West Nile cases dropped sharply in 2004

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Dec 29, 2004 (CIDRAP News) – West Nile virus continued its relentless blanketing of the United States in 2004, marching westward but leaving far fewer dead and ill people in its wake than it did last year.

A total of 2,448 cases have been reported to the Centers for Disease Control and Prevention (CDC) in Atlanta so far this year. That includes 87 deaths, 888 cases of neuroinvasive disease (West Nile meningitis and West Nile encephalitis), 1,011 West Nile fever cases, and 549 other clinical or unspecified cases.

Although the epidemic had as broad a geographic reach this year as last year, the 2004 case total represented a sharp drop from last year's 9,862 cases. The 2003 total included 6,830 fever cases, 2,866 cases of neuroinvasive disease, and 166 unspecified cases. There were 264 deaths.

The hardest-hit states this year were in the West and Southwest, as measured by the numbers of neuroinvasive cases. California had 154 cases, Arizona 128, Texas 105, and Colorado 39. In contrast, last year California and Arizona had only 2 and 7 neuroinvasive cases respectively, while Colorado had 621. Texas was about the same last year, with 108 cases.

Because West Nile fever is less severe, and because reporting of fever won't be nationally mandated until 2005, experts focus on the neuroinvasive cases, explained Dan O'Leary, DVM, a medical epidemiologist in CDC's Division of Vector-Borne Infectious Diseases in Fort Collins, Colo.

Aside from the change in the number of people afflicted, other aspects of the disease this year were generally consistent with past years. The 87 deaths this year represent a case-fatality rate for neuroinvasive disease of about 9.8%, compared with a rate of about 9.2% in 2003. O'Leary said the neuroinvasive case-fatality rate has remained steady at about 9% to 10% since the virus landed on US soil.

Only about 20% of people infected with the virus get sick and only about 1 in 150 infected people has neurologic involvement, according to information on CDC's Web site.

Once a state has the virus, it remains endemic from season to season, O'Leary said. Only Washington state has reported finding the virus in animals (a bird and a horse in 2002) without seeing any subsequent animal or human cases. Cooler weather appears to reduce the spread of the virus, which may contribute to Washington's situation, he added.

As in past years, some severe localized outbreaks occurred this year, often on the leading edge of the West Nile wave or in places where the virus arrived within the past year, O'Leary said. Last year Colorado had a severe epidemic with 2,947 total cases, but this year the center of West Nile activity moved west, he said. Phoenix, Ariz., and Riverside, San Bernardino, and Los Angeles counties in California saw more than 100 cases each. However, Mesa County, Colo., had about 125 cases this year, he said.

A mosquito-borne pathogen, West Nile virus was discovered in New York City in 1999 and spread steadily westward in the ensuing years. Now some 55 species of mosquito carry the virus, although Culex mosquitoes are considered the primary vectors, O'Leary said.

Although the epidemic's severity dropped in 2004, O'Leary urged people to continue to use precautions against West Nile.

"States where the virus has occurred previously and continues to occur, there is ongoing human risk. People shouldn't let down their guard because it wasn't a big epidemic," he said. He recommended checking local West Nile activity on state health department Web sites and remembering to use effective mosquito sprays for outdoor activities.

One reason to stay vigilant is that researchers are learning more about how serious West Nile is in people, O'Leary added. Ongoing studies of neuroinvasive cases show some patients suffer "lingering neurologic problems months or even years after the initial disease."

See also:

2004 WNV activity by state

2003 WNV activity by state

Sep 8, 2004, CIDRAP News story on lingering effects of West Nile fever

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