December 1, 2015
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West Nile Virus

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West Nile virus facts

  • West Nile virus (WNV) is a virus capable of causing disease in humans.
  • Symptoms and signs of West Nile virus include fever, headache, body aches, skin rash, and swollen lymph nodes.
  • Severe symptoms and signs may include stiff neck, sleepiness, disorientation, coma, tremors, convulsions, and paralysis.
  • Most cases of West Nile virus infection 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. In 2015 alone, all 48 continental states and the District of Columbia have reported West Nile virus infections in people, birds, or mosquitoes.
  • 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. The most common causes of encephalitis are 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 42,000 people in the U.S. have been reported to be infected with West Nile virus. More than 1,700 have died.

As of November 2015, 1,732 cases of West Nile virus disease in people have been reported to the CDC for the year. This is the highest number of reported West Nile virus cases in a single year since the virus was first detected in the U.S. in 1999. Of these, 65% were classified as neuroinvasive disease (such as meningitis or encephalitis) and 35% were classified as non-neuroinvasive disease. The CDC states that West Nile virus in people was reported by 44 states and the District of Columbia. Approximately half of the cases of neuroinvasive WNV cases have been reported from six states (California, Texas, Colorado, Illinois, North Dakota, and Oklahoma). Arizona also experienced several cases in 2015.

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. The virus is a type of arbovirus ("arbo" comes from ARrthropod-BOrne, as many insects are arthropods). It is a member of the Flavivirus genus and the family Flaviviridae.

Medically Reviewed by a Doctor on 11/20/2015


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