West Nile Virus

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West Nile virus facts

  • West Nile is a virus capable of causing disease in humans.
  • Symptoms and signs include fever, headache, body aches, skin rash, and swollen lymph glands.
  • Severe symptoms may include stiff neck, sleepiness, disorientation, coma, tremors, convulsions, and paralysis.
  • Most cases of West Nile virus are mild and go unreported.
  • A key feature of neuroinvasive West Nile virus disease is encephalitis, an inflammation of the brain.
  • The virus is carried from infected birds to people by mosquitoes.
  • There is no evidence for transmission from person to person.
  • West Nile virus first gained attention in the U.S. in 1999 after an outbreak in New York City. Since then, outbreaks have occurred in 47 states.
  • Use of insect repellents may help reduce the risk of becoming infected with the West Nile virus.

What is the history of West Nile virus?

West Nile encephalitis is an infection of the brain that is caused by a virus known as the West Nile virus. First identified in Uganda in 1937, the virus is commonly found in Africa, West Asia, and the Middle East. "Encephalitis" means inflammation of the brain. One of the causes of encephalitis is viral and bacterial infections, including viral infections transmitted by mosquitoes.

West Nile virus had not been previously reported in the U.S. prior to an outbreak in New York in September 1999. According to the U.S. Centers for Disease Control and Prevention (CDC), since 1999 more than 30,000 people have been reported with West Nile virus. More than 1,200 have died.

As of August 2012, an outbreak of 1,118 cases of West Nile virus were confirmed by the CDC. This is the highest number of reported West Nile virus cases since the virus was first detected in the U.S. in 1999. Of those, 56% of patients developed neuroinvasive disease (involvement of the brain and nervous system), the most severe form of West Nile virus infection. The CDC reports neuroinvasive disease was reported by 47 states. Approximately 75% of the cases have been reported from five states (Texas, Mississippi, Louisiana, South Dakota, and Oklahoma), and almost half of all cases have been reported from Texas, many in the Dallas area.

Picture of a Culex pipiens mosquito
Picture of a Culex pipiens mosquito; SOURCE: CDC

Among all people who become infected with West Nile virus, most have mild symptoms that do not get reported. Typically, less than 1% will actually develop severe neuroinvasive disease, according to the CDC.

West Nile virus infection is also called West Nile fever or West Nile encephalitis.

Medically Reviewed by a Doctor on 8/24/2012

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West Nile Virus Symptoms and Risk Factors

West Nile virus infection cannot be spread by casual contact such as touching or kissing an infected person. In addition to transmission via mosquito bites, less common modes of transmission of the virus include organ transplantation, blood donation, and from mothers to their fetuses in the womb or to infants via breast milk.

Humans normally acquire the viral infection through a mosquito bite. The early fall, from late August to early September, is the most common time for infection to occur in the U.S. West Nile virus has the potential to cause a very serious illness, although 80% of people infected will not develop any symptoms at all. The others (about 20% of infected people) most commonly develop a mild illness, sometimes termed West Nile fever with symptoms of:

  • fever,
  • tiredness,
  • headaches,
  • body aches,
  • swollen lymph nodes, and
  • sometimes a rash.

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