EPIDEMIOLOGY OF CHLAMYDIAL INFECTIONS IN KOALAS

2.1 Prevalence and distribution

Of the two described species of Chlamydia that infect koalas, C. pecorum consistently remains the most preva­lent and pathogenic organism, often implicated as the primary cause of chlamydial disease in koalas (Griffith et al.

A range of studies encompassing most of Australia’s mainland koala populations over the past three decades suggest that chlamydial infections are relatively common (Marsh et al. 2011; Patterson et al. 2015; Speight et al. 2016), with C. pecorum and C. pneumoniae readily detected at ocular, nasal, urogenital and rectal sites of infected animals. Many infected animals can shed large numbers of chlamydial organisms. Approaches using qPCR have detected high levels of C. pecorum DNA from

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Fig. 35.1. Chlamydial cell cycle and taxonomy. (a) Different stages of the chlamydial developmental cycle involving the interconversion of elementary bodies (EB), reticulate bodies (RB) and aberrant bodies (AB). (b) Bayesian phylogenetic analyses of the Order Chlamydiales 16S rRNA gene sequences. Sequences for the Chlamydiales 16S rRNA gene were obtained from GenBank, NCBI. Phylogenetic tree was generated in Geneious (http://www.geneious.com/) with MrBayes plugin using a Hasegawa-Kishino-Yano (HKY) nucleotide substitution model.

the conjunctival and urogenital mucosa of koalas with clinical disease, both acute and chronic, as well as asymp­tomatically infected animals (Wan et al.

Although sample sizes and detection methods have varied, the prevalence of chlamydial infection can range from very low, and possibly 0% in some island popula­tions, to >70%, particularly in koala populations with evidence of ocular and/or urogenital tract disease (Polk- inghorne et al. 2013). In those populations where the identity of the chlamydial organisms involved has been determined, C. pecorum infections were the most preva­lent. Although fewer studies have been conducted for C. pneumoniae, reports of infection in Australia’s free- ranging and managed koala populations have declined since the late 1990s (Wardrop et al. 1999). In those studies where speciation was performed, C. pneumoniae infec­tion prevalence was much lower (averaging 18%) than that of C. pecorum (averaging 57%) (Polkinghorne et al. 2013). Most recently, a study of Victorian koalas found only one C. pneumoniae-positive sample (conjunctival) from a survey of 459 koalas (Legione et al. 2016a).

Anecdotal and scientific evidence suggests that regional differences in the prevalence of chlamydial infec­tion and disease may occur. Populations in the north­eastern regions of Australia (e.g. Qld and northern NSW) generally show higher levels of chlamydial infection and more evidence of clinical disease than populations in the southern states, with C. pecorum being the predominant species detected by molecular methods (Marsh et al. 2011; Higgins et al. 2012; Polkinghorne et al. 2013). Recent epi­demiological studies assessing prevalence (as detected by PCR) and clinical significance of chlamydial infection in island and mainland populations of koalas from six differ­ent regions across Vic. revealed that C. pecorum was the primary species detected, with prevalence ranging from 15% to 41%. In these animals, C. pecorum was mainly detected at urogenital sites. Notably, some mainland Vic. koala populations were Chlamydia-free (Patterson et al. 2015; Legione et al. 2016a; Legione et al. 2016b).

The current prevalence of chlamydial infection in SA koalas is largely unknown, with only few clinical cases of mild chlamydial ocular disease reported (Funnell et al. 2013). Recent necropsy examination of 65 rescued wild koalas from the Mount Lofty (n = 62) and Eyre Peninsula (n = 3) populations in SA revealed infection with C. pecorum in 50 ocular and 40 urogenital swabs from 57 koalas, with 34 koalas positive at both ocular and urogenital sites. Of the 57 PCR-positive koalas, 16 were subclinical with no pathological lesions and the remaining 41 animals had either overt clinical disease or pathological lesions detected at necropsy or on histo­pathology consistent with chlamydiosis (Speight et al. 2016). The results from this and ongoing studies suggest there is a high prevalence of chlamydial infection in koala populations in SA, despite an apparent low inci­dence of overt disease.

Early reports suggested that several small ‘isolated’ koala populations, including St Bees Is., Magnetic Is., French Is. and Kangaroo Is., may be free of chlamydial infection. These are all artificial island populations, cre­ated by translocation of presumed healthy, Chlamydia- free animals (Polkinghorne et al. 2013). However, in recent studies C. pecorum was detected in ocular and/or urogenital samples from seemingly healthy koalas from isolated populations on St Bees Is., Qld (Jelocnik et al. 2013), French Is., Vic. (Legione et al. 2016b) and Raymond Is., Vic. (Jelocnik et al. 2015), further highlighting the widespread and subclinical nature of some koala chlamydial infections.

Besides the koala, in Australia C. pecorum infects and causes disease in livestock (sheep and cattle) (Walker et al. 2016) and C. pneumoniae infects amphibian, reptile and mammalian (including human) hosts (Roulis et al. 2013). As chlamydiosis is highly prevalent among koalas, it is not surprising that chlamydial infections (including C. pecorum) have also been detected in a variety of other Australian marsupials such as the greater glider (Petau- roides volans), mountain brush-tailed possum (Trichosu- rus cunninghami), western barred bandicoot (Perameles bougainville) (Bodetti et al.

2.2 Molecular epidemiology

Molecular epidemiological studies and comparative genomics of koala-associated C. pecorum isolates indicate the strains involved are highly genetically diverse (Marsh et al. 2011; Jelocnik et al. 2013; Bachmann et al. 2014a). The observed genetic diversity of C. pecorum isolates extends from the individual to the population level, with repeated molecular evidence suggesting that (a) an indi­vidual animal can be infected at the same or different anatomical sites with genetically distinct C. pecorum strains and (b) that discrete populations of koalas can contain genetically diverse strains (Jelocnik et al. 2013; Bachmann et al. 2015; Legione et al. 2016a; Fernandez et al. 2019). A repeated observation was that certain koala C. pecorum strains share more similarity to C. pecorum strains from livestock than they do to other koala strains, raising questions over the role of historical and/or con­temporary spillover from domestic livestock (Burnard and Polkinghorne 2016). In contrast, the current genetic diversity of koala C. pneumoniae remains unknown, although molecular typing studies suggest that the animal strains are diverse and distinct from human strains (Roulis et al. 2013).

2.3 Transmission

To date, no studies have evaluated modes of chlamydial infection transmission between koalas. High C. pecorum infection loads were detected from both vaginal and penile urethral swabs of sexually active female and male koalas, suggesting it is primarily sexually transmitted (Polking- horne et al. 2013). Older, sexually active koalas with clini­cal signs of chlamydiosis made up the most frequent admission group to a koala rehabilitation facility over a 30-yr period, further supporting sexual transmission as the primary mode of transmission (Griffith et al. 2013). As Chlamydia can be detected in young, sexually inactive ani­mals, transmission from dam to joey is suspected and most likely occurs via the faecal-oral route during pap feeding (Blanshard and Bodley 2008; Nyari et al.

2.4 Co-infections

Chlamydial infections and associated clinical disease in koalas could occur concurrently with other bacterial, viral and parasitic pathogens, most notably koala retrovi­rus (KoRV) (Tarlinton et al. 2005), koala herpesvirus (Stalder et al. 2015) and haemoprotozoa (Trypanosoma spp.) (McInnes et al. 2011). KoRV has been proposed as an underlying cause of susceptibility of koalas to chlamydial disease, because of its immunosuppressive effect, with KoRV B potentially having a more significant association than other genotypes (see Chapter 34) (Tarlinton et al. 2005; Waugh et al. 2017).

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