This year's outbreak of West Nile virus is the worst since the illness was first observed in the United States in 1999, officials from the U.S. Centers for Disease Control and Prevention said Wednesday. The number of confirmed cases rose by 25% last week to 1,993 -- although only an estimated 2% to 3% of cases are reported to the government. Those are generally the most serious infections: Most people who contract the virus do not develop severe symptoms, and many never even know they were infected. The most common symptoms are a fever and neck stiffness. Severe cases can lead to encephalitis or meningitis.
The number of deaths rose to 87, up from 65 a week ago. The death total seems unlikely to break the record of 260 set in the 2002-03 season, however.
Almost half of the reported cases (888) are in Texas, which has seen 35 deaths from the disease. Other states with high rates include Mississippi, South Dakota, Oklahoma and Louisiana. California has had 55 confirmed cases and two deaths.
The outbreak probably peaked in August, CDC officials said, although the number of new cases will most likely continue to grow until October -- at least in part because of lags in reporting them. Officials said a mild spring, very hot summer and heavy rains in some areas probably contributed to this year's increases.