More than half of the koalas that are kept in zoos in Japan have been infected with a new virus that may have been responsible for baby koalas falling out of their mother's pouch, a research team discovered.

A team led by Kyoto University Associate Professor Takayuki Miyazawa analyzed blood samples from 51 koalas (40 northeast Australian-born and 11 southeast Australian-born koalas and their offspring) that were kept in Japanese zoos from July 2008 to January 2009. It was found through this study that 27 out of the 40 northeast Australian born koalas and their offspring, or 67.5 percent, were confirmed to carry the new type of retrovirus. The 27 account for nearly 53 percent of all the koalas covered by the study.

Koalas are known to be prone to retroviral infection that causes diseases such as leukemia, with which some 90 percent of koalas in Japan are believed to be infected. The newly discovered virus, however, has revealed a different nucleic acid sequence from that of the common retrovirus.

Meanwhile, though the cause remains unconfirmed, the number of baby koalas falling out of their mother's pouches seems to be increasing, according to Japanese Association of Zoos and Aquariums official Masami Kurobe.

Female koalas don't have placenta, and babies stay in their mothers' pouches for around six months after birth. The opening to the mothers' pouches faces down when they sit, however, and baby koalas sometime fall out of the pouch and die.

Miyazawa said it is possible that the baby koalas became infected with the virus through their mother's milk, and fell out of the pouch after becoming weakened through disease caused by the virus.

The research team will carry out follow-up studies on those koalas from which blood samples were taken, and will examine the bodies of the deceased baby koalas to study the relationship with the virus. All of the eight zoos in Japan that keep the animal are set to cooperate with the study.