Chlamydial disease
Of the two species of chlamydia that infect koalas (Chlamydia
pecorum and C. pneumoniae), C. pecorum is consistently more
prevalent (i.e., higher percentage individuals infected) and more
pathogenic . C. pecorum is therefore the chlamydia species that
researchers and veterinarians focus on . At each veterinary examination,
koalas were (1) tested for the presence of C. pecorum , and (2)
assessed for clinical signs of chlamydial disease. Tests for C.
pecorum were conducted using either quantitative real-time PCR (qPCR)
targeting a fragment of the C. pecorum 16s rRNA gene , or using
the Clearview® Chlamydia MF test kit, which has a 60%
sensitivity (Inverness Medical, Unipath Ltd). Clinical signs of
chlamydial disease are mostly found in the ocular and urogenital
regions. The ocular form of the disease varies from inflammation of the
mucosal surfaces of the eye, or conjunctival hyperplasia, to the
complete opacification and scarring of the cornea, which can cause
blindness . In urinary disease, inflammation of the bladder (cystitis)
leads to incontinence and staining of the fur around the cloaca, which
is also coupled with alopecia and ulceration in severe cases . Finally,
infection of the reproductive tract in females causes bursal cysts
surrounding the ovary and upper reproductive tract pathology, while in
males orchitis and epididymitis can occur . In this study, veterinary
diagnostic techniques for assessing the presence of chlamydial disease
involved observation of external signs of disease, cystocentesis with
observation of the urine sediment, and ultrasound examination of the
kidneys, reproductive tract, and bladder (as described in ). We
initially aimed to independently analyse whether koalas tested positive
for the presence of C. pecorum , and whether koalas were diagnosed
with clinical signs of disease. However, in our data, the majority of
individuals that tested positive also displayed clinical signs of
disease (87%). We therefore opted to use whether koalas tested positive
(‘chlamydia status’) as our response variable in all analyses.
Although individuals were monitored for multiple years (average = 1.2
years, max = 4 years) and had repeated measures of disease status
gathered at scheduled veterinary exams (N = 1182 observations of N = 342
koalas, average = 3 observations per koala), we selected to only use
chlamydia status at their first capture. This was because during the
project individual koalas were treated for any illness or injury they
presented at veterinary examinations (including chlamydial disease), and
some individuals were also included in a chlamydia vaccination trial .
Individual disease status at subsequent veterinary examinations could
therefore have been affected by the veterinary treatment, vaccine trial
and management of the population.