ATLANTA (AP) _ The United States is headed for another record number of West Nile cases this year, with the total shooting up by more than a third in the past week alone, the government said Thursday.
Nationwide, 4,137 human cases had been reported by Thursday, just 19 shy of last year's total of 4,156, the Centers for Disease Control and Prevention said.
Despite the high number of infections, there have been far fewer deaths. So far, 80 have been reported; last year, 284 died from the virus. Health officials said one reason for the drop is that people with mild symptoms are now more likely to be tested.
The number of reported infections climbed by more than 1,200 in the past week. But health officials said even that may be far lower than the number of actual cases, because many mild infections are not diagnosed as West Nile.
Dr. Lyle Petersen, who heads the CDC's studies of mosquito-borne diseases, said at a conference this week that about 100,000 Americans could get a nasty three-day bout of fever from the virus, and 500,000 people could be infected in all, most showing no symptoms.
Last week's jump came at the peak of the season for the mosquito-spread virus.
Dr. Raoult Ratard, state epidemiologist in Louisiana, said the number of reported cases is sure to grow, particularly in the South.
``In Louisiana it grows until mid-November to early December,'' he said. Louisiana has at least 52 cases and one death from the virus this year.
On Thursday, CDC director Dr. Julie Gerberding said that two people this season have become infected with the virus through a blood transfusion. Last year, 23 people got the virus through the blood supply.
``Our transfusion supply is far safer than it was a year ago,'' she said. ``But we're still on the lookout for the occasional case that could slip through.''
The nation's blood suppliers have been screening for West Nile since July.
Most of this year's reported cases have been in the South and West, with more than a third from Colorado (1,542), followed by 580 in South Dakota and 543 in Nebraska. The agency's count may be lower than state figures, however, since there often is a lag time in reporting.
Nevada and Oregon are the only states in the continental United States that have not yet reported signs of West Nile virus.
The virus has quickly swept westward since 1999, when it was found for the first time in the Western Hemisphere, in New York City.
The virus is passed by bites from mosquitoes that have fed on infected birds. A small fraction of people infected become seriously ill with encephalitis or meningitis.