Pathological Findings in Koala Retrovirus-positive Koalas (Phascolarctos cinereus) from Northern and Southern Australia

Abstract

Koala retrovirus (KoRV) infection shows differences in prevalence and load between northern and southern Australian koala populations; however, the effect of this on diseases such as lymphoma and chlamydial disease is unclear. This study compared clinicopathological findings, haematology and splenic lymphoid area of KoRV-positive koalas from northern (Queensland [Qld], n = 67) and southern (South Australia [SA], n = 92) populations in order to provide further insight into KoRV pathogenesis. Blood was collected for routine haematology and for measurement of KoRV proviral load by quantitative polymerase chain reaction (qPCR). Plasma samples were assessed for KoRV viral load by reverse transcriptase qPCR and conjunctival and cloacal swabs were collected for measurement of the load of Chlamydia pecorum (qPCR). During necropsy examination, spleen was collected for lymphoid area analysis. Lymphoma was morphologically similar between the populations and occurred in koalas with the highest KoRV proviral and viral loads. Severe ocular chlamydial disease was observed in both populations, but urinary tract disease was more severe in Qld, despite similar C. pecorum loads. No associations between KoRV and chlamydial disease severity or load were observed, except in SA where viral load correlated positively with chlamydial disease severity. In both populations, proviral and viral loads correlated positively with lymphocyte and metarubricyte counts and correlated negatively with erythrocyte and neutrophil counts. Splenic lymphoid area was correlated positively with viral load. This study has shown further evidence for KoRV-induced oncogenesis and highlighted that lymphocytes and splenic lymphoid tissue may be key sites for KoRV replication. However, KoRV infection appears to be highly complex and continued investigation is required to fully understand its pathogenesis.

Koala Retrovirus Diversity, Transmissibility, and Disease Associations

“…Overall, koalas with leukemia/lymphoma or other cancers had significantly higher VLs in both plasma and proviral DNA compared to those that were alive or those that died from other causes. A new report also showed that koalas with neoplasia had significantly higher pVLs and plasma VLs than other disease categories but KoRV subtyping was not reported (32), similar to the results of Tarlinton et al (30). Quigley et al…”

“…al who reported higher pVLs and plasma VLs in koalas with neoplasia compared to other disease categories in wild koalas by using a generic KoRV polymerase qPCR assay (32). These discrepancies could be from the different populations studied and methods used but highlight the importance of additional studies to evaluate disease associations of KoRV infection.…”

“…A recent report also did not find an association of KoRV VL with chlamydial disease in a longitudinal study of 16 wild koalas, including in koalas infected with subtypes B, D, and F (15), but the same group reported an association of KoRV-B infection with chlamydial disease in a larger cross-sectional study (14). Fabijan et al using generic polymerase qPCR reported an association of increasing severity of chlamydial disease with increasing KoRV VLs in koalas from South Australia but not from those in Queensland (32). They did not report KoRV subtype in this study but in an earlier study they reported no association of KoRV-A infection with chlamydial infection and disease in the same koala population (33).…”

Koala Retrovirus Diversity, Transmissibility, and Disease Associations

“…The rst study reported transmission in captivity of subtype C but not B to a joey from the KoRV-B and -C-infected parents (19). The second study found two dam-joey pairs in wild koalas for which the dam was KoRV-A-positive but the joeys were KoRV-negative and three pairs for which the joeys were KoRV-A-positive and the dams were KoRV-negative suggesting KoRV-A may be exogenously transmitted in this population (33).…”

“…The original study that identi ed KoRV-B reported damto-joey transmission of KoRV-B but only small numbers of captive koalas were studied at one zoo (22). While these results were supported by another study with a larger number of koalas (14), two recent studies identi ed discordant dam-to-joey transmission results (19,33). The rst study reported transmission in captivity of subtype C but not B to a joey from the KoRV-B and -C-infected parents (19).…”