by Steve Christensen

Created on: June 16, 2010

West Nile virus is a mosquito-borne arbovirus (arthropod-borne virus) which causes neurologic disease in humans. Meningitis – inflammation of the membranes surrounding the brain and spinal cord – is only one manifestation of this disease.

West Nile virus was first recognized in the West Nile district of Uganda in 1937 and was a relatively common cause of illness – characterized by fever, meningitis, encephalitis, and a polio-like paralysis – in Africa, Asia, Europe, and Australia until 1999, when the virus infected 59 people in New York City and thereby first entered the Western hemisphere.

By the end of 2001, West Nile virus had spread throughout the southern and Midwestern US and had established itself in mosquitoes, humans, and other animals in 28 states. In 2002, West Nile spread rapidly westward and caused the largest epidemic of human arboviral encephalitis in US history. As of 2008, only Alaska and Hawaii had not reported the virus' presence in either birds, mosquitoes, or mammals (including humans). A natural reservoir for West Nile virus now exists in birds, with mosquitoes remaining the only known vector (i.e., the mode of bird-to-bird or bird-to-mammal transmission).

Clinical Syndromes Caused by West Nile Virus

Among persons who are infected by West Nile virus, 80 percent remain symptom-free; approximately 20 percent have a self-limited illness with fever, headache, and flu-like symptoms. Conjunctivitis, rash, hepatitis, pancreatitis, and myocarditis are less common manifestations of infection.

Less than one percent of infected individuals – about 1 in 150 – develop meningitis, encephalitis, or paralysis.

West Nile virus infection appears to equally affect all ages and both genders. However, people who are older than 50 demonstrate a twenty-fold higher risk of complications and death than younger individuals.

Neurologic syndromes caused by West Nile virus include:

-Meningitis: Inflammation of the meninges (membranes surrounding the brain) is often heralded by fever and a headache and progresses to neck stiffness, irritability, and extreme light sensitivity.

-Encephalitis: Inflammation of the brain tissue itself, encephalitis is characterized by changes in mental status (confusion, dementia, delirium, etc.). Encephalitis may be accompanied by symptoms of meningitis; severe cases may progress to include convulsions and coma.

-Flaccid paralysis: Patients with West Nile-induced paralysis typically

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