California reported one-third of the 2005 U.S. total of West Nile virus cases through December 1 and 25% of the cases of the viral neuroinvasive disease, the Centers for Disease Control and Prevention reported.
A total of 2,744 human cases of West Nile virus (WNV) disease were reported in 42 states in 2005, compared with 2,359 in 2004. There was a spread of the disease in both humans and animals into counties not previously affected, as well as recurrence where the disease had previously been reported. “The increase … suggests that endemic transmission of WNV in the United States will continue for the foreseeable future,” the CDC said (MMWR 2005;54:1253–6).
WNV infections in humans, birds, mosquitoes, and nonhuman mammals are reported to the CDC through ArboNET, an Internet-based arbovirus surveillance system managed by state health departments and the CDC.
Of the cases reported, 1,165 (43%) were WNV neuroinvasive disease (WNND), which includes meningitis, encephalitis, or acute flaccid paralysis; 1,434 (52%) were West Nile fever (WNF); and 145 (5%) were unspecified illnesses.
WNND had its highest incidence in the central United States, including South Dakota, Nebraska, and North Dakota, but there were also focal outbreaks in Illinois, Texas, and Louisiana.
“Nationally, reports of WNV disease began in late May, peaked during the third week in August, and lasted into November,” the CDC said.
The median age of WNND patients was 57 years (range 3 months to 98 years); 85% of the 1,165 patients were hospitalized and 7% died. The median age of those with WNND who died was 75 years (range 36–98 years).
The median age of patients with WNF was 48 years (range 1–92 years); 23% of the 1,434 patients were hospitalized and 0.3% died. The median age of the four who died was 89 years (range 44–92 years).
“Approximately 80% of all WNV infections are asymptomatic, approximately 20% cause WNF, and less than 1% cause WNND. The large percentage of WNND among reported cases reflects underreporting of WNF and lack of reporting of asymptomatic infections,” the CDC said.
“A total of 11,263 mosquito pools from 410 counties in 43 states and the District of Columbia tested positive for WNV.” Of these, 64% were made up of Culex mosquitoes, and “control of Culex mosquitoes remains critical to reducing risk for human WNV disease,” the CDC said.