West Nile virus (WNV) was first detected in the Western Hemisphere in 1999 in New York City and has since caused seasonal epidemics of febrile illness and neurologic disease across the United States, where it is now the leading cause of arboviral encephalitis. This report updates a previous report and summarizes WNV activity in the United States reported to CDC in 2009. A total of 38 states and the District of Columbia (DC) reported 720 cases of WNV disease. Of these, 33 states and DC reported 386 cases of WNV neuroinvasive disease, for an incidence of 0.13 per 100,000 population. The five states with the highest incidence of WNV neuroinvasive disease were Mississippi (1.05 per 100,000), South Dakota (0.74), Wyoming (0.73), Colorado (0.72), and Nebraska (0.61). Neuroinvasive disease incidence increased with increasing age, with the highest incidence among persons aged ≥70 years. A total of 33 WNV deaths were reported, 32 from neuroinvasive disease. Calculating from the number of neuroinvasive disease cases and projections from 1999 serosurvey data, CDC estimated that 54,000 persons were infected with WNV in 2009, of whom 10,000 developed nonneuroinvasive WNV disease. The continuing disease burden caused by WNV affirms the need for ongoing surveillance, mosquito control, promotion of personal protection from mosquito bites, and research into additional prevention strategies.
Since introduced into the United States in 1999, WNV has become the leading cause of arboviral encephalitis in the country. However, in 2009, the reported incidence of WNV neuroinvasive disease in the United States was 0.13 per 100,000 population, the lowest recorded since 2001 . During 2004–2007, WNV had appeared to reach a stable incidence of approximately 0.4 per 100,000, but incidence dropped to 0.2 per 100,000 in 2008 and continued to decline in 2009. This trend might be attributed to variation in populations of vectors and vertebrate hosts, accumulation of immunity in avian amplifying hosts, human behavior (e.g., use of repellents and protective clothing), community-level interventions, reporting practices, or environmental factors (e.g., temperature and rainfall).
In 2009, evidence of WNV human disease again was detected in all geographic regions of the continental United States. The highest incidence of WNV neuroinvasive disease continued to occur mainly in the west-central United States, likely because of the high efficiency of Cx. tarsalis as a WNV vector. Mississippi (31 cases, 1.05 cases per 100,000) continued to be among those states with the highest incidence of WNV neuroinvasive disease. Arizona, which had the second highest incidence of WNV neuroinvasive disease in 2008 (62 cases, 1.0 per 100,000), reported an 81% decrease in cases with 12 cases and an incidence of only 0.18 per 100,000 in 2009. After reporting its first two neuroinvasive disease cases in 2008, Washington reported the seventh highest state incidence in 2009 (26 cases, 0.39 per 100,000). These findings illustrate the wide annual variability and focality of WNV transmission.
The findings in this report are subject to at least two limitations. First, ArboNET is a passive surveillance system that depends on clinicians to consider the diagnosis of an arboviral disease, obtain the appropriate diagnostic test, and report any positive results. Diagnosis and reporting likely are incomplete, leading to underestimation of the true incidence of disease. Second, arboviral surveillance programs, testing capacity, and reporting can vary by county, state, or region, affecting incidence estimates.
In the absence of an effective human vaccine, prevention of WNV disease depends on community-level mosquito control and promotion of personal protective measures. Such measures include use of mosquito repellents, barrier protection (e.g., long-sleeved shirts, long pants, and socks), avoiding outdoor exposure, or using personal protection from dusk to dawn. Household measures, such as window screens and covering or draining peridomestic water-holding containers can further decrease the risk for WNV exposure.
Additional information on prevention of WNV infection is available from CDC at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm . An overview of current year WNV transmission activity is available at http://diseasemaps.usgs.gov/wnv_us_human.html .
To read the full report, go to http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5925a1.htm?s_cid=mm5925a1_e .
(Source: CDC Morbidity and Mortality Weekly Report, July 2, 2010 / 59(25); 769-772.)