Putative koala retrovirus-associated diseases in the Japanese captive koala (Phascolarctos cinereus) population. In Proceedings of the Second Koala Retrovirus Workshop, ed. D. E. Alquezar-Planas, D. P. Higgins, C. L. Singleton, and A. D. Greenwood

Japan began housing koalas (Phascolarctos cinereus) in 1984, increasing from six individuals in 1984 to a peak of 96 koalas in 1997. However, the number of koalas has almost halved since and as of 2020, 54 koalas remain in zoos in Japan. Although records of 330 koala deaths have been accumulated over 37 years, there have been no comprehensive reports on the relationship between the causes of death and koala retrovirus (KoRV) in the Japanese captive population. Based on the koala studbook updated by the Japanese Association of Zoos and Aquariums, we have investigated causes of death in the Japanese captive koala population. The most common cause of death was joeys falling. When combined with stunted joey growth, one-third of the koalas died within a year of birth. Deaths due to malignant neoplasms and opportunistic infections cannot be directly associated with KoRV infection because no test for KoRV had been performed before or during disease onset. It is suspected that KoRV may be associated with deaths due to the large number of cases of neoplasms, which accounted for 16.4% of all deaths.