Dec 16, 2005 (CIDRAP News) – The number of West Nile virus (WNV) cases in the United States rebounded slightly this year after a sharp drop in 2004, signaling that the disease is here to stay, according to figures released by the Centers for Disease Control and Prevention (CDC).
A total of 2,744 WNV cases were reported to the CDC from January through Dec 1 this year, compared with 2,359 for the same period in 2004, a 16% increase, the CDC says in today's Morbidity and Mortality Weekly Report. Both totals are far below the 9,862 cases reported in 2003.
"The increase in reported cases of WNV disease in 2005 compared with 2004 suggests that endemic transmission of WNV in the United States will continue for the foreseeable future," the report states.
Of this year's cases, 42.5%, or 1,165, were West Nile neuroinvasive disease (WNND) (meningitis, encephalitis, or acute flaccid paralysis), the CDC said. Another 52.2% (1,434) of cases were West Nile fever, and 5.3% (145) were unspecified. West Nile caused 98 deaths, CDC spokeswoman Christine Pearson told CIDRAP News.
Almost 19% of the nation's counties—596 counties in 42 states—reported cases.
California was hit hard this year, with 854 cases, 31% of the national total, which included 285 cases of WNND, or 25% of the national total, the CDC says. Other states with high numbers of WNND cases were Illinois, with 133; Texas, 107; and Louisiana, 100.
The incidence of neuroinvasive cases was highest in the central part of the country, with 4.8 cases per 100,000 people in South Dakota, 2.1 per 100,000 in Nebraska, and 1.9 per 100,000 in North Dakota.
The report did not address whether Louisiana and other Gulf Coast states saw a big increase in WNV cases this year in the wake of Hurricane Katrina and other storms. Health agencies feared that standing water left by the storms would breed unusual numbers of mosquitoes, leading to a wave of West Nile cases.
Pearson couldn't give a specific assessment of the hurricanes' effects on case counts, but she said the numbers in the Gulf states in the relevant period "were consistent with typical late summer activity" for WNV.
Louisiana had an increase in cases, but there was no surge related to the hurricanes, the Louisiana Department of Health and Hospitals reported in a news release last month. As of Nov 17, the state counted 155 cases, compared with 114 in 2004 and 122 in 2003, the department said. (Because of lags in reporting, the CDC often lists lower state case numbers than state health departments do.)
"Despite seeing a few cases in the affected areas, there was no spike in West Nile because of Hurricanes Katrina and Rita," Louisiana State Eipdemiologist Dr. Raoult Ratard said in the news release. "The majority of cases this year have occurred in North Louisiana, which was not directly affected. Overall, the 2005 West Nile case count is similar to what we have seen in the past two years."
In Texas, the Department of State Health Services reported 158 WNV cases as of Nov 28, as compared with 119 cases in 2004. A CDC map of WNV activity as of Dec 6 shows 70 cases in Mississippi, 10 in Alabama, and 21 in Florida.
The CDC estimates that about 80% of all WNV infections cause no symptoms, about 20% cause West Nile fever, and fewer than 1% cause WNND. The high percentage of WNND cases this year (42.5%) reflects underreporting of West Nile fever and lack of reporting of asymptomatic infections, the report says.
WNND was made a nationally notifiable disease in 2002, but West Nile fever was not listed as nationally notifiable until this year. "The true incidence and public health impact of WNF [West Nile fever] remains underestimated by national surveillance," the article says.
The virus has been found in more than 300 species of birds since it first surfaced in North America in 1999, the CDC reports. Sixteen species were newly identified as carriers this year.
CDC. West Nile virus activity—United States, January 1–December 1, 2005. MMWR 2005 Dec 16;54(49):1253-6 [Full text]
CDC's West Nile virus site
Dec 29, 2004, CIDRAP News story "West Nile cases dropped sharply in 2004"
Nov 17 Louisiana news release