West Nile cases rose sharply in 2006

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Jun 7, 2007 (CIDRAP News) – The number of illnesses related to West Nile virus (WNV) in the United States rose in 2006 for the second year in a row, after a dramatic decline in 2004, suggesting that the virus will remain endemic, according to the Centers for Disease Control and Prevention (CDC).

In today's issue of Morbidity and Mortality Weekly Report, the CDC reported a total of 4,261 cases of WNV disease in 2006, compared with 3,000 cases in 2005, for a 42% increase.

Of the 2006 cases, 34.9% (1,491) were West Nile neuroinvasive disease (WNND) (meningitis, encephalitis, or acute flaccid paralysis), the CDC said. Another 61.3% (2,612) of cases were West Nile fever, and 3.7% (158) were unspecified. There were 161 deaths among the WNND cases, the report said.

The CDC looks at the number of WNND cases as the best indicator of human WNV disease trends, because health departments are more likely to report them, Mark Duffy, an Epidemic Intelligence Service officer with the CDC division of vector-borne diseases in Ft. Collins, Colo., told CIDRAP News. Compared to 2005, the number of WNND cases was up 14% in 2006.

WNV cases were reported in 23.3% of the nation's counties—731 counties in 43 states—in 2006. The state of Washington reported the disease in humans for the first time.

One of the hot spots in 2006 was Idaho, which saw its WNND count jump from just 4 cases between 2003 and 2005 to 139 in 2006, which accounted for 9.3% of the national total, the CDC report said. Other states that had high WNND case counts were Texas, with 229, Illinois, 127, Louisiana, 91, and Mississippi, 89.

Counties that had the highest incidence of WNND cases were mainly in the west-central United States, with 9.9 cases per 100,000 people in Idaho, 4.9 per 100,000 in South Dakota, and 3.2 per 100,000 in North Dakota, according to the CDC.

Among bird surveillance findings, the CDC said WNV infection has been identified in 300 species, including 11 in which the disease was identified for the first time in 2006.

The agency said that about 140 cases of WNV infection occur for every case of WNND and about 80% of people who are infected never have symptoms. On that basis, the CDC estimated that the nation had a total of 208,700 cases of WNV infection in 2006, including 41,750 cases of West Nile fever.

Past yearly WNV reports have shown that the incidence of WNND and death from WNND increase with age, particularly among those who are older than 60. In 2006 the median age for fatal WNND was 58, similar to previous years, the CDC said.

With the lack of an effective human WNV vaccine, prevention of the disease continues to depend on community-level mosquito control and personal protection against mosquito bites, the report says.

CDC. West Nile virus activity—United States, 2006. MMWR 2007 Jun 8;56(22):556-59 [Full text]

See also:

CDC's West Nile virus site
http://www.cdc.gov/ncidod/dvbid/westnile/index.htm

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