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This August, West Nile virus has been detected in mosquitoes across the five boroughs of New York City, as well as in Erie, Nassau, Rockland and Suffolk counties. But it isn’t content to simply hang out with its insect host. Sooner or later, an infected mosquito will bite a human, resulting in an illness called West Nile fever.
According to the Centers for Disease Control and Prevention (CDC), the majority of people who get infected—roughly 80 percent—don’t develop symptoms. However, 20 percent aren’t quite so lucky. They may experience some or all of the following:
A small number of people may develop severe neurological symptoms, and an even smaller number may succumb to life-threatening illness.
Unfortunately, there’s no West Nile vaccine and no antiviral treatment capable of mitigating the effects of the disease, especially in its severe form.
“When an infected mosquito bites a human, the person can develop West Nile fever within the next 2 to 14 says,” says Dr. Ole Vielemeyer, Associate Professor of Clinical Medicine, Medical Director of the Travel Medicine Clinic and an infectious disease specialist at Weill Cornell Medicine.
See Dr. Vielemeyer’s answers to your FAQs below.
The virus is found in many parts of the world, including Europe, Africa, Asia and Australia. It was first reported in North America in 1999, when a cluster of cases was seen in New York City. Now, all 48 continental states have had reported cases.
“It isn’t easy to get a clear sense of how common the disease is in the New York Metro Area, given that not all infections cause symptoms,” he says. “What I can say is that the risk of acquiring a severe type of West Nile infection is very, very low.”
Nearly all human cases of West Nile virus infection can be traced to a mosquito bite. There’s no known transmission from human to human.
Dr. Vielemeyer goes on to explain that transmission can theoretically occur through blood transfusion or organ transplantation. However, blood supplies and organs ready for transplantation are regularly screened for West Nile, among other viral infections, and thus remain safe.
In New York City, mosquitos are most active between April and the end of October. With the warmer temperatures we’ve experienced in recent years, though, they may stick around well into the late fall.
The mosquito carrying West Nile virus usually bites after dark, but when it’s low on energy sources—when it needs a “blood meal” from a human—it may also bite during the day, mainly in the morning.
People at the extremes of age—infants and young children as well as those over 65—and people with immunocompromised status are at increased risk for severe disease.
“A diagnosis of West Nile virus infection requires a high degree of suspicion and testing of blood samples,” he says, “and sometimes of cerebrospinal fluid (CSF) taken through a lumbar puncture. But there is no quick test.
“People with symptoms of severe disease who end up in the emergency room should undergo testing for West Nile fever and for illnesses that mimic its symptoms, such as Guillain-Barré syndrome, stroke or bacterial meningitis,” he adds.
Treatment is mostly supportive, including rest, fluids and over-the-counter medication.
“Patients with mild symptoms don’t need to seek medical care,” Dr. Vielemeyer says, “but for moderate and more severe symptoms, or for rapidly worsening symptoms, prompt evaluation is necessary to ensure an accurate diagnosis is made and another condition isn’t missed—especially bacterial meningitis, which can be treated successfully with the appropriate antibiotic when administered without delay.”
“The best way is to not get bitten by a mosquito in the first place,” he says. “When you’re indoors, have screens on your windows to keep them out, or close them and use air conditioning.
“Avoid creating an easy breeding ground for mosquitoes,” he continues, “by cleaning up small pools of rainwater created by plastic waste, discarded old tires and other trash. And when you’re outdoors during biting season, use insect repellent on exposed skin.”
Community-based mosquito control programs can also be very effective in preventing mosquito-borne infections, including West Nile.
“However, there are many benefits to spending time outdoors, whether engaging in physical activity or meeting up with family and friends. With proper clothing and insect repellent, you can and should continue to reap these benefits,” Dr. Vielemeyer advises.
If you’ve been experiencing any of the symptoms listed above or have questions, consider making an appointment with a Weill Cornell Medicine physician.
