West Nile Virus
Last updated 9/13/2024.
West Nile virus (WNV) is a mosquito-borne disease found in the United States and many other places in the world. Cases of WNV happen during mosquito season, which starts in the summer and continues through the fall.
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There are no vaccines or medicines to prevent WNV.
The best way to prevent WNV is to protect yourself from mosquito bites.
- Use insect repellent,
- Wear long-sleeved shirts and pants,
- Treat clothing and gear, and
- Take steps to control mosquitos indoors and outdoors, such as keeping screens on windows and doors in good repair.
Mosquitoes that spread WNV lay their eggs on standing or stagnant water. Keep mosquitos from breeding around your home by dumping and draining any containers that hold water, including:
- Buckets,
- Old car tires,
- Birdbaths, and
- Pet bowls.
Swimming pools that are not maintained can also be a breeding source for mosquitoes. Contact the Vector Control Program if you notice a green swimming pool in your neighborhood, or know of an unattended home that has a swimming pool. Technicians will treat the pool to prevent it from breeding mosquitos.
WNV is most commonly spread to people by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. WNV is not spread from person to person.
In a very small number of cases, WNV has been spread through exposure in a laboratory setting; mother to baby during pregnancy, delivery, or breast feeding; blood transfusion; and organ transplant. It is recommended that anyone recently diagnosed with WNV infection should not donate blood or bone marrow for 120 days following infection.
After getting bitten by a mosquito with WNV, symptoms may develop within 3 to 14 days. The incubation period may be longer in persons with weakened immune systems
Illness from WNV can range from no symptoms, mild symptoms, to severe illness. Most people (8 out of 10) who are infected with WNV do not develop any symptoms.
Some people (about 1 in 5 infected with WNV) develop a fever with other symptoms, such as:
- Headache,
- Body aches,
- Joint pains,
- Vomiting,
- Diarrhea, or
- Rash.
Most people will recover completely, but fatigue and weakness can last weeks or months.
A few people (about 1 in 150 infected with WNV) will become very sick. The virus can affect:
- The central nervous system and can cause encephalitis (inflammation of the brain), or
- Meningitis (inflammation of the membranes that surround the brain and spinal cord).
Symptoms can include:
- High fever,
- Headache,
- Neck stiffness,
- Stupor (almost unconscious),
- Disorientation,
- Tremors,
- Convulsions,
- Muscle weakness,
- Vision loss,
- Numbness,
- Coma, and
- Paralysis.
Recovery from severe illness may take several weeks or months. Some effects may be permanent. Severe illness may happen in people of any age, however, people over 60 years of age are at greater risk for severe illness if they are infected, as well as people with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants.
WNV can be fatal for about 1 out of 10 people who develop severe illness affecting the central nervous system.
If you think you may have WNV, talk with your healthcare provider.
Your healthcare provider may diagnose infection based on:
- Signs and symptoms,
- History of possible exposure to mosquitoes that can carry WNV, and
- Laboratory testing of blood or spinal fluid.
There are no specific medicines available to treat WNV. Antibiotics do not treat viruses. Rest, fluids, and over-the-counter pain medications may relieve some symptoms. In severe cases, patients may need to be hospitalized to receive supportive treatment.
Most people infected with WNV are believed to have lifelong immunity, or protection, from getting the disease again. Some people who have weakened immune systems from certain conditions or medications may not have a strong immune response to the initial infection or their immunity may wane over time.
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