Immunopathological characterisation of infectious diseases of the koala and the platypus

Abstract

This study characterised the pathological and immunopathological features of selected infectious diseases in the koala and the platypus. Originally, lymphosarcoma, cryptococcosis and chlamydiosis in the koala were chosen. Lymphosarcoma was included because of the putative retroviral involvement. Another infectious disease of Australian wildlife, mucormycosis of the platypus, was also included in the study. One hundred and ten koalas were necropsied throughout the study to determine the main cause of death, other pathological conditions present and to provide a source of case material. Fifty-six koala lymphoid neoplasia cases were obtained and the clinical features and clinical pathology were described. Cases were classified according to the tissues affected and the morphology of the neoplastic cells. The technique of immunohistochemistry was successfully applied to immunophenotype koala lymphoid neoplasms. Approximately half the cases were of the T cell immunophenotype, one quarter of B cell immunophenotype and one quarter did not stain. Multiple organ involvement and/or lymphoid leukaemia were common, probably reflecting presentation of koalas at advanced stages of disease. In order to improve understanding of the dynamics of progression from asymptomatic carriage of Cryptococcus neoformans to cryptococcosis, a preliminary study was undertaken to determine the prevalence, extent, biotype and seasonality of nasal and skin colonisation in the koala by Cryptococcus neoformans. Over a 22-month period, sequential nasal and skin swabs were obtained from 52 healthy captive koalas from the Sydney region. Prevalence of nasal colonisation varied seasonally from 12 to 38%. Cryptococcus neoformans var. gattii was cultured from 37, var. neoformans from 22 and both varieties from 5 nasal swabs. One case of cryptococcosis in a captive koala was diagnosed, and the treatment and response to therapy was described. The applicability of a streptavidin biotin-horseradish peroxidase immunohistological staining method to determine the variety and serotype of Cryptococcus neoformans in histological sections of infected koala tissues was assessed. A preliminary study was undertaken to assess the proliferative responses of koala lymphocytes to various mitogens and to chlamydial and cryptococcal antigen in infected and non-infected koalas. The proliferative response of cultured koala lymphocytes varied with the individual animal, the mitogen or antigen used and its concentration, but were invariably greater with separated peripheral blood mononuclear cells than with whole blood. Prior to investigating the immunopathogenesis of mucormycosis in the platypus, the use of various immunomarkers was validated using normal platypus lymphoid tissue. The gross structure and histology of lymphoid tissues obtained from 15 platypuses was described; including spleen, thymus, lymphoid nodules, gut-associated lymphoid tissue and bronchus-associated lymphoid tissue. With the exception of lymphoid nodules, the lymphoid tissue of the platypus was comparable in histological structure to that of therian mammals. Cross-reactive and specific antiplatypus antibodies were successfully applied to histological sections of platypus lymphoid tissue. The immunohistological appearance of the lymphoid tissues in the platypus was similar to that of eutherian and metatherian mammals, except for the detection of fewer B lymphocytes. In order to improve understanding of the pathogenesis of Mucor amphibiorum infection in the platypus, the gross, histological and immunohistological features of cutaneous lesions from 14 platypuses were described. For comparative purposes, normal platypus skin was also examined histologically and immunohistologically. Cases of mucormycosis were confirmed by cytology, histology, mycology and/or ELISA. Skin lesions varied in size, and ranged from raised red nodules or plaques, to ulcerated lesions. Lesions could be either granulomatous or pyogranulomatous, and were commonly diffuse. T cells were the predominant infiltrating lymphoid cell in the lesions and few B cells were observed in all cases. Presumptive plasma cells were observed in about half the cases.