West Nile Virus from A to Z
In “The Great Gatsby,” F. Scott Fitzgerald wrote that “life starts all over again when it gets crisp in the fall,” but New York’s mosquito population may not have gotten the memo. Although mosquitoes are most active from May through October, they tend to hang around for weeks and even months after peak season. That’s why the city conducts its mosquito prevention, surveillance and control efforts all year long, including periodic spraying.
West Nile virus, a potentially serious disease, belongs to the group of viruses called arboviruses. Closely related viruses are those that cause dengue fever and yellow fever. All 3 of these are mosquito-borne.
Here’s how West Nile virus spreads: Infected mosquitos bite animals (often birds), the virus replicates itself in the animal and another mosquito picks up the virus by taking a blood meal from the infected animal. That’s how the cycle works to the virus’s benefit.
Infected mosquitos also transmit the virus to the next generation of mosquitos; in other words, to their eggs.
When an infected mosquito bites a human, the person can develop disease within the next 2 to 14 days, says Dr. Ole Vielemeyer, Associate Attending Physician, Associate Professor of Clinical Medicine and Medical Director of the Travel Medicine Clinic at Weill Cornell Medicine.
See Dr. Vielemeyer’s answers to your FAQs below.
How prevalent is West Nile virus in humans?
West Nile virus is found in many parts of the world, including Europe, Asia, Africa and Australia. It was first discovered in 1937 in the West Nile region of Uganda. The first cases in North America were reported in 1999, when a cluster of cases was seen in New York City. Now, all 48 continental states in the United States have had reported cases.
“It isn’t easy to get a clear idea of how common the disease is in the New York Metro Area, given that not all infections cause symptoms,” he says. “So far, reported cases in the city this year total well under 50, including 5 positive samples identified in blood donors. The risk of acquiring a severe type of this infection is very, very low.”
Can humans contract it from other humans? Are there other ways it can spread among humans?
Virtually all human cases of West Nile virus infection are due to a mosquito bite. There is no known transmission from human to human via casual or sexual contact.
Dr. Vielemeyer goes on to explain that transmission can theoretically occur through blood transfusion, organ transplantation or even from mother to child via the placenta. However, blood supplies and organs ready for transplantation are regularly screened for West Nile, among other viral infections, and thus remain safe.
Neither is it possible to get the infection through direct contact with animals, including birds. As well, it’s perfectly safe to stay in contact with loved ones after they’ve been diagnosed with West Nile virus and to provide them with direct care.
Even in the hospital setting, providers don’t use special precautions when caring for patients with suspected or documented West Nile virus infection, he adds.
Is mosquito season still with us? When do mosquitoes usually bite?
In New York City, mosquitos are most active between April and the end of October. However, even later in the year, mosquitos may stick around and take a blood meal from animals or humans, especially with the warmer temperatures we’ve experienced in recent years. The mosquito carrying West Nile virus usually bites after dark, but when it’s low on energy sources, they also bite during the day—mainly in the morning.
What are the best ways to protect ourselves from West Nile virus?
“The best way is to not get bitten by a mosquito in the first place,” he says. “When indoors, have screens on your windows to keep them out, or close them and use air conditioning.
“Avoid creating an easy breeding ground for mosquitos,” 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.”
There is no available vaccine for West Nile, even though yellow fever, a closely related disease that’s found only in sub-Saharan Africa and South America, can be prevented with a live vaccine. Unfortunately, the yellow fever vaccine doesn’t cross-protect against West Nile virus infection.
Community-based mosquito control programs can also be very effective in preventing mosquito-borne infections, including West Nile.
What are the symptoms of mild-to-moderate illness? Severe illness?
- Not all mosquitos carry the virus, so not all mosquito bites carry risk of this infection.
- 20 to 40 percent of West Nile virus infections are asymptomatic and harmless.
- There is no known chronic or latent form of the infection, and no post-viral syndrome associated with West Nile.
Symptomatic individuals may develop abrupt onset fever and headaches and feel very weak, he says. They may develop a rash or eye symptoms; an ophthalmologist may diagnose inflammation of the eye.
Additionally, an infected individual may have an upset GI tract. Sometimes, the brain can be affected, and people can become confused, develop muscle weakness and nerve damage or even be unable to walk. On rare occasions, the disease can be life-threatening.
Who may be at risk for severe illness?
People at the extremes of age—infants and young children as well as those over 65—and people with an impaired immune system are at increased risk of severe disease.
Is there a test for it? If so, who should receive it?
“A diagnosis of West Nile virus infection requires a high level of suspicion and testing of blood samples, and sometimes of cerebrospinal fluid (CSF) taken through a lumbar puncture,” he says. “But there is no quick test that gives results within 24 hours. People with symptoms of more severe disease who end up in the emergency room or hospital should undergo testing for West Nile virus infection and for mimics of the illness that would require treatment with antimicrobial agents.”
What are the best ways to treat mild-to-moderate West Nile fever? Severe illness?
Treatment for the condition is mostly supportive: rest, fluids and over-the-counter medications. No antivirals with activity against West Nile virus have been discovered as yet. Antiviral drugs used to treat HIV or to manage herpes, including acyclovir, aren’t effective and are therefore not used.
When should a person seek medical treatment?
As there is no specific treatment, no concern for chronic infection and no concern for person-to-person spread, patients with mild symptoms don’t need to seek medical care, says Dr. Vielemeyer. 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.”
West Nile virus infection, especially the severe type, remains uncommon in the New York Metro Area. The best way to prevent it is to reduce the risk of mosquito bites by using insect repellent, wearing dark-colored clothing and avoiding going out in the early morning or evening hours.
“On the other hand,” he says, “there are many benefits to spending time outdoors, engaging in physical activity and meeting up with family and friends. With proper clothing and insect repellent, you can and should continue to reap these benefits.”
If you’ve been experiencing any of the symptoms listed above or have questions, consider making an appointment with a Weill Cornell Medicine physician.