West Nile Virus

Imperial, California - West Nile infection is caused by a virus transmitted by mosquitoes. Most people infected with West Nile virus don't experience any signs or symptoms, or may experience only minor ones, such as fever and mild headache. However, some people who become infected with West Nile virus develop a life-threatening illness that includes inflammation of the brain.

Mild signs and symptoms of a West Nile virus infection generally go away on their own. But severe signs and symptoms - such as a severe headache, fever, disorientation or sudden weakness - require immediate attention.

Exposure to mosquitoes where West Nile virus exists increases your risk of getting West Nile virus infection. Protect yourself from mosquitoes by using mosquito repellent and wearing clothing that covers your skin to reduce your risk.

Most have no signs or symptoms

Most people infected with the West Nile virus have no signs or symptoms.

Mild infection signs and symptoms

About 20 percent of people develop a mild infection called West Nile fever. Common signs and symptoms of West Nile fever include:

  • Fever
  • Headache
  • Body aches
  • Fatigue
  • Back pain
  • Skin rash (occasionally)
  • Swollen lymph glands (occasionally)
  • Eye pain (occasionally)

Serious infection signs and symptoms

In less than 1 percent of infected people, the virus causes a serious neurological infection. Such infection may include inflammation of the brain (encephalitis) or of both the brain and surrounding membranes (meningoencephalitis). Serious infection may also include infection and inflammation of the membranes surrounding the brain and spinal cord (meningitis), inflammation of the spinal cord (West Nile poliomyelitis) and acute flaccid paralysis — a sudden weakness in your arms, legs or breathing muscles. Signs and symptoms of these diseases include:

  • High fever
  • Severe headache
  • Stiff neck
  • Disorientation or confusion
  • Stupor or coma
  • Tremors or muscle jerking
  • Lack of coordination
  • Convulsions
  • Pain
  • Partial paralysis or sudden muscle weakness

Signs and symptoms of West Nile fever usually last a few days, but signs and symptoms of encephalitis or meningitis can linger for weeks, and certain neurological effects, such as muscle weakness, may be permanent.

When to see a doctor

Mild symptoms of West Nile fever usually resolve on their own. If you experience signs or symptoms of serious infection, such as severe headaches, a stiff neck, disorientation or confusion, seek medical attention right away. A serious West Nile virus infection generally requires hospitalization.

Infection transmitted by mosquitoes

Typically, West Nile virus spreads to humans and animals via infected mosquitoes. Mosquitoes become infected when they feed on infected birds. You can't get infected by touching or kissing a person with the virus.

Most West Nile virus infections occur during warm weather, when mosquito populations are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — ranges from three to 14 days.

West Nile virus is present in areas such as Africa, parts of Asia and the Middle East. It first appeared in the United States in the summer of 1999 and since then has been found in all 48 contiguous states.

Other possible routes of transmission

In a few cases, West Nile virus may have been spread through other routes, including organ transplantation and blood transfusion. However, blood donors are screened for the virus, substantially reducing the risk of infection from blood transfusions.

There have also been reports of possible transmission of the virus from mother to child during pregnancy or breast-feeding, but these have been rare and not conclusively confirmed.

Additional information