Neuroinvasive disease cases, refers to severe cases of disease that affect a person’s nervous system. These include encephalitis which is an inflammation of the brain, meningitis which is an inflammation of the membrane around the brain and the spinal cord and acute flaccid paralysis which is an inflammation of the spinal cord that can cause a sudden onset of weakness in the limbs and/or breathing muscles.
Click here for further explanation of WN meningitis and/or encephalitis.
Click here for further explanation of acute flaccid paralysis
Non–neuroinvasive disease cases refers to typically less severe cases that show no evidence of neuroinvasion–primarily West Nile fever. 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.
See the case definition (2011) for Neuroinvasive and Non-Neuroinvasive Domestic Arboviral Diseases (includes diseases caused by California serogroup viruses; eastern and western equine encephalitis viruses; and Powassan, St. Louis encephalitis, and West Nile viruses).
Presumptive viremic blood donors (PVDs) are people who had no symptoms at the time of donating blood (people with symptoms are deferred from donating) through a blood collection agency, but whose blood tested positive in preliminary tests when screened for the presence of West Nile virus. Some PVDs do go on to develop symptoms after donation, at which point they would be included in the count of human disease cases by their state.
Total Human Cases Reported to CDC: These numbers reflect both neuroinvasive and non-neuroinvasive confirmed and probable human disease cases occurring in 2012 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 2012 virus/disease activity is posted when such cases are reported to CDC.
In 2012, a total of 5,674 cases of West Nile virus disease in people, were reported to CDC. Of these, 2,873 (51%) were classified as neuroinvasive disease (such as meningitis or encephalitis) and 2,801 (49%) were classified as non-neuroinvasive disease. Seven hundred and three WNV presumptive viremic blood donors (PVDs) were reported.
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 seroseroepidemiological survey. Lancet 2001;358:261-264.