West Nile Virus: A Dangerous Season Returns
West Nile virus season is upon us. Each year, reported infections of the disease tend to occur mainly between late July and early September. Cases of the mosquito-spread illness—a couple of them lethal—have already cropped up in states as far apart as California, Mississippi, and North Dakota. But it's not yet clear where, if anywhere, the highly unpredictable virus will be big this year. "From one year to the next, we don't know where it will break out," says Gary Nabel, director of the Vaccine Research Center at the National Institute of Allergy and Infectious Diseases.
"We start seeing infections peak in August," says Emily Zielinski-Gutierrez, a behavioral scientist at the Centers for Disease Control and Prevention. Since the virus incubates for two to 15 days in people, she says, now is the time when "a lot of those infections are happening." It's also the time when prudent people are on guard. Experts urge everyone to take precautions, including using mosquito repellent, especially around dawn and dusk, wearing long-sleeved protective clothing, repairing window screens that have holes or gaps, and emptying containers of standing water. The Culex mosquitoes that usually spread West Nile tend to bite at sunup and sundown, which is why risk is heightened at those times of day.
Rural areas of the Midwest are particularly prone to outbreaks in part because Culex tarsalis, which breeds in the region's abundant irrigated farmland, is an efficient spreader of West Nile. But other areas of the country aren't immune. The virus made its U.S. debut in New York City, in 1999, and has since caused outbreaks in Chicago, Detroit, and New Orleans, among other places. In Bakersfield, Calif., where a 96-year-old woman died of West Nile on July 13, a drought has driven mosquitoes to urban and suburban areas, where they've taken up residence in storm drains, for example, and their favorite breeding ground—residential swimming pools.
In some places, especially those where West Nile has arrived recently, large numbers of dead birds can indicate that the virus is active—and likely to hit the human population hard. The virus replicates quickly in birds, especially crows and jays, which makes them particularly likely to transmit it to fresh mosquitoes, which may in turn pass it on to people. An absence of dead birds, however, can be falsely reassuring, especially in areas with long-established West Nile activity. Birds that have survived an infection can be immune, so they may not get sick even when infected mosquitoes are biting. The virus's familiarity as a summer hazard throughout the U.S., moreover, may have led people to be apathetic about reporting dead birds. That reporting is one of the key ways that public health experts track the virus's activity.
Even when the virus infects people, it often escapes notice, in part because 80 percent of infections aren't symptomatic. Others fall ill but never get the cause diagnosed: The 4,261 known West Nile cases in 2006 represent only a fraction the estimated 43,000-plus human illnesses that occurred that year. Only about 1 percent of infections develop into one of the serious "neuroinvasive" conditions associated with West Nile.