Chlamydial infections in animals
Infections in koalas
C. pecorum
Free-living populations of a marsupial, the
koala,
are seriously affected by chlamydial infections that cause blindness and
infertility. Conjunctivitis and chemosis
had been noted for many decades in koala (Studdert,
1976). "Wet tail" or "dirty bottom" is also a
characteristic sign [see external link: koala
threats]. Major mortality among koala populations
has also been observed as far back as 1887. These were apparently caused by severe eye infections
which lead to impaired vision, reduced food intake and death (Cockram and Jackson,
1981). Respiratory tract infections have also been
observed in koala populations. Strains infecting koala and at the time designated as Chlamydia psittaci
were divided as type I - those causing conjunctivitis and systemic disease in
kola, and type II - those causing ocular and urogenital infections (Girjes,
1988). Using molecular methods, Timms et al., (1988) found that the koala conjunctivitis
organisms were different from the chlamydiae causing avian disease, ruminant abortion, feline pneumonitis
or sporadic bovine encephalomyelitis. Chlamydophila pecorum strains have been isolated from koalas (Girjes et al.,
1993; Glassick et al.,
1996; Jackson et al., 1997) in association with reproductive
and urinary tract disease and infertility. The majority of koala with C. pecorum infection
had overt clinical disease, and some of the strains were closely related to the encephalomyelitis subtype of C. pecorum.
This organism poses a serious risk to the long-term survival of free-living koala.
The chlamydial strains infecting koala proved
to be genetically diverse (Timms et al.,
1996), explaining some of the clinical and immunological diversity
observed. Diversity was particularly found in the C. pecorum strains from
koala conjunctivitis, urinary tract infection and infertility. This suggests that
koala may become infected from various animal sources, such as cattle and sheep,
possibly pigs, but not birds. C. pecorum may thus cross host barriers easily,
perhaps transmitted though faecal contamination of pastures and leaves. Indeed, C. pecorum infection
may have been introduced in the last 200 years when sheep and cattle were
imported into Australia.
Symptomatic chlamydial infections in the koala
range from keratoconjunctivitis to reproductive and urinary tract disorders and
female infertility (Martin and Cross, 1996). Reproductive tract infection is
probably sexually transmitted. Tetracycline is effective in eradicating the infection and has been used to save koala populations that otherwise
were threatened with extinction.
C. pneumoniae
It is now clear that koala are also infected
with C. pneumoniae.
In free-range koala populations C. pneumoniae infection is rarely associated with respiratory, ocular or urogenital tract
disease. However, in one captive koala population C. pneumoniae infection caused a severe and extended respiratory episode.
Wardrop et al. (1999) demonstrated that koalas could be infected with C. pecorum and C. pneumoniae, both of which
would have been formerly designated Chlamydia psittaci. Based on
sequences of the gene encoding the chlamydial major outer membrane protein, MOMP, C. pecorum isolates from koalas showed diversity relating to
geographical region and possibly to their origin from other hosts. C. pneumoniae isolates from koala were
considered sufficiently distinct from
the human and equine biovars of C. pneumoniae to warrant separate koala biovar status.
In a study of two separate free-range koala populations, Jackson et al., (1999) found a high level of infection in one population, with most
infection due to C. pecorum rather than C. pneumoniae. C.
pecorum appeared to be the more pathogenic of the two chlamydial species in
the koala, with the C. pneumoniae infections generally low grade.
[MEW comment: In humans, C. pneumoniae
has been linked to coronary artery disease and is thought to disseminate to
atherosclerotic plaque from an initial respiratory focus of infection via
peripheral blood monocytes [see: C. pneumoniae
and CAD]. In the koala, which is also a natural host for C. pneumoniae,
C. pneumoniae genome or antigens were detected at similar frequency in
the blood of 30% of 20 koalas tested (Bodeti
& Timms, 2000). This was considered to suggest that the ability of C.
pneumoniae to disseminate from the respiratory site may be a characteristic
of this chlamydial species rather than of the human host. As in humans, C.
pneumoniae infection of peripheral monocytes in the koala increases lipid
uptake into these cells (Coles et
al., 2001). How important this is to the pathogenesis of ateriosclerosis
and whether C. pneumoniae is in any way related to heart or vascular disease in
the koala remains unknown.
Bibliography
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