The number of cases so far this year is the highest recorded through August since the disease was first detected in the United States in 1999, according to the Centres for Disease Control and Prevention (CDC).

"The number of West Nile disease cases in people has risen dramatically. We are in the midst of one of the largest West Nile outbreaks ever seen," Lyle R. Petersen, director of the CDC's division of vector-borne diseases, said at news teleconference Wednesday.