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.
Highlights
Koala (Phascolarctos cinereus) populations are distributed down the eastern and southeastern coast of Australia and are broadly separated into two groups, northern koalas in the states of Queensland (Qld) and New South Wales, and southern koalas in South Australia (SA) and Victoria (DSEWPC, 2012)
In Qld, five koalas presented with lymphoid neoplasia (7.5%, two with lymphoma and three with lymphoid leukaemia), one with mesothelioma (1.5%) and three with osteochondroma (4.5%; two craniofacial and one costal)
In the SA koalas with lymphoma, the median mitotic rate was 4 mitotic figures per 10 ×400 high-power fields (HPFs), but mitotic rate was not evaluated in the affected Qld koalas
Summary
Koala (Phascolarctos cinereus) populations are distributed down the eastern and southeastern coast of Australia and are broadly separated into two groups, northern koalas in the states of Queensland (Qld) and New South Wales, and southern koalas in South Australia (SA) and Victoria (DSEWPC, 2012). Southern koalas are less genetically diverse than northern koalas (Neaves et al, 2016), which likely arose from historical translocation conservation efforts post-European settlement (Robinson, 1978). These translocations may have altered the prevalence of disease in southern populations. Northern populations are recognised as vulnerable to extinction, as these populations are declining at a considerable rate, unlike southern populations, where koalas were introduced into previously unoccupied areas and are considered overabundant (DSEWPC, 2012) This is due partly to differences in the prevalence of disease between the two regions. Chlamydia pecorum and koala retrovirus (KoRV) are highly prevalent and have been associated with a high prevalence of disease in northern populations, while in southern populations there is a lower prevalence of infection and disease (Tarlinton et al, 2005; Polkinghorne et al, 2013; Quigley et al, 2018a)
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