CDC: West Nile Virus - Statistics, Surveillance, and Control

2008 West Nile Virus Activity
in the United States
(Reported to CDC as of October 7, 2008)

West Nile encephalitis and West Nile meningitis are forms of severe disease that affect a person’s nervous system. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord.
Click here for further explanation of WN meningitis and/or encephalitis.

West Nile fever refers to typically less severe cases that show no evidence of neuroinvasion.
WN fever is considered a notifiable disease, however the number of cases reported
(as with all diseases) may be limited by whether persons affected seek care, whether
laboratory diagnosis is ordered and the extent to which cases are reported to health
authorities by the diagnosing physician.

Other Clinical includes persons with clinical manifestations other than WN fever, WN encephalitis or WN meningitis, such as acute flaccid paralysis. Clinical/Unspecified cases are those for which sufficient clinical information was not provided.

See the case definition (2004) for Neuroinvasive and Non-Neuroinvasive Domestic Arboviral Diseases. From the CDC Epidemiology Program Office.

Total Human Cases Reported to CDC: These numbers reflect both mild and severe human disease cases occurring between January 1, 2008 to December 31, 2008 as reported through October 7, 2008 to ArboNET by state and local health departments. ArboNET is the national, electronic surveillance system established by CDC to assist states in tracking West Nile virus and other mosquito-borne viruses. Information regarding 2008 virus/disease activity is posted when such cases are reported to CDC.

Of the 1030 cases, 465 (45%) were reported as West Nile meningitis or encephalitis (neuroinvasive disease), 532 (52%) were reported as West Nile fever (milder disease), and 33 (3%) were clinically unspecified at this time. Please refer to state health department web sites for further details regarding state case totals.

Note: The high proportion of neuroinvasive disease cases among reported cases of West Nile virus disease reflects surveillance reporting bias. Serious cases are more likely to be reported than mild cases. Also, the surveillance system is not designed to detect asymptomatic infections. Data from population-based surveys indicate that among all people who become infected with West Nile virus (including people with asymptomatic infections) less than 1% will develop severe neuroinvasive disease. See: Mostashari F, Bunning ML, Kitsutani PT, et al. Epidemic West Nile Encephalitis, New York, 1999: Results of a household-based seroepidemiological survey. Lancet 2001;358:261-264.

For Case Information:
1999|2000|2001|2002|2003|2004|2005|2006|2007|2008



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