West Nile Virus: The Leading Mosquito-Borne Disease in the Contiguous U.S.

West Nile virus is the leading cause of mosquito-borne disease in the contiguous United States, and every state there has reported cases. Most people it infects never notice. But each year more than 1,300 people develop severe illness affecting the central nervous system, and more than 130 die — and there is no licensed vaccine or medicine to prevent or treat it.

West Nile Virus: It Starts with a Mosquito Bite. CDC.

A mosquito of the kind that carries West Nile virus, on a person's arm. Image from the Centers for Disease Control and Prevention's West Nile virus page.

How it spreads, and when

Mosquitoes pick up the virus when they feed on infected birds, then pass it on through their bites — by far the most common route to people. The virus can also spread through blood transfusions and organ transplants, and rarely through laboratory exposure or from mother to baby during pregnancy, delivery or breastfeeding.

People are infected during mosquito season, which runs from summer through fall — typically June through October. Diagnoses usually peak in late August to early September. About 2,000 people in the United States are diagnosed each year, an undercount, because most infected people have no symptoms or blame mild ones on something else.

What it does

Symptoms usually begin 2–6 days after the bite, sometimes later, especially in people with a weakened immune system. Anyone living in or traveling to an area where the virus circulates is at risk, and anyone infected can become mildly or severely ill, but the risk of severe illness rises with age, certain chronic medical conditions and a weakened immune system.

Getting diagnosed and treated

If you think you or someone close to you might have West Nile, talk to a healthcare provider, who will diagnose it based on several factors. No medicine treats the virus, and antibiotics do not work on viral infections.

• Mild illness can often be managed at home: rest, stay hydrated, and take over-the-counter medicine such as acetaminophen for fever, pain and headaches. If you live in an area with dengue — also spread by mosquitoes — avoid ibuprofen and other non-steroidal anti-inflammatory drugs (NSAIDs).
• Severe illness often needs a hospital stay for supportive care, such as intravenous fluids, pain medication and nursing care.

Most people who have been infected are believed to be protected for life. Some people whose immune systems are weakened by certain conditions or medications may not respond strongly to the first infection, or their immunity may fade over time.

Prevention: avoid the bite

The best way to protect yourself and the people you love is to prevent mosquito bites. A Georgia man living with long-term complications of severe West Nile illness described how it began for him in a CDC video:

"Mosquito bites are part of life in the South. I was wearing flip flops while I was working, and I never remembered to spray my feet. I took a nap one Saturday evening, and when I woke up, I couldn't move my legs."

Sources

Based on "About West Nile Virus," Centers for Disease Control and Prevention; a work of the United States government in the public domain. The video, "West Nile Virus: It Starts with a Mosquito Bite," is also from CDC.

• The film is the agency's own low-resolution copy, recovered from the Internet Archive's record of cdc.gov.