KoRV and Chlamydia: are they co-culprits?

Abstract

There are two main infectious disease threats for the koala; Chlamydia and KoRV. A major question is whether or not KoRV predisposes koalas to more severe chlamydial disease. In the only study to date that has examined co-infections, KoRV load (as determined by qPCR) and chlamydial load (as determined by qPCR) and chlamydial disease were examined in wild koalas. While there was a statistically significant correlation between Chlamydia infection load and Chlamydia clinical disease score, there was no significant correlation between KoRV load and either Chlamydia infection load or Chlamydia clinical disease score, however the groups were not ideally constructed and hence additional comparisons are needed. If KoRV does predispose koalas to more severe chlamydial disease, one would expect it to do this via an effect on the koala immune system. A series of Chlamydia vaccine trials in captive as well as wild koalas are showing that koalas in fact appear to make perfectly normal antibody and cytokine responses to vaccine antigens, even if they have high circulating KoRV loads, arguing against an immune suppressive effect by KoRV. Timms, Peter. 2014. KoRV and Chlamydia: are they co-culprits? In The Koala and its Retroviruses: Implications for Sustainability and Survival, ed. Geoffrey W. Pye, Rebecca N. Johnson and Alex D. Greenwood. Technical Reports of the Australian Museum, Online 24: 89–90. In Australia, wild koala (Phascolarctos cinereus) populations in many areas, particularly Queensland and NSW, are declining for many reasons. One of the main causes of these declines is infection and disease due to Chlamydia (Polkinghorne et al., 2013). While Chlamydia cause similar disease syndromes in their non-koala hosts, the koala seems to have a higher than expected level of disease. This raises the question as to whether or not KoRV is somehow contributing to chlamydial disease. This brief overview will focus first on what we know about Chlamydia in koalas and then look at the very limited data regarding KoRV and Chlamydia. Overview of Chlamydia Chlamydia is an obligate intracellular bacterium with a unique two-phase developmental cycle. Immunity to chlamydial infections requires both a strong, neutralising antibody response as well as an interferon-gamma directed T cell response. Of the nine species present in the genus Chlamydia, two, C. pecorum and C. pneumoniae, cause infections in koalas (Jackson et al., 1999; Deveraux et al., 2003). The frequency of chlamydial infections (measured by a range of techniques, but utilizing PCR mostly of late) varies between populations, ranging from nil (on just a few island populations) to 90% in several populations (Polkinghorne et al., 2013). Disease levels also vary, but usually represent 25% or so of the infection level at any time point sampled. Animals are infected at ocular and urogenital sites mainly. Of the two chlamydial species, C. pecorum is by far the most common and is thought to be the species responsible for the symptoms observed (Glassick et al., 1996). Even though it is C. pecorum that is responsible for most infection and disease in koalas, there is considerable genetic diversity between sub-strains (Jackson et al., 1997). A range of gene markers have been used to assess C. pecorum strain diversity and while there are some minor differences, they all show that the various koala C. pecorum genotypes cluster together, but show considerable strain diversity.