TOKYO, April 12, 2009 (AFP) - Nearly 90 percent of koalas in Japanese zoos are infected with a virus believed to cause leukemia in the marsupials, a report said on Sunday.

Researchers at the Institute for Virus Research Kyoto University and the Japanese Association of Zoos and Aquariums tested the blood of 50 of the 62 koalas kept in Japan, the Daily Yomiuri said.

The koala retrovirus - believed to cause leukemia and lymphoma - was detected in all 39 northeastern koalas, and in four of 11 southern koalas, the English version of the mass-circulation Yomiuri Shimbun reported.

There are two main subspecies of wild koala - the northeast Australian koala and the south Australian koala. The retrovirus poses no danger to humans, but there is concern that it could lead to a dramatic decline in the number of koalas in captivity in Japan, the daily said.

While the sources of the infections were not identified, it is thought most likely they already were infected before being imported from Australia, it said.

Scientists say it is difficult to prevent koalas becoming infected with the virus as it infects the reproductive cells of the koalas and is passed down to their offspring, the daily said.

Of the 38 koalas born in Japan, 36 were found to have been infected, it added.