The type of swine fever virus recently detected at a pig farm in the city of Gifu had not been found before in Japan, the National Agriculture and Food Research Organization said.
The variant, seen elsewhere in Asia and in Europe, was confirmed through genetic analysis, the institute said Friday, adding that it is highly likely it came from abroad.
Also on Friday, the Gifu Prefectural Government said the carcass of a wild boar found about 8 km from the farm Thursday also tested positive for swine fever, which is also known as hog cholera.
The infection was confirmed by a detailed examination at the institute and may be spreading through the boar population, the prefectural government said Saturday.
The farm ministry urged all prefectures to carry out swine fever tests if any more dead boars turn up.
The prefectural governments were also instructed to inspect all pig farms within 10 km of places where dead boars are found.
Hog cholera is endemic in Asia but does not affect humans even if an infected animal is consumed. The disease, which only affects pigs and wild boars, has a fatality rate of nearly 100 percent.
It is the first time since 1992 that a hog cholera infection has been confirmed in Japan. The government declared the virus eradicated in 2007.
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