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West Nile Virus (cont.)

What Causes West Nile Virus and West Nile Encephalitis?

West Nile virus is transmitted to humans by the bite of an infected mosquito. Mosquitoes become infected by biting birds (or other animals like horses or dogs) that harbor the virus; thus, mosquitoes are the vectors of West Nile encephalitis (WNE). The virus is not spread from person to person nor is it spread from infected birds to humans without a mosquito bite. The virus has now been found in 111 bird species and about a dozen mammals.

  • How West Nile virus entered New York in 1999 is not entirely clear. The most likely explanation is that the virus was introduced by an imported infected bird or by an infected human returning from a country where West Nile virus is common. Before the 1999 New York outbreak, West Nile encephalitis had been identified previously only in Africa, Asia, the Middle East, and only rarely in Europe.
  • Most cases of West Nile occur during the warm weather months when mosquitoes are most active. Nonetheless, the mild climate in southern U.S. states is expected to sustain the mosquitoes beyond those months.

What Are the Risk Factors for West Nile Virus and West Nile Encephalitis?

The major risk factor for West Nile virus and West Nile encephalitis is being exposed to mosquitoes that may be carrying the virus. Such individuals are those who spend time outdoors and have skin exposed for mosquitoes to bite (for example, campers, hikers, those engaged in outdoor working conditions). Individuals who are is 50 years of age or older and individuals who have a weakened immune systems (cancer patients, diabetics, for example) are at higher risk for both infection and encephalitis.

What Are the Symptoms and Signs West Nile Virus and West Nile Encephalitis?

Signs and symptoms of the West Nile virus infection range from no symptoms at all to a rapidly fatal brain infection. The incubation period ranges from two to 14 days although two to six days is the most common range. In areas where the virus is common, people are more likely to show no symptoms of the infection or have only a mild, flu-like illness rather than a severe brain infection. The following describes symptoms and their frequency of occurrence:

  • According to the CDC, the majority of people (70%-80%) who become infected show no symptoms and recover completely.
  • When symptoms develop, West Nile virus infection typically begins with the abrupt onset of fever, chills, muscle aches, headache, and flu-like symptoms. Headache is particularly common and may be severe. The person may have sensitivity to light with pain behind the eyes, and some patients may also develop vomiting, diarrhea, or a rash. Although fatigue and weakness may last for weeks or even months, most patients recover completely. The set of symptoms described here may occur in about 20% of infected patients.
  • In others, particularly the elderly, the disease can progress to cause encephalitis or meningitis. These patients may show neurological changes such as disorientation, tremors, seizures, and develop other symptoms such as headache, high fever, and neck stiffness. Some of the neurological effects will become permanent, and about 10% of people who develop severe neurological infections will die. People with certain medical conditions (cancer, diabetes, hypertension, and kidney disease) are at higher risk for serious West Nile virus infections.
Last Reviewed 11/21/2017

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West Nile Encephalitis »

West Nile encephalitis (WNE) is distinguished from other arthropod-borne causes of viral encephalitis (eg, western equine encephalitis, eastern equine encephalitis [EEE], Japanese encephalitis, Venezuelan encephalitis) based on its geographic distribution, clinical features, and laboratory findings.

Read More on Medscape Reference »

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