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

Formalin-fixed conjunctival and urogenital (urinary bladder, urogenital sinus and prostate or penis, or both) samples obtained from 29 free-living koalas were examined histopathologically and immunohistochemically for the presence and character of inflammation and for evidence of chlamydiae. Five koalas had no inflammation at any site examined and were chlamydia-negative. Twenty-four koalas had inflammation at one or more sites and 18 were positive for chlamydiae by serology or antigen detection tests, or both. Histopathological and immunohistochemical findings were consistent with those seen in chlamydial infections in other species. The inflammatory infiltrate was most dense in the lamina propria-submucosa and, in most tissues, predominantly consisted of lymphocytes and plasma cells. Neutrophils and macrophages were variable in number. Lymphocytes in uninflamed and inflamed tissues consisted largely of T cells. B lymphocytes and plasma cells were mainly distributed immediately beneath the epithelium, but T cells were widely distributed. MHC H-positive lymphoid cells were present in variable numbers and, in some inflamed tissue samples, epithelial cell labelling was also seen. Chlamydial inclusion bodies were uncommon in routinely stained sections. Immunoperoxidase labelling for chlamydiae was positive in 16 of 52 inflamed tissues. The proportion of positive test results for chlamydial antigen by any method was directly related to the severity of inflammation.