Chlamydial infection has been described as the main cause of clinical signs of disease for koala populations. The clinical signs are commonly conjunctivitis and urogenital tract infection, the latter of which can cause the condition referred to as 'dirty tail' or 'wet bottom'. It has been shown that the occurrence of clinical signs does not, however, provide an accurate estimation of infection rates, as koalas can become infected without presenting these signs (see Weigler et al(1988) J Wildl Dis 24: 282 ), and that infection may not be confirmed by some acceptable techniques in koalas with signs that indicate chlamydial infection (see Canfield et al(1991) Aust Vet J 68:167).
With the incursion of European settlement in Australia, substantial modification of the habitat of animals in general, and of that of koalas in particular, has taken place. Because agricultural and pastoral development has concentrated on the coastal and alluvial flats which provided the habitat for the most dense populations of koalas, the effects of these developments have been pronounced for koala populations. More recently, modifications to the habitat of remaining koala populations has occured as a result of urban development, the ensuing change in the habitat of the koala also causing habitat alienation.
Much of the information about chlamydial infection in koalas has come either from mortality surveys and clinical presentations or from studies of near-urban populations that may be under some or considerable pressure in the form of habitat alienation.
Many chlamydial detection tests have been used to determine infection rates for populations of koalas. Recently, polymerase chain reaction (PCR) techniques have been used for chlamydial detection in humans, because of their high sensitivity. These techniques have advantages for field workers as they require fewer chlamydial elementary bodies (EB) than gene probe (GP) techniques, and unlike tissue culture, the EB do not have to be viable for culture. PCR was employed in conjunction with other proven detection methods in our investigation of a free-ranging koala population located near Springsure in central Queensland, to determine the prevalence of chlamydial infection in a relatively remote area. Although not a pristine koala habitat, there has been little recent modification to the habitat of the koalas residing in the study area.
Samples we collected were tested using a complement fixation test (CFT), which employed guinea pig complement and rabbit haemolysin for sheep red blood cells. Samples samples were also screened for anti-chlamydial (koala strain) antibodies, using our immuno-dot blot technique (IDBT).
Of the koalas tested, one male had an ocular discharge consistent with chlamydial infection, and all others appeared to be free from infection. There were 3 positive CFT results (titre >16) and 2 'suspect' (titre 8) results. The male with clinical signs of infection was negative. Five animals, including the male mentioned, were positive in IDBT. GP analysis of swab material showed one animal to be positive, while the same five animals, which were positive in IDBT, gave positive PCR results.
This investigation, which employed two tests for the detection of Chlamydiafrom blood samples (CFT and IDBT) and two tests for identifying chlamydia from swab material (GP and PCR), enabled comparison of the tests for screening free-ranging populations of koalas. The CFT had previously been found to have low sensitivity and our results confirmed that the CFT was unsuitable for screening populations of koalas for chlamydial infection. Previous analysis of methods for detection of Chlamydiain free-ranging koalas suggested that procedures for the collection and transport of swabs could affect the sensitivity of tests using swabs. Since neither GP or PCR rely on viable EB from swab material, both have advantages for field sample collection. This study was the first to compare the use of PCR with other methods for detecting chlamydial infection in a population of free-ranging koalas. The results of this investigation indicated that PCR was useful for screening such populations of koalas.
The role of habitat alienation in the epidemiology of chlamydial infections in populations of koalas needs to be addressed in order that the priorities for conservation can be properly assessed. While habitat alienation has become a popular catch phrase for conservation groups, the evidence linking alienation to disease outbreaks is not available, and the true nature of chlamydial infection in the wider population has not been studied. Our results indicated that a relatively undisturbed population showing no overt clinical signs of infection could harbour chlamydial infection.
It is now clear that the koala has a variable response to chlamydial infection, and that, at least in in-vitro experiments, koalas are able to produce antibodies capable of neutralizing chlamydial infection. It is therefore possible that in populations with endemic chlamydial infection, overt clinical disease may not be normally produced, and it may be relatively unimportant to the health status of the koalas, as found in our work. Nonetheless, the population will include koalas from which chlamydia can be isolated, and from which anti-chlamydial antibodies can be detected.
A comprehensive survey by Brown et al (1984) indicated
that prevalence of infection was likely to be lower in areas where koalas
were under least pressure from habitat alienation. Using a radiographic
technique, the prevalence of cystitis was 5% at Springsure, compared with
increased levels in populations in near-urban environments. It would
appear that while infection rates may be similar in different areas, the
likelihood of infection manifesting itself as overt ocular or urogenital
infection, or as reduced fertility, increases as habitat alienation
pressure increases.