Retroviral endogenization of the koala genome has been ongoing since the 19th century.1 Koala retrovirus (KoRV) genotypes (KoRV-B, KoRV-J) distinct from the originally described koala retrovirus A (KoRV-A) have been discovered recently.2 Retroviral infection in koalas have been associated with significant morbidity and mortality, including leukemia, lymphoma, myelodysplasia, and immunosuppression leading to opportunistic infections, particularly with Chlamydia sp. or Cryptococcus sp.3 One important sequela of some retroviral-infected koalas is myelodysplasia, specifically ineffective hematopoiesis with consequent peripheral cytopenias, often leading to severe nonregenerative anemia.4,5 Myelodysplasia may be a direct or indirect result of retroviral infection.5 This case series presents a description of clinical signs, duration and severity of illness, and treatment attempts of six KoRV-infected koalas with fatal anemia. Age at presentation ranged 0.6–12.8 years (average 5.8 years). Packed cell volume at initial presentation ranged 3–25% (average 17%). Average time from presentation to death was 38 days (range 10–82 days). Treatments with erythropoietin and/or blood transfusion were unsuccessful. Mortality in each case was attributed to the effects of bone marrow dyscrasia and consequent severe nonregenerative anemia. Although koalas presented in this case series either tested positive for or were presumed positive for KoRV-A, a causative relationship between retroviral infection and development of nonregenerative anemia has not been definitively verified to date. Early diagnosis of this hematologic manifestation is obtainable through blood film review and concurrent cytologic examination of bone marrow aspirates,5 both of which provide guidance for clinical case management decisions and should be considered an essential part of koala preventive medicine examinations.
The authors thank the pathology staff and Laura Keener from San Diego Zoo Global for assistance with the cases and data retrieval.
1. Avila-Arcos MC, Ho SYW, Ishida Y, Nikolaidis N, Tsangaras K, Honig K, Medina R, Rasmussen M, Fordyce SL, Calvignac-Spencer S, Willerslev E, Gilbert MTP, Helgen KM, Roca AL, Greenwood AD. One hundred and twenty years of koala retrovirus evolution determined from museum skins. Mol Biol Evol. 2012;30(2):299–304.
2. Kinney ME, Pye GW. Koala retrovirus: a review. J Zoo Wildl Med. 2016;47:387–396.
3. Shimode S, Nakagawa S, Yoshikawa R, Shojima T, Miyazawa T. Heterogeneity of koala retrovirus isolates. FEBS Lett. 2014;588:41–46.
4. Singleton CL, Mulot B, Rideout BA. Retrospective review of mortality in Queensland koala (Phascolarctos cinereus) in zoological facilities in North America (1976–2016) and Europe (1987–2017). In: Proc Eur Assoc Zoo Wildl Vet, Am Assoc Zoo Vet, Leibniz Instit Zoo Wildl Res. 2018:47.
5. Stacy NI, Rideout B, Harvey JW, Pye GW, Gillett A, Miller CL, Garner MM. Cytologic characterization of myelodysplasia in retroviral-infected koalas (Phascolarctos cinereus). In: Proc Am Assoc Zoo Vet, Eur Assoc Zoo Wildl Vet, Leibniz Instit Zoo Wildl Res. 2016:109–110.