The living koala population in North America is predominantly descended from koalas imported from a single Australian facility in 1976 and 1981, with several smaller imports from other facilities between 1985 and 2013. Koala retrovirus subtype B (KoRV-B) entered the North American population via imports in 2005, 2008, and 2013. The 2005 and 2008 KoRV-B positive lineages are deceased, but the 2013 KoRV-B positive lineage has seven surviving koalas, including one female of breeding age. Three koalas born to KoRV-B negative dams were documented as being KoRV-B positive at 15 months of age after nursing from a KoRV-B positive female. The prevalence of koala retrovirus subtype A (KoRV-A) detection in North America is 100% but the prevalence of KoRV-B detection is 17%. Lymphoid neoplasia is a common cause of mortality, dating back to founder koalas. Most cases of lymphoid neoplasia have occurred in presumptive KoRV-B negative koalas, and many cases have occurred between the ages of four and nine years. Familial clusters of lymphoid neoplasia are apparent. Additionally, myelodysplasia and fatal peripheral cytopenias are important putative KoRV-associated diseases that cause mortality in koalas in North America, with higher prevalence in koalas younger than two years of age. Since 2013, breeding of known KoRV-B positive koalas has been managed, to maintain separation from the remainder of the KoRV-B negative population.
Singleton, Cora L., and Chris Hamlin-Andrus. 2023. Koala retrovirus status and putative koala retrovirus-associated diseases in captive koalas (Phascolarctos cinereus) in North American zoos. In Proceedings of the Second Koala Retrovirus Workshop, ed. D. E. Alquezar-Planas, D. P. Higgins, C. L. Singleton, and A. D. Greenwood. Technical Reports of the Australian Museum Online 38: 49–52.