The government is cautioning travelers planning to visit North America to be on the alert for West Nile fever.
The Health, Labor and Welfare Ministry recently issued the warning for the summer travel season — some 3.4 million Japanese travelers visited the United States and Canada last year.
When the first case of the virus was identified in the United States in New York City in 1999, it moved quickly, causing 264 deaths in 2003 across the country, according to the health ministry.
The ministry and other government agencies are increasing their watch for mosquitoes on international flights and screening imported birds.
Any traveler who returns home with a fever should seek immediate medical attention, they said.
Officials at the National Institute of Infectious Diseases said the virus’ incubation period is from three to 15 days and symptoms include fever, headache, back pain and skin rash.
An infected person is likely to recover in about a week, and about 80 percent of patients shake off the virus without experiencing any symptoms.
But in serious cases, people can suffer paralysis or spasms, or even fall into a coma. Some can develop meningitis or encephalitis and die.
Health experts say elderly people have accounted for the most serious cases in the United States and about 5 percent of the patients have died.
West Nile fever was first discovered in Africa and the Middle East.
It has not yet been determined how the virus appeared suddenly in the U.S. There has been speculation that mosquitoes carrying the virus made their way to New York aboard airplanes or that infected birds were brought in to the country.
The number of West Nile patients last year totaled 9,862 in the United States and more than 1,000 in Canada.
Outbreaks occur in early summer and later when mosquitoes become more active.