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West Nile Virus 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 WNV. 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.

West Nile virus occurs in late summer and early fall in temperate zones, but can occur year round in southern climates. Usually, the West Nile virus causes mild, flu-like symptoms. However, the virus can cause life-threatening illnesses such as encephalitis (inflammation of the brain), meningitis (inflammation of the lining of the brain and spinal cord), or meningoencephalitis (inflammation of the brain and its surrounding membrane).


WNV is transmitted to humans by the bite of a mosquito primarily the Culex genus), that is infected with the virus. They are considered the primary vector for WNV and the source of the August 1999 outbreak in New York, New Jersey, and Connecticut. Mosquitoes become infected by feeding on birds contaminated with the virus. The mosquitoes may then transmit the virus to humans and animals when taking a blood meal.

Ticks infected with WNV have been found in Africa and Asia only. The role of ticks in the transmission and maintenance of the virus remains uncertain; to date, ticks haven't been considered a vector for transmission in the United States.

The CDC has reported that there is no evidence that a person can contract the virus by handling live or dead infected birds. However, people should be instructed to use gloves or double plastic bags to place the carcass of any dead bird or animal in a garbage can; the finding should be reported to the nearest Emergency Management Office.

Signs and symptoms

Symptoms occur around 3 to 15 days after being bitten by an infected mosquito. West Nile virus usually causes a mild infection with flu-like symptoms that include fever, headache, and body aches. Some people may also develop a skin rash and swollen lymph glands. In more severe cases, the patient develops meningoencephalitis, which is an inflammation of the spinal cord and brain. Symptoms can include severe headache, high fever, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. West Nile virus results in death in approximately 3 to 15 percent of all cases, primarily among the elderly. It appears that those who develop West Nile virus are then immune to any future West Nile infections.

Diagnostic tests 

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 treatment for West Nile virus-related diseases, although you must be diagnosed to rule out forms of meningitis such as meningococcal meningitis which can be treated with antibiotics. For the severe cases, the patient may be hospitalized to provide intravenous fluids, keep them breathing, provide good nursing comfort care, and prevent getting other infections such as pneumonia or urinary tract infections.


There is no approved vaccination currently available for West Nile Virus. To reduce your risk of becoming infected, take these steps to avoid mosquito bites:

  • Stay indoors at dawn, dusk, and in the early evening when mosquitoes are most active.
  • Wear long-sleeved shirts and long pants when you are outdoors.
  • NEVER apply insect repellants to children under 3 years old.
  • Limit the number of places available for mosquitoes to lay their eggs by eliminating standing water sources from around your home.

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