CDC reports on West Nile virus activity in 2009

Although incidence of West Nile virus-related disease was significantly lower in 2009 than in previous years, the disease’s unstable epidemiology demands continued surveillance and better interventions, according to CDC researchers.

In 2009, 720 cases of West Nile virus disease in 262 counties across 38 states and Washington, D.C., were reported to ArboNET, an Internet-based arbovirus surveillance system managed by state health departments and the CDC. Neuroinvasive disease represented 54% of reported cases, whereas the remaining 46% were considered non-neuroinvasive.

The incidence rate of these cases was the lowest since 2001, at 0.13 per 100,000 population in the United States. Twenty-four percent of cases occurred in Texas and 17% in California. Although the rate in Washington remained lower, a rise from two cases in 2008 to 26 in 2009 alerted the researchers to the disease’s capacity for change. “These findings illustrate the wide annual variability and focality of [West Nile virus] transmission,” CDC officials wrote in the Morbidity and Mortality Weekly Report.

Results also indicated that states with the highest incidence were Mississippi (1.05 cases per 100,000 residents), South Dakota (0.74), Wyoming (0.73), Colorado (0.72) and Nebraska (0.61).

An analysis of seasonality also showed that West Nile virus-related neuroinvasive disease was also most commonly reported in mid-August, with 91% of cases appearing during July and September.

Fifty-nine percent of neuroinvasive disease cases occurred in males. The median age of patients was 60 years, with data indicating higher incidence rates among older age groups. Most of these patients were hospitalized, and most cases were classified as encephalitis.

The researchers said about 20% of West Nile virus disease cases are non-neuroinvasive. Surveillance data on animals indicated that equines accounted for most of veterinary cases. Evidence of infection, however, was often found in dead birds — usually corvids — even in counties that reported no cases of human disease.

The researchers also included data on mosquito activity and infection in the report. In 2009, 6,646 mosquito pools in 40 states and Washington, D.C., tested positive for West Nile virus, they said, with Culex mosquitoes acting as the principal vectors of transmission. However, the first evidence of West Nile virus infection in Aedes epactius was found in Texas.

“The continuing disease burden caused by [West Nile virus] affirms the need for ongoing surveillance, mosquito control, promotion of personal protection from mosquito bites, and research into additional prevention strategies,” the researchers wrote. – by Melissa Foster

CDC. MMWR. 2010;59:769-772.

I have two comments about this report, which is timely and interesting. The passive surveillance system is clearly prone to underreporting, as the authors concede. The most striking contract is between Mississippi, which had the highest U.S. incidence, and the adjacent state of Alabama, which reported no cases. Overall, the decline in cases over the last few years is most gratifying: Why this is the case is unexplained. We can postulate introduction of better vector control and personal protective measures, but do they really explain the decline?

– Lowell S. Young, MD

Kuzell Institute for Arthritis and Infectious Disease San Francisco, CA

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