What is the prevalence of West Nile virus (WNV) infection and encephalitis (WNE)?

Updated: Apr 05, 2021
  • Author: David J Cennimo, MD, FAAP, FACP, AAHIVS; Chief Editor: Michael Stuart Bronze, MD  more...
  • Print

CDC presented a data update analyzing 21,869 cases of WNV disease from 2009-2018. Of these, 12,835 (59%) were WNV neuroinvasive disease cases and were reported to CDC from all 50 states, the District of Columbia, and Puerto Rico. 89% of all WNV patients had onset of disease during July–September with later cases in more northern lattitudes. Neuroinvasive disease incidence, case-fatalities, and hospitalization rastes increased with increasing age.  WNV neuroinvasive disease peaked in 2012 (0.92 cases per 100,000 population) and remained relatively stable during 2013–2018 (average annual incidence: 0.44; range: 0.40–0.51). In contrast, state level incidence varied from year to year. During 2009–2018, the highest average annual incidence of neuroinvasive disease occurred in North Dakota (3.16 cases per 100,000 population), South Dakota (3.06), Nebraska (1.95), and Mississippi (1.17), and the largest number of total cases occurred in California (2,819), Texas (2,043), Illinois (728), and Arizona (632). [3]

Among neuroinvasive WNV cases, 53% were reported as encephalitis, 37% as meningitis, 7% as AFP, and 3% as an unspecified neurologic presentation. The median age of patients with encephalitis (66 years; IQR: 54–75 years), AFP (60 years; IQR: 51–70 years), or an unspecified neurologic presentation (64 years IQR: 53–74 years) was higher than that of patients with meningitis (52 years; IQR: 38–64 years). Case-fatality ratios were higher among patients with encephalitis (14%) or AFP (13%) compared with those with an unspecified neurologic presentation (5%) or meningitis (2%). [3]

The peak month for WNV disease is August, with 83% infections occurring from July-September and 95% of infections occurring from July through the end of October. [8]

The three largest outbreaks in the United States occurred in 2002 (2,946 cases), 2003 (2,866 cases), and 2012 (2,873 cases). The 2012 outbreak saw an above-average incidence of neuroinvasive disease throughout the country. [17] More than half of the WNE cases in 2012 were reported from only four states, and 29% of the overall WNE cases were reported from Texas alone. [17] That year, the incidence of WNE was 7.3 per 100,000 in Dallas County, Texas, significantly higher than the previous record rate for the county (2.91 per 100,000). [18] This outbreak was associated with an unusually warm winter and correlated strongly with the local infected-mosquito population. [18]


Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!