Immunohistochemical identification of aetiological agents of systemic bovine zygomycosis

The reactivity of a panel of rabbit hyperimmune antisera raised against somatic antigens of three zygomycetes, Absidia corymbifera, Rhizopus oryzae and Rhizomucor (Mucor) pusillus was assessed by crossed immunoelectrophoresis (XIE) and indirect immunofluorescence (IIF) staining of experimentally infected murine tissues. Despite a widespread cross-reactivity of the neat antisera, heterologous absorption rendered antisera monospecific as assessed by XIE. Heterologous absorption also rendered the antisera against R. oryzae and R. (Mucor) pusillus specific by IIF, whereas heterologous absorption of Abs. corymbifera antiserum did not abolish reactivity with R. oryzae. The reactivity of the heterologously absorbed antisera and a murine monoclonal IgG1 antibody against Abs. corymbifera enabled zygomycetes within bovine lesions to be identified. Of 113 zygomycotic lesions of 95 cattle examined, 82 (72.5 per cent) showed a pattern of reactivity compatible with Abs. corymbifera, 21 (18.6 per cent) with R. oryzae, and 5 (4.4 per cent) with R. (Mucor) pusillus. Fungi in four lesions reacted with the monoclonal antibody only, and fungal elements in one lesion showed no reactivity at all. Each of the bovine lesions contained only a single fungal species, but one animal was infected by Abs. corymbifera in the rumen and by R. oryzae in the other stomach compartments. Apart from being the main cause of systemic bovine zygomycosis in Denmark, Abs. corymbifera also seems to be the most pathogenic due to its frequent haematogenous and lymphatic spread (87.8 per cent) as compared with R. oryzae (1.4 per cent) and R. (Mucor) pusillus (5.4 per cent).