West Nile Encephalitis
West Nile encephalitis is an infectious disease that primarily causes inflammation, or encephalitis, of the brain. The etiology stems from the West Nile virus (WNV), a flavivirus commonly found in humans, birds, and other vertebrates in Africa, West Asia, and the Middle East. This disease is part of a family of vectorborne diseases that also includes malaria, yellow fever, and Lyme disease.
The virus was first documented in the Western Hemisphere in August 1999, when a virus found in numerous dead birds in the New York, New Jersey, and Connecticut region was identified by genetic sequencing as WNY. Scientists in the United States first discovered the rare strain in and around the Bronx Zoological Park and believe imported birds may have carried the disease, which spread by mosquitoes that fed on the infected birds.
In temperate areas of the world, West Nile encephalitis cases occur mainly in late summer or early fall. In climates where temperatures are milder, West Nile encephalitis can occur year-round.
As of mid-November 1999, health officials at the Centers for Disease Control and Prevention (CDC) reported 56 cases of WNV infection (31 confirmed and 25 probable), including 7 deaths.
The risk of contracting West Nile encephalitis is greater for residents of areas where active cases have been identified. Individuals older than age 50 and those with compromised immune systems have the greatest risk. At this time, there is no documented evidence that a pregnant woman's fetus is at risk due to an infection with WNV. The mortality rate for West Nile encephalitis ranges from 3% to 15%; the mortality rate is higher in the elderly population.
This virus is transmitted to humans by the bite of an infected mosquito. Mosquitoes become infected by biting birds that harbor the virus. The virus is not spread from person to person or from infected birds to humans without a mosquito bite. The virus has now been found in 111 bird species and about a dozen mammals.
Symptoms and Signs
Common symptoms includes:
The enzyme-linked immunosorbent assay (ELISA), the MAC-ELISA, is the test of choice for obtaining a rapid definitive diagnosis. The major advantage of MACELISA lab analysis is the high probability of an accurate diagnosis of WNV infection. An accurate diagnosis is possible only when serum or cerebrospinal fluid specimens are obtained while the patient is still hospitalized with acute illness.
When developing a differential diagnosis, another condition to consider is St. Louis encephalitis, which causes similar symptoms. Inflammation of the brain can be caused by numerous viral and bacterial infections, so all data must be examined to make a definitive diagnosis.
There is no specific therapy to treat West Nile encephalitis and no known cure. Treatment is generally aimed at controlling the specific symptoms. Supportive care measures such as I.V. fluids, fever control, and respiratory support are rendered when necessary.
There is no vaccine at present to prevent the transmission of West Nile encephalitis.
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