Health A-Z

West Nile virus

Clinical Definition

West Nile virus is an arthropod-borne flavivirus that is transmitted by mosquitos. Most infected individuals are asymptomatic, but some develop a mild febrile illness; more rarely, individuals develop serious illness such as West Nile meningo-encephalitis.

In Our Own Words

The West Nile virus is an infection spread by mosquitos. Endemic to Africa, the Middle East and some parts of Asia, West Nile has been in the United States for a decade or more. Most patients never even suspect they have West Nile virus because symptoms are so mild. Some cases cause West Nile fever, which will produce fever and body aches. Rarely, the virus will infect the lining of the brain and spinal cord (meninges) or create inflammation in the brain itself  (meningo-encephalitis) to produce symptoms such as stiff neck, decreased mental abilities, reduced alertness, tremors, and other neurological symptoms. Less than 1 percent of people who are infected become seriously ill from the disease. Elderly people and those with weak immune systems are more likely to become ill. There is no vaccine for West Nile virus, but the mainstay of prevention is reducing the risk of mosquito bites with insect repellent.

Symptoms and Side Effects

None, or:
  • Rash
  • Headache
  • Fever
  • Body aches
  • Swollen glands
  • Neurologic symptoms in less than 1 percent
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