So far, 1,405 serious illnesses and 118 deaths from West Nile have been reported across the country. The bulk are in Texas but Louisiana, Mississippi, Oklahoma, South Dakota and Michigan have also seen substantial numbers.
The worst year for the mosquito-borne disease was 2002, which saw nearly 3,000 severe cases and 284 deaths, according to the Centers for Disease Control and Prevention.
This year's count of severe cases rose by more than 30 per cent in the past week, and is on track to surpass 2002, the CDC's Dr. Lyle Petersen said in a call with reporters on Wednesday.
While the height of mosquito season has passed, infections are expected to continue into October, and severe illness and death reports are expected to keep coming in for months, CDC officials said.
West Nile virus was first diagnosed in Uganda in 1937, but no cases were reported in the U.S. until 1999 in New York. The virus gradually spread across the country.
Only about 1 in 5 infected people get sick. Early symptoms can include fever, headache and body aches. Some recover in a matter of days. But 1 in 150 infected people will develop severe symptoms including neck stiffness, disorientation, coma and paralysis.