Atlanta ? 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 — including 26 in Kansas — had been reported by Thursday, just 19 shy of last year’s total of 4,156, the Centers for Disease Control and Prevention said. One reason for the higher numbers is a widely available test to diagnose the virus, health officials have 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.

The number of reported infections climbed by more than 1,200 in the past week. But health officials said even that might 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 this week that about 100,000 Americans could get a nasty bout of fever from the virus, and 500,000 people could be infected in all.