A Kawasaki man in his 30s has come down with West Nile fever, the first confirmed case in Japan, sources said Monday.
It is suspected the man was infected in the United States during his stay there from late August to early September, the sources said.
The man visited Los Angeles between Aug. 28 and Sept. 4.
He came down with a fever when he returned to Japan and initially received treatment at a hospital in Kawasaki, according to the sources.
The National Institute of Infectious Diseases and other institutions then conducted tests on the suspicion he was infected with West Nile, they said. On Monday, the institute determined he contracted the virus.
West Nile fever can be transmitted by mosquitoes or birds, and humans can be infected if they are bitten by a mosquito carrier, medical experts said. It is believed the disease is not transmitted directly person-to-person.
About 80 percent of people who are infected show no symptoms, and less than 1 percent fall seriously ill, they said.
Serious cases, however, can lead to extremely high fever and encephalitis, and can prove fatal.
West Nile fever was long believed to be endemic to mainly Africa and the Middle East. But it broke out in Europe and Russia in the late 1990s.
The first U.S. case was discovered in 1999, and the disease spread nationwide within about three years.
The disease killed about 100 of at least 2,500 people stricken with the disease in the United States last year.