Encephalitis: Viral - Causes

West Nile Virus (WNV). Until 1999, the West Nile virus was generally restricted to Africa, the Middle East, southwestern Asia, eastern Europe, and Australia. It emerged in the United States with the first outbreak in New York City in 1999. As of 2004, WNV had been found in birds and mosquitoes in every state except Alaska and Hawaii. Human cases of West Nile encephalitis have been reported throughout the continental United States.

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How WNV Is Transmitted. WNV, discovered in Uganda in 1937, circulates primarily between birds and mosquitoes and can be carried long distances by migrating birds. In a given geographic area, the appearance of the virus among birds and mosquitoes generally precedes infection in humans. WNV has infected over 110 species of birds. In addition to mosquito-to-human transmission, other causes of human infection include blood transfusions, organ transplantation, and breast milk. In 2005, the FDA announced the first West Nile virus blood test to screen donors of blood, organs, cells, and tissues. The test will be used by blood banks throughout America to help protect patients who receive transfusions.

Severity of WNV. Approximately 80% of people infected with WNV will not have any symptoms; 20% will develop West Nile fever (which includes fever, headache, and occasional skin rash). Less than 1% of infected people will develop neuroinvasive disease, the most severe form of WNV. Neuroinvasive disease affects the nervous system and includes encephalitis, meningitis, and poliomyelitis. People over age 50 and those with weakened immune systems are at the greatest risk for neuroinvasive disease; the fatality rate for those afflicted ranges from 3 - 15%. According to the U.S. Centers for Disease Control (CDC), neuroinvasive disease symptoms include high fever, headache, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. Preliminary research is currently being conducted on vaccines to prevent WNV and antiviral drugs to treat it.

Tick-Borne Encephalitis Viruses

Tick-borne encephalitis (TBE) is commonly found in many countries throughout Europe, Asia, and the former Soviet republic, but it is reported only rarely in the U.S. Powassan encephalitis is the main tick-borne encephalitis found in the United States and Canada. Cases of tick-borne encephalitis have also been reported from Rocky Mountain spotted fever, but this is a bacterial (not viral) infection.

Enteroviruses

Enteroviruses account for between 10 - 20% of viral encephalitis cases. For example, the group A coxsackievirus has been detected in infants and children with encephalitis and is among the important viruses in the class. (It should be noted that the enteroviruses are nearly as common as cold viruses and are rarely serious.) Enteroviruses can be spread through food or water contaminated by trace amounts of fecal material and through sneezing and coughing.