West Nile Virus: Who's at Risk?

Kathleen Doheny

August 23, 2012

August 22, 2012 — The West Nile virus outbreak is spreading, threatening to make this the worst year ever in the U.S., with 47 deaths now reported, according to the latest statistics from the CDC.

The mosquito-borne virus is circulating in 47 states.

WebMD turned to three experts and asked them to address the West Nile virus questions most commonly asked.

Who is most at risk?

"Looking at the risk of getting infected, anyone who is outdoors and participating in activities is,'' says Erin Staples, MD, PhD, medical epidemiologist at the CDC.

That's especially true at dusk and dawn, she says, when mosquitoes carrying the virus are most active. "Only 1 in 5 people who get infected will develop symptoms," she says.

In most cases, the symptoms are flu-like and fleeting. But not always. Some people develop severe neurological problems.

"Less than 1% of persons who get infected will go on to develop encephalitis or meningitis," she says. These severe neurological symptoms raise the death risk from the virus.

Anyone 50 or older is more at risk for developing the more severe symptoms, she says.

"Clearly the older you are, starting at 50 or so, severity [of the infection] is likely to increase as are death rates," says William Schaffner, MD, an infectious disease specialist and chair of preventive medicine at the Vanderbilt University School of Medicine in Nashville.

Using data reports from cases, the CDC has identified certain medical conditions that raise your risk of becoming infected, Staples says. They include:

  • Cancer

  • Diabetes

  • High blood pressure

  • Kidney disease

  • Organ transplant

"As far as we know, pregnant women are not necessarily at higher risk of getting infected," she says. "The data we have is somewhat limited. We have not had a lot of pregnant women being infected."

The cases they have looked at have been few, she says. It is difficult to draw any conclusions.

"I know of no data that says if they are infected it will imperil their pregnancy," Schaffner says. Much needs to be learned, he says. "This is a relatively new infection and we should all stay tuned."

Information on infants is limited, Staples says. While there have been periodic reports of infants infected, she could not cite an exact number since the virus was first identified.

"So far this year, we are only aware of one [infected] infant that has been reported," she says.

Infants don't seem to be at increased risk for severe illness, she says, ''but that is based on limited data."

The virus was first identified in the U.S. in 1999.

What is a typical time frame from bite to symptoms?

From bite to symptoms can take three to 12 days, Staples says. "It's usually three to six."

Typically, first symptoms include fever, chills, feeling poorly, and lacking an appetite, Schaffner says.

At that point, especially if you live in a highly affected area, it is reasonable to call your doctor, he says.

It could well be something else, such as a urinary tract infection, he says.

If you are infected, what is the usual course?

"The people who develop the less severe illness report feeling sluggish for weeks or [sometimes] months," Staples says.

"For those who develop the more serious complications, about 10% will die."

Those who survive can have long-lasting neurological effects. One example is a condition called acute flaccid paralysis. The patient is unable to move arms or legs. That is rare, she says.

Long-lasting kidney effects may occur, says Schaffner. Preliminary research has found a gradual decline in kidney function in some patients.

"It is a slight decline, but it is progressive," he says. "There is no sense yet this will lead to kidney failure."

More research and follow-up is needed, he says.

There is no specific antiviral medicine for the infection, Schaffner says. Doctors instead treat the individual symptoms of each person.

What is the best way to avoid getting infected with West Nile virus?

After avoidance, experts advise using insect repellents with DEET, but not in infants less than 2 months old.  

Schaffner has some more suggestions:

  • Parents can avoid taking walks with infants early in the morning or at dusk.

  • Put mosquito netting over baby carriages and apply repellent to the carriage itself.

  • When outdoors, wear long pants and long sleeves.

  • Remove standing water from around your house, including in gutters, kiddie pools, and potted plant saucers.

What is in the aerial spray being used in some areas, and is it safe?

The exact insecticides used depend on the region, says William Reisen, PhD, director of the Center for Vectorborne Diseases at the University of California, Davis.

Most common, he says, are synthetic pyrethroids, derived from petroleum derivatives, or organophosphate compounds.

Synthetic pyrethroids are often found in garden insecticides.

"Everything they are applying is approved by the EPA," he says. The vehicles dropping it account for the wind and other factors, he says.

"All the health departments and mosquito departments involved always announce when they are spraying. They suggest people remain indoors and limit time outdoors while the spray is occurring."


William Reisen, PhD, director, Center for Vectorborne Diseases, University of California, Davis.

William Schaffner, MD, chair of preventive medicine, Vanderbilt University School of Medicine, Nashville; former board of directors member, Infectious Diseases Society of America.

Erin Staples, MD, PhD, medical epidemiologist, CDC, Fort Collins, Colo.


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