What is West Nile Virus?
West Nile virus is the leading cause of mosquito-borne disease in the contiguous United States. First discovered in Uganda in 1937, it didn't appear in the U.S. until 1999 in New York City. Since then, it has spread across the country, becoming a regular,
seasonal public health concern during mosquito season, which typically starts in the summer and continues through the fall. The virus is maintained in a cycle between birds and mosquitoes. Mosquitoes get infected when they feed on infected birds, and then they can transmit the virus to humans and other animals when they bite.
How It Spreads—and Doesn't Spread
The primary way people get West Nile virus is through the bite of an infected mosquito. It is not spread through casual contact, like touching or kissing a person who has the virus. Humans are considered “dead-end hosts” because, unlike in birds, the level of virus in our blood doesn't become high enough to infect a mosquito that bites us. This means you can't pass the virus to another person or to a mosquito. In very rare instances, the virus has been transmitted through blood transfusions, organ transplants, and from mother to baby during pregnancy or breastfeeding, which is why blood donations are screened.
Recognizing the Symptoms
The vast majority of people—about 80%—who get infected with West Nile virus show no symptoms at all. About 20% of infected people develop what is known as West Nile fever, with milder, flu-like symptoms. These can include fever, headache, body aches, joint pains, vomiting, diarrhea, or a rash. While most people recover completely, feelings of fatigue and weakness can last for weeks or even months. Less than 1% of those infected develop a serious, neuroinvasive illness, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes surrounding the brain and spinal cord). Severe symptoms can include high fever, severe headache, neck stiffness, disorientation, coma, tremors, seizures, and paralysis. People over 60 and those with certain medical conditions like cancer, diabetes, or kidney disease are at a higher risk for severe illness.
Treatment is Supportive, Not Curative
There are no specific vaccines or antiviral medications to treat West Nile virus in humans. For those with mild symptoms, treatment focuses on rest, staying hydrated, and using over-the-counter pain and fever relievers. However, anyone experiencing symptoms of severe illness, such as an unusually bad headache, confusion, or neck stiffness, should seek medical attention immediately. Patients with severe neuroinvasive disease often require hospitalization for supportive care, which can include IV fluids, pain management, and breathing assistance.
How to Protect Yourself: Repel and Reduce
Prevention is the most effective strategy. The best way to avoid West Nile virus is to prevent mosquito bites. Use an EPA-registered insect repellent containing an active ingredient like DEET, picaridin, or oil of lemon eucalyptus when you go outdoors. Wear long-sleeved shirts and long pants, especially during dawn and dusk when many mosquito species are most active. You can also mosquito-proof your home. Empty any standing water from flowerpots, gutters, buckets, and birdbaths at least once a week, as this is where mosquitoes lay their eggs. Ensure you have tight-fitting screens on all windows and doors to keep mosquitoes outside.
