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Mice carrying alien pathogens have taken up residence in Japanese ports, apparently after arriving inside freight containers from foreign countries, according to a three-year study by the Ministry of Health, Labor and Welfare.

A team of researchers led by Yasuhiro Yoshikawa, a professor at the University of Tokyo, conducted the study in 34 seaports and airports across Japan between 1998 and 2001 and found that some of the mice carry antibodies of a virus that causes lymphocytic choriomeningitis (LCM), a disease not previously found in Japan.

According to a report on the study, Yoshikawa’s team captured some 2,300 rats and mice of six different species. Foreign-species mice were found in eight ports: Otaru in Hokkaido, Nagoya, Osaka, Kobe, Tokyo, Yokohama, Kagoshima and Shibushi in Kagoshima Prefecture.

Five house mice carrying the antibody of the LCM virus were captured in container storage sites at Nagoya and Kobe ports.

The LCM virus can cause symptoms similar to influenza in humans, including fevers and respiratory problems. In serious cases, it can lead to cerebral meningitis and encephalitis, a fatal disease.

Yoshikawa’s team also captured 62 mice in the eight ports and two airports that carried hantavirus, a pathogen of hemorrhagic fever with renal syndrome (HFRS).

The illness causes fever and bleeding from the skin and kills between 5,000 and 20,000 people around the world every year.

HFRS was reported in Japan in the 1960s and 1970s.

Through genetic analysis, the team found that 47 of the 150 mice collected in seven ports came from abroad. All 31 black rats captured at Otaru port came from overseas.

The team believes the house mice came either from southeast Asia or from Europe, while the black rats are from Russia.