Officials at the Centers for Disease Control and Prevention said about 75 percent of the cases were in five states: Texas, Mississippi, Louisiana, South Dakota and Oklahoma.
"The number of West Nile cases in people has risen dramatically in the last few weeks and indicates that we are in one of the biggest West Nile virus outbreaks we have ever seen in this country," Lyle Petersen, director of the CDC's Division of Vector-borne Infectious Diseases, told USA Today.
This year, 47 states have reported West Nile virus in humans, birds or mosquitoes, while 38 states reported infections in people, the CDC said.
Only about 20 percent of people who contract West Nile virus develop symptoms such as fever, headache, body ache, swollen lymph glands and occasionally a rash on the trunk of the body three to 12 days after infection from a mosquito bite.
Of those who develop a fever from West Nile, fewer than 1 percent develop neuroinvasive disease, which causes inflammation of the brain, spinal cord or the tissue surrounding the brain, which can be fatal, Petersen said.
People age 50 and older with compromised immune systems are more likely to become infected and develop the neuroinvasive disease, the CDC said.