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Histopathological and immunohistochemical investigation of naturally occurring chlamydial conjunctivitis and urogenital inflammation in koalas (Phascolarctos cinereus)

Hemsley, S & Canfield, PJ 1997, Journal of Comparative Pathology, vol. 116, no. 1, pp. 273-290.

This study reports the findings of a histopathological (changes in tissue as a result of disease) and immunohistochemical (using antibodies to test for antigens in tissue) examination of the conjunctiva and lower urogenital tract of koalas exhibiting symptoms of chlamydial disease. Antibodies were used to label T and B lymphocytes, plasma cells, major histocompatibility complex class II (MHC II)-positive cells and chlamydial antigen in order to improve our knowledge of the immune response and pathogenesis of chlamydiosis.

  From 29 deceased wild koalas, samples of the conjunctiva, urinary bladder, urogenital sinus, and prostate and/or penis were collected. Following examination of these samples, any inflammation of the tissue and evidence of chlamydiae were characterised in great detail. Of the 29 koalas, five displayed no tissue inflammation and tested negatively for Chlamydia. The remaining 24 koalas exhibited some degree of inflammation at one or more sample sites, although six of these 24 did not test positively for Chlamydia. It is not known whether these six koalas were truly uninfected by the pathogen, or whether the tests were not sensitive enough to detect it. The extent of tissue inflammation was categorised as either uninflamed, mild, moderate or severe based primarily upon the abundance and formation of leucocytes present. Infection was more likely to be detected when inflammation was more severe. The histopathological and immunohistochemical descriptions of the effects of Chlamydia on the conjunctiva and lower urogenital tract in koalas were similar to those documented for other hosts of the pathogen. Within the mucous membranes examined, infiltration of inflammatory cells, most commonly lymphocytes and plasma cells, was most dense in the lamina propria-submucosa. Less commonly, neutrophils and macrophages also occurred. The lymphocytes that were present in both inflamed and uninflamed tissues were predominantly T cells, which were widely distributed compared to B cells and plasma cells that were mainly present beneath the epithelium. Lymphoid cells positive for MHC II were found in variable numbers, while epithelial cell labelling was also observed in some samples of inflamed tissue. Although uncommon, chlamydial inclusion bodies were also detected in some samples.

  The findings of this study add to the growing body of knowledge about chlamydial infection and disease in koalas, which is vital given the severity of the threat to free-ranging populations. Based on their findings here, the authors suggest further investigations be conducted into other factors that contribute to immune response to infection and disease progression in the koala such as cell-mediated responses, humoral responses, inflammatory mediators, hypersensitivity and autoimmunity.


Summarised by Joanna Horsfall


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