The number of West Nile virus cases in the U.S. jumped dramatically in one week, increasing to 1,221, with 43 deaths, the Centers for Disease Control and Prevention reported today.

The report marked a substantial increase from last week's tally of 693 cases and 26 deaths.

Approximately 75% of the cases have been in five states: Texas, Mississippi, Louisiana, South Dakota and Oklahoma, the CDC said.

Texas has been hardest hit, accounting for almost half of all cases.

As of Wednesday, 23 deaths and 640 cases had been reported there, said Christine Mann with the Texas Department of State Health Services.

"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," said Lyle Petersen, director of the CDC's Division of Vector-borne Infectious Diseases.

So far this year, 47 states have reported West Nile virus in humans, birds or mosquitoes, the CDC said. Thirty-eight states have reported cases of the human disease.

The 1,221 cases are the highest number of West Nile virus disease cases reported to the CDC through the third week in August since the virus was first detected in the United States in 1999, the agency said.

Up to 20% of people who contract West Nile virus develop symptoms that include fever, headache, body ache, swollen lymph glands and occasionally a rash on the trunk of the body. Symptoms appear within two to 15 days and then disappear within a few days.

About 80% of people infected with the West Nile virus have no symptoms.

Of those who develop a fever, fewer than 1% develop West Nile neuroinvasive disease which causes inflammation of the brain, spinal cord or the tissue surrounding the brain. About 10% of those will die, said Marc Fischer, medical epidemiologist with CDC's mosquito-borne illness division in Ft. Collins, Colo.

People older than 50 and those with compromised immune systems are more likely to develop this form.

The West Nile virus first appeared in the United States in 1999, from Africa.