Luckily, less than 1% of people who get the virus from a mosquito develop serious symptoms.

By Amanda Gardner
July 13, 2018

Since it first arrived in the United States almost 20 years ago, West Nile virus has infected people across the country. Transmitted to humans by mosquitoes, West Nile is virtually impossible to track in people: In 80% of cases, the virus causes no symptoms. Symptoms are usually mild in the other 20% and include fever and headache, but in less than 1% of infected people, more serious consequences can occur.

Here’s what you need to know about West Nile virus this mosquito season.

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West Nile virus causes

West Nile virus is most commonly spread through mosquito bites. But humans are actually not a mosquito’s “preferred lunch” says William Schaffner, MD, an infectious diseases specialist at Vanderbilt University Medical Center in Nashville. “They prefer to bite birds. It’s not a virus that wants to make us sick. We are an incidental host.”

Although West Nile virus carried by mosquitoes infects and kills birds (especially crows and blue jays), it’s not possible for humans to catch it by handling dead birds. (That said, it’s still not a good idea to handle dead birds without gloves, according to the Centers for Disease Control and Prevention.)

When West Nile first came to the U.S., a few people did get it through blood transfusions and organ transplants, but that isn’t likely to happen today.

“You could potentially get it through a blood transfusion but–this is a big 'but'–our blood [supply] is screened for West Nile virus,” says Dawn Wesson, PhD, associate professor at Tulane University School of Public Health and Tropical Medicine in New Orleans.

There’s an outside chance that pregnant and breastfeeding women infected with West Nile can pass the virus to their babies, but mothers should continue to breastfeed even if they become sick. There are so many benefits to breastfeeding that it outweighs the unknown risk of transmission, says Wesson.

The risk for transmitting West Nile during pregnancy also seems to be low. “There is some possibility if a woman is infected very close to delivery and she becomes very sick, the infant could become infected,” says Wesson. However, “it’s nothing like Zika infection.”

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West Nile virus symptoms

Eighty percent of people who are infected with West Nile virus have no symptoms. Most of the remaining 20% develop what’s called West Nile fever, which, in addition to a fever, can come with aches and pains, vomiting, diarrhea, and possibly a rash.

“It goes away,” says Dr. Schaffner. “That’s the milder end of the spectrum. There are a few people who develop encephalitis, and they tend to be older.”

Encephalitis is potentially fatal inflammation of the brain. West Nile virus can also cause meningitis, inflammation of the membranes that surround and protect the brain and spinal cord.

Symptoms of these conditions can include a high fever, neck stiffness, headache, feeling disoriented, convulsions, paralysis, vision loss, weakness, and even a coma. People 60 years or older are more at risk, as are people whose immune systems have been compromised either because of another illness or because of medications they’re taking. Fortunately, symptoms this severe only happen to about one out of every 150 people who are infected with West Nile, according to the CDC. However, about 10% of the people who develop these serious symptoms die.

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West Nile virus treatment

There is no vaccine for West Nile virus, although there are researchers working on developing one. There’s also no specific antiviral or other treatment for either West Nile fever or its more serious complications.

“Basically, we’re treating aches and pains if you have them,” says Wesson. “If you’re hospitalized, we’re giving supportive care.”

That can mean over-the-counter pain medicine for milder symptoms and, for people who are hospitalized, stronger pain medication and IV fluids.

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West Nile virus prevention

The best way to avoid West Nile is to avoid getting bitten by mosquitoes. There are a number of ways to do this.

“One of the most important things you can do [around your home] is look for any standing water, which might become a breeding site for mosquitoes,” says Dr. Schaffner.

Look at the gutters on your house to make sure they’re draining properly. Change birdbath water frequently, and make sure any containers in your yard, including toys, are turned over so they don’t collect water. Pools usually aren’t a problem as long as they’re regularly maintained and the water keeps circulating rather than staying still.

When you’re outside in areas with mosquitoes, you can avoid bites by covering up with long sleeves and pants, especially in the mornings or evenings, when mosquitoes are most active. Use insect repellent on any exposed areas of skin.

“I recommend DEET; for my own personal use, it’s been a very good option,” says Wesson. “Other approved repellents are oil of lemon eucalyptus and picaridin.”

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“Some of these work better than others against specific mosquito species, and some work better on some people, so you may have to try more than one,” Wesson adds. “It also depends on how long you need to be protected, if it’s a multi-day camping trip or if it’s an hour or two.”

It’s okay to use DEET on children older than 2 months, according to the CDC. Oil of lemon eucalyptus should not be used on kids under 3. You can also protect kids of any age with long sleeves, and, if you’re using a stroller, cover the stroller with mosquito netting.