West Nile virus cases up in 2006

The number of West Nile virus infections in the U.S. rebounded last year, reaching the highest point since 2003, the federal Centers for Disease Control and Prevention reported Thursday.

The CDC recorded nearly 4,300 West Nile cases, including about 1,500 patients who developed neuroinvasive disease -- a more severe form of infection that encompasses encephalitis, meningitis and muscle weakness or paralysis.

The highest of number of infections occurred in Idaho, with 996 cases; Texas, with 354 cases; and Colorado, with 345 cases. The agency reported 177 deaths related to West Nile virus last year.

The total number of infections last year represents a significant increase over 2004 and 2005, when the CDC recorded 2,539 and 3,000 cases respectively.

It is unclear why the numbers have jumped back up, said Mark Duffy, a veterinarian in the CDC’s Arbovirus Diseases Branch in Fort Collins, Colo.

“West Nile virus does not appear to be going away, and we have to deal with it for the foreseeable future,” Duffy said.

West Nile virus, which is primarily transmitted by mosquitoes, is named after the district in Uganda where the disease was first isolated. It circulates mostly among birds, but has also been found in some mammals and reptiles.

In humans, the virus can cause fever, encephalitis and other severe nervous-system diseases.

West Nile virus first appeared in New York in 1999, and quickly spread west, reaching California in 2002.

The highest number of West Nile virus infections occurred in 2003, when the CDC recorded 9,862 cases, of which 2,866 involved a neuroinvasive disease. There were 264 deaths that year.

Duffy said the resurgence of West Nile virus underscored the need for people to protect themselves with insect repellent or clothing during mosquito season, and to clean up pools of standing water where mosquitoes like to breed.

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jia-rui.chong@latimes.com