What is West Nile Virus?

West Nile virus (WNV) is spread to humans by infected mosquitoes. Most people have no symptoms; some develop a mild, flu-like illness; and a small percentage (<1%) develop a severe illness that affects the central nervous system. WNV is not contagious.

Symptoms
  • No symptoms in most people: About 80% of infected individuals never develop symptoms.
  • Mild illness (~20% of cases):
    • Fever
    • Headache
    • Body and joint aches
    • Vomiting, diarrhea, or rash
    • Fatigue and weakness, which can last weeks to months
  • Severe illness (<1% of cases):
    Infections may affect the central nervous system, leading to:
    • High fever
    • Neck stiffness
    • Muscle weakness
    • Confusion or disorientation
    • Tremors or seizures
    • Hospitalization is often required.

Symptoms typically begin 2–6 days after a mosquito bite (range: 2–14 days; longer in people with weakened immune systems).

When to Seek Medical Care
  • Talk with your primary healthcare provider if you think you or a family member may have West Nile virus.
  • Seek immediate medical attention for high fever, neck stiffness, muscle weakness, confusion, or tremors.
  • Call 911 right away if you are very sick or experiencing severe symptoms.

Treatment

There are no specific antiviral medicines for West Nile virus. Antibiotics are not effective. Care is supportive:

  • Mild illness:
    • Acetaminophen (Tylenol) for fever, pain, and headache
    • Rest and fluids
    • Avoid NSAIDs (ibuprofen, aspirin, naproxen) if living in an area with dengue activity, unless a doctor rules out dengue
  • Severe illness:
    • Hospitalization for supportive care
    • Intravenous fluids
    • Pain management
    • Nursing support and monitoring
Preventing West Nile Virus

There is no vaccine or specific medicine to prevent West Nile virus disease. The best protection is avoiding mosquito bites and reducing mosquito habitats.

Personal Protection

  • Use EPA-registered insect repellents with one of the following active ingredients:
    • DEET
    • Picaridin (KBR 3023 / icaridin)
    • IR3535
    • Oil of lemon eucalyptus (OLE) – plant-derived
    • Para-menthane-diol (PMD)
    • 2-undecanone – plant-derived
  • Wear protective clothing:
    • Loose-fitting, long-sleeved shirts and long pants.
    • Use permethrin-treated clothing and gear (boots, pants, socks, tents, equipment).
      • Permethrin kills or repels mosquitoes and remains effective through multiple washings.
      • Do not apply permethrin directly to the skin.

Safe and effective when used as directed, including during pregnancy and breastfeeding.

Environmental Protection

  • Control mosquitoes indoors:
    • Use screens on windows and doors; repair holes.
    • Use air conditioning when available.
  • Control mosquitoes outdoors:
    • Stop mosquitoes from breeding:
      • Once a week, empty, scrub, turn over, cover, or dispose of items that hold water, such as:
        • Tires, buckets, toys, kiddie pools, birdbaths, flowerpot saucers, trash containers.
      • Even small amounts of water (as little as a bottle cap) can support mosquito breeding.
Risk

Risk of infection

  • Anyone who lives in or travels to an area where WNV circulates is at risk.
  • All states in the contiguous United States have reported cases.
  • In rare cases, West Nile virus has spread from a pregnant woman to the fetus, through breast milk, through a blood transfusion, or through an organ transplant.

Risks of severe illness

  • Anyone infected can develop mild or severe illness, but the risk is higher for:
    • Older adults (people 65+ are three times as likely to develop neurologic illness as those under 65).
    • People with chronic medical conditions (e.g., cancer, diabetes, hypertension, kidney disease).
    • People with a weakened immune system, including those taking certain medications for cancers, autoimmune or inflammatory conditions, or to prevent transplant rejection.
Additional Resources