Number of West Nile virus cases surges

Lisa Schnirring

Jul 26, 2007 (CIDRAP News) The number of West Nile virus (WNV) cases reported so far this season has dramatically outpaced the number reported at about this time last summer, the US Centers for Disease Control and Prevention (CDC) said today.

The CDC, in tomorrow's issue of Morbidity and Mortality Weekly Report (MMWR), said 122 cases of WNV illness have been reported as of Jul 24. Last summer, the CDC had reported only 15 cases as of the middle of July.

Of the total cases reported so far this year, 34% (42) were West Nile neuroinvasive disease (WNND) (meningitis, encephalitis, or acute flaccid paralysis). Another 58% (71) were West Nile fever, and 7% (9) were unspecified. The virus has caused three deaths.

The focus of the WNND and West Nile fever cases appears to be in California and the Dakotas.

Lyle Peterson, director of the CDC's vector-borne infection division, said the high number of cases so far is a warning that individuals and communities should be extremely vigilant, according to a New York Times report today.

Ninety percent of WNV cases typically occur in August and early September, so it's difficult to say if the trend will continue, he told the Times.

South Dakota reported its first WNV death 2 days ago, according to a press release from the South Dakota Department of Health (SDDH). The death, which is not yet reflected in the CDC tally, occurred in a patient in the 80- to 89-year-old age-group in whom WNV encephalitis developed.

Lon Kightlinger, PhD, South Dakota's state epidemiologist, told CIDRAP News that the Culex tarsalis mosquito, an efficient vector for the disease, is abundant in South Dakota, even in severe drought years.

"Our cities, towns, and Indian reservations are putting up a good fight against the Culex mosquito," he said, adding that before 2002 few of the state's communities had mosquito-control programs. "Now, nearly two thirds of our population lives in a community with some type of mosquito control program."

Despite the growth of control programs, personal precautionssuch as wearing insect repellent, staying indoors during mosquito active periods, and eliminating standing waterare still vital for preventing WNV illnesses, Kightlinger said.

In 2006 the number of WNV illnesses in the United States rose for the second year in a row, after a dramatic decline in 2004, suggesting that the virus will remain endemic, the CDC said in a Jun 8 MMWR report.

CDC. West Nile virus updateUnited States, January 1July 24, 2007. MMWR Jul 27;56(29):740-1 [Full text]