Diseases & Conditions



Understanding the West Nile Virus

What is the West Nile virus?

A virus is a tiny life form that can grow inside a person or animal and may cause illness. West Nile virus is a type of virus endemic in Africa, the Middle East, and parts of Asia. It is now found in the United States, emerging in the summer of 2002.

How do you get West Nile virus?

The virus spreads by mosquitoes that become infected when they bite birds carrying the virus. When an infected mosquito then bites a person or animal, the virus is injected through the mosquito’s saliva into the bite wound.

In addition, four novel (non-mosquito) modes of transmission have been documented: through the placenta from mother to baby, through breast-feeding, through the transplantation of infectious organs, and through transfusion of infectious blood products.

What are the symptoms of West Nile virus?

In most cases, West Nile virus causes no illness or a mild self limiting febrile illness. In some cases, the virus can cause a mild illness called West Nile fever. The symptoms of West Nile fever resemble flu symptoms and may include:

  • Fever
  • Headache
  • Body aches
  • Skin rash
  • Swollen glands

Less frequently, West Nile virus can cause a more serious illness, such as West Nile meningo-encephalitis – which is inflammation of the brain – or meningitis – which is inflammation of the protective membrane surrounding the brain and spinal cord. Symptoms of serious illness may include:

  • Headaches
  • High fever
  • Stiff neck
  • Decreased mental abilities
  • Reduced alertness
  • Involuntary movement of muscles (tremors or convulsions)
  • Muscle weakness
  • Paralysis
  • Coma

In rare cases, West Nile virus can cause death.

How common is West Nile virus?

Many people get mosquito bites, but few get ill from West Nile virus. Most people infected with the virus have very mild or no symptoms. Less than 1 percent of people infected with West Nile virus get seriously ill.

Who is at risk for West Nile virus?

People who spend time outdoors in warm weather are at risk for mosquito bites, but only a small percentage of mosquitoes carry the West Nile virus. Among those that are bitten by infected mosquitoes, less than 30 percent will develop even mild symptoms. However, elderly people and those with weakened immune systems may be less able to fight the virus and, therefore, more likely to become ill from it.

Mosquitoes spread the virus, so the risk is greatest when mosquitoes are active and reproducing, Mosquitoes are most active in warm weather, and between dusk and dawn. In addition, female mosquitoes lay their eggs in standing or very slow moving water. By avoiding these conditions, you can reduce your risk of West Nile virus infection.

Blood products and solid organs transplanted from infected individuals have been shown to transmit West Nile virus. However, the nation’s blood supply is now screened for West Nile virus, and the number of transfusion associated cases has declined significantly.

How do I know if I have West Nile virus?

Most symptoms of West Nile virus are mild and will go away as your body fights the infection. However, see your doctor if you have symptoms that don’t go away or if they concern you. The doctor may order a blood test to look for evidence of the virus in your blood.

How is West Nile virus treated?

There is no specific treatment for West Nile virus. Treatment plans focus on relieving any bothersome symptoms, such as body aches or headache. People with severe illness may need to stay in the hospital to receive supportive therapies—such as intravenous fluids and breathing support—while their bodies work to fight the virus.

Is there a vaccine for West Nile virus?

There is currently no vaccine approved for use in humans; however, researchers are actively seeking to develop a vaccine.

What is the outlook for people with West Nile virus?

Most people with West Nile virus recover fully with no complications.

Can West Nile virus be prevented?

The virus itself cannot be prevented, but you can take steps to reduce your risk of mosquito bites. The US Centers for Disease and Control offer these suggestions:

  • Apply insect repellent with the ingredient DEET to your exposed skin before going outdoors. (Use care when applying insect repellent to children. Do not put repellent on their hands, and be careful to avoid their mouths and eyes.)
  • Spray your clothing with insect repellent.
  • Wear shoes and long-sleeve shirts and pants while outdoors, whenever possible
  • If possible, stay indoors between dusk and dawn.
  • Avoid activities in areas where mosquitoes are active and plentiful, such as near standing water.
  • Fix or install window and door screens to keep mosquitoes out of buildings and homes.
  • Drain standing water that may collect in items such as flowerpots, pet bowls, birdbaths, swimming pool covers, clogged rain gutters, buckets or barrels.
  • Report dead or dying birds to your state health department.

Where can I learn more?

CDC Hotline: 1.800.232.4636


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