A man scratches at a mosquito bite in the woods.
A man scratches at a mosquito bite in the woods.
A man scratches at a mosquito bite in the woods.

West Nile Virus

What is the West Nile virus?

The West Nile virus is spread by mosquitoes. The West Nile virus can infect humans, birds, mosquitoes, horses, and some other mammals. Very rarely, the virus can spread in transfused blood, a transplanted organ, or through the placenta to a fetus.

West Nile virus occurs in late summer and early fall in mild zones. It can also occur year-round in southern climates. Most often, the West Nile virus causes mild, flu-like symptoms. But, the virus can cause life-threatening illnesses, such as:

  • Encephalitis (inflammation of the brain)
  • Meningitis (inflammation of the lining of the brain and spinal cord)
  • Meningoencephalitis (inflammation of the brain and its surrounding membrane)

What causes West Nile virus?

West Nile virus is spread to humans through the bite of an infected female mosquito. The mosquitoes get the virus when they bite an infected bird. Crows and jays are the most common birds linked to the virus. But at least 110 other bird species also have the virus.

West Nile virus isn't spread between humans. However, in a few cases it has spread through organ transplant. Health officials think the organ donor acquired the virus through a blood transfusion. All blood is screened for the virus. The risk for getting West Nile virus from blood is much lower than the risk of not having any procedure that would call for a blood transfusion.

What are the symptoms of West Nile virus?

Most people infected with West Nile virus have only mild, flu-like symptoms that last a few days. Symptoms usually appear within 3 to 14 days of infection.

About 20% of the people who become infected will develop West Nile fever. These are the most common symptoms of West Nile fever:

  • Fever
  • Headache
  • Body aches
  • Skin rash on trunk of body
  • Swollen lymph glands

The more severe form of the West Nile virus affects mostly older adults. It occurs when the virus crosses the blood-brain barrier and can cause:

  • Headache
  • High fever
  • Neck stiffness
  • Stupor (a state of impaired consciousness, extreme lethargy, and reduced reactivity to outside stimuli)
  • Disorientation
  • Coma
  • Tremors
  • Convulsions
  • Muscle weakness
  • Paralysis

The symptoms of West Nile virus may look like other conditions or health problems. Always see your healthcare provider for a diagnosis.

What are the risk factors for West Nile virus?

Certain things can increase the risk for getting West Nile virus. You are more likely to get the virus if you are exposed to mosquito bites during the summer months.

Most people who are infected have a minor illness and recover fully. But, older people and those with weak immune systems are more likely to get a serious illness from the infection.

How is West Nile virus diagnosed?

Your doctor will order a blood test to check for antibodies to the West Nile virus. He or she may also do a lumbar puncture to test cerebrospinal fluid for signs of infection.

How is West Nile virus treated?

Your healthcare provider will figure out the best treatment based on:

  • How old you are
  • Your overall health and medical history
  • How sick you are
  • How well you can handle specific medications, procedures, or therapies
  • How long the condition is expected to last
  • Your opinion or preference

There's no specific treatment for West Nile virus-related diseases. If a person gets the more severe form of the disease, West Nile encephalitis or meningitis, treatment may include intensive supportive therapy, such as:

  • Hospitalization
  • Intravenous (IV) fluids
  • Breathing support (ventilator)
  • Prevention of other infections (such as pneumonia or urinary tract infections)
  • Nursing care

What are the complications of West Nile virus?

Usually, the West Nile virus causes mild, flu-like symptoms. However, the virus can cause life-threatening illnesses, such as

  • Encephalitis (inflammation of the brain)
  • Meningitis (inflammation of the lining of the brain and spinal cord)
  • Meningoencephalitis (inflammation of the brain and its surrounding membrane)

Can West Nile virus be prevented?

At this time, there's no vaccine available to prevent West Nile virus. The CDC recommends taking these steps to avoid mosquito bites and West Nile virus:

  • Apply insect repellent containing DEET (N,N-diethyl-meta-toluamide) when you're outdoors. (If you spray your clothing, there's no need to spray repellent containing DEET on the skin under your clothing.)
  • When possible, wear long-sleeved shirts and long pants treated with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing. (Don't directly apply repellents containing permethrin to exposed skin.)
  • Consider staying indoors at dawn, dusk, and in the early evening. These are peak hours for mosquito bites, especially those mosquitoes that carry the West Nile virus.
  • Limit the number of places for mosquitoes to lay their eggs by getting rid of standing water sources from around your home.

Mosquitoes are drawn to people’s skin odors and the carbon dioxide you breathe out. Many repellents contain a chemical, N, N-diethyl-m-toluamide (DEET), which repels the mosquito. Repellents are effective only at short distances from the treated surface, so mosquitoes may still be flying nearby. Always follow the directions on the insect repellent to determine how often you need to reapply repellent. To boost your protection from insect repellent, remember:

  • Sweating, or water may call for reapplication of the product.
  • If you aren't being bitten, you don't need to reapply repellent.
  • Use enough repellent to cover exposed skin or clothing. Don't apply repellent to skin that's under clothing. Heavy application isn't needed for protection.
  • Don't apply repellent to cuts, wounds, or irritated skin.
  • After returning indoors, wash treated skin with soap and water.
  • Don't spray aerosol or pump products in enclosed areas.
  • Don't apply aerosol or pump products directly to your face. Spray your hands and then rub them carefully over the face, avoiding your eyes and mouth.

Repellents containing a higher concentration of active ingredient (such as DEET) provide longer-lasting protection. Read the directions to find out how long your product will last.

The American Academy of Pediatrics advises using care when putting insect repellent on children:

  • Use products with a low concentration of DEET, 30% or less, on children between ages 2 and 12. (Some experts suggest that it's acceptable to apply repellent with low concentrations of DEET to infants older than age 2 months. For children younger than age 2, only one application per day of repellent containing DEET is recommended.)
  • When using repellent on a child, apply it to your own hands and then rub them on your child.
  • Avoid children's eyes and mouth and use the repellent sparingly around their ears.
  • Don't apply repellent to children's hands because children tend to put their hands in their mouths.
  • Don't allow a young child to apply his or her own insect repellent.
  • Keep repellents out of reach of children.
  • Don't apply repellent to skin under clothing. If repellent is applied to clothing, wash treated clothing before wearing again.

Always consult your healthcare provider for more information.

When should I call my healthcare provider?

Most people infected with the West Nile virus will have only mild symptoms. However, if any of the following serious symptoms develop, seek medical attention right away:

  • High fever
  • Severe headache
  • Stiff neck
  • Confusion
  • Muscle weakness                                                                                                                   
  • Vision loss
  • Numbness
  • Paralysis
  • Tremors
  • Seizures
  • Coma

Key points

  • Humans get West Nile from the bite of an infected mosquito.
  • Usually, the West Nile virus causes mild, flu-like symptoms.
  • The virus can cause life-threatening illnesses, such as encephalitis, meningitis, or meningoencephalitis.
  • There is no vaccine available to prevent West Nile virus. So, it is important to avoid mosquito bites.

Next steps

Tips to help you get the most from a visit to your healthcare provider:

  • Know the reason for your visit and what you want to happen.
  • Before your visit, write down questions you want answered.
  • Bring someone with you to help you ask questions and remember what your provider tells you.
  • At the visit, write down the name of a new diagnosis, and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you.
  • Know why a new medicine or treatment is prescribed, and how it will help you. Also know what the side effects are.
  • Ask if your condition can be treated in other ways.
  • Know why a test or procedure is recommended and what the results could mean.
  • Know what to expect if you do not take the medicine or have the test or procedure.
  • If you have a follow-up appointment, write down the date, time, and purpose for that visit.
  • Know how you can contact your provider if you have questions.

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