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West Nile epidemic could be on the horizon

Infection Control Monitor, July 27, 2007

A dramatic rise in cases of West Nile virus in the United States may be a forecaster for a large future epidemic of the disease, CDC researchers reported this week.

The number of West Nile virus cases is nearly four times what it was a year ago, meaning that hospitals may be in store for a large epidemic, CDC officials told The New York Times. "It's certainly a warning sign that we need to be extremely vigilant. The worst is yet to come," Lyle Petersen, MD, director of the CDC's division of vector-borne infections told the newspaper Wednesday.

West Nile virus is carried by mosquitoes. It causes a mild, flu-like illness in 20% of those infected, and no symptoms in about 80% of cases. However, in about 1% of cases, the virus progresses to a brain infection that can be fatal.

Last year, 4,269 cases of West Nile virus were reported in the United States, including 1,495 brain infections, and 177 people died, the Times reported. So far this year, 122 cases have been reported, with the most in California and the Dakotas. There have been 42 brain infections and three deaths this year. The risk of severe illness increases with age.

This is early in the season for West Nile, with 90% of cases usually occurring in August and early September. It is impossible to predict whether the trend will continue, Petersen told the Times, adding that the increase may be weather related. If people keep getting infected at the current rate, "we could see the largest epidemic ever," he told the newspaper.

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