in 2003 came from cases of West Nile fever, a relatively mild, flulike illness, Dr. Brown says. This suggests that increasing numbers of states may be reporting mild cases as well as the most serious ones, she says.
By contrast, the number of very serious cases has changed little since 2002, Dr. Brown says. In these cases — nearly one-third of the 2003 total — people became very ill with inflammation of the brain or the tissues that cover it (encephalitis or meningitis). Some of the most serious cases also result in a type of paralysis that resembles polio.
Thousands of other Americans probably have been infected with West Nile virus but don't know it because they have continued to feel fine. Symptoms can vary tremendously.
"Most people who acquire West Nile don't get sick, and most who do get sick don't become profoundly ill," Pollack says. The CDC estimates that less than 1 percent of people infected with West Nile become severely ill.
How West Nile Spreads
West Nile is spread through the saliva of mosquitoes. In this country, the main culprits are Culex mosquitoes, although other kinds of mosquitoes also may be involved.
Only the female mosquito feeds on blood. Culex mosquitoes tend to prefer bird blood, but they are not always so picky, Pollack says.
"If given the choice, they'll probably go after a bird. Mostly, West Nile virus bounces back and forth between birds and Culex mosquitoes. But if they're hungry and you're in the way, some of these mosquitoes will also feed on people, dogs, raccoons, just about any vertebrate animal."
Once inside a suitable bird, the virus may multiply "to an incredible extent," Pollack says. "Some birds succumb to the infection and will die, but others tolerate it quite well and may then infect many more mosquitoes."
In addition to mosquito transmission, in 2002 nearly two dozen people got sick from West Nile virus spread by blood transfusion. This was the first time that West Nile virus transmission by blood transfusion ever had been reported.
In 2003, blood banks began testing for the virus to reduce the risk of transfusion-related infections. This testing prevented hundreds of potentially infected units from entering the blood supply, Dr. Brown says. Two transfusion-related cases were confirmed and reported to the CDC in 2003.
Changes in blood screening procedures are possible for the future. Dr. Brown noted that it took a while to refine screening tests for other viruses, such as hepatitis C, that now are considered highly effective.
How Did It Get Here?
The West Nile outbreak in the United States began in the late summer of 1999 in the New York City area.
Originally, CDC officials thought they were dealing with St. Louis encephalitis, another virus transmitted by mosquitoes — until they realized that birds also were dying. St. Louis virus normally doesn't kill birds. Tests revealed that the culprit was something unexpected — West Nile virus.
How did this virus, well known in Africa, Europe, the Middle East and part of Asia, get to the United States?
Researchers don't really know. "We do have a strong indication as to where it actually came from," Pollack says. "The strain that was isolated in New York was virtually identical to a strain that was isolated the previous year in Israel, but less closely related to isolates from outbreaks from other countries."
Pollack says the virus could have been carried by a person who was not aware of being infected, a migrating bird blown off-course by a storm, or a bird (or other animal) smuggled into the country.
His favorite theory is that a mosquito hitched a ride inside an airplane.
"I think it's probably a mosquito that disembarked in the New York area, and then fed upon one of our local birds and transmitted the infection," he says. "Perhaps then some of our local mosquitoes picked up the slack — and helped perpetuate and spread the outbreak."
Future Cycles
Future outbreaks in any particular area are likely to occur in cycles, Pollack says.
"When West Nile arrives at a new region you have a tremendous population of what we call naïve hosts — birds that have never seen this infection before and are susceptible," he says. "So it may infect lots of birds." The birds infect more mosquitoes, which in turn may spread the virus to people and other animals.
"But a certain proportion of the survivors are essentially immune, and they will moderate the infection in the future," Pollack says. Once the immune birds in an area get old and die, and are replaced by other birds that are not immune, another local outbreak becomes more likely, he says.
Prevention
Public health officials across the country have been dealing with West Nile, in part, by monitoring bird and mosquito populations. They test a proportion of the dead birds found in an area to detect the virus, and they also may sample and test mosquitoes.
Officials also are making more intense efforts to eliminate standing water or treat areas that can breed mosquitoes. These can include anything from catchment basins and ponds to backyard breeding grounds such as open garbage cans and birdbaths.
In many areas, officials have used insecticides to kill mosquito larvae. If an outbreak has occurred, sometimes they have sprayed to kill adult mosquitoes.
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