Immunocytochemistry of ovine sporozoan encephalitis and encephalomyelitis.

An indirect immunoperoxidase procedure was developed to detect Sarcocystis spp meronts in paraffin wax sections of infected ovine tissues. Ten rabbits seronegative to Sarcocystis spp and Toxoplasma gondii were injected intradermally or subcutaneously over 2 months with varying quantities (150 to 750 micrograms) of purified soluble cystozoite antigen derived from ovine heart microcysts. Serum from each rabbit was tested against paraffin wax preparations of S. ovicanis cysts, second generation S. ovicanis meronts and T. gondii cysts. Sera from three rabbits reacted with S. ovicanis meronts without any cross reaction to T. gondii antigens. Toxoplasma gondii and Sarcocystis spp-specific sera were used to identify the sporozoa in six cases of naturally-occurring ovine encephalitis or encephalomyelitis. Sarcocystosis was diagnosed in four lambs and toxoplasmosis in one lamb. The generic identity of the sporozoan in the sixth case was not established as meronts reacted with both Sarcocystis spp- and T. gondii-specific sera. The study demonstrates the value of the immunohistological technique of visualizing sparsely distributed meronts in naturally-infected tissues and shows that sporozoa of the genus Sarcocystis and T. gondii, and possibly also sporozoa the identity of which is as yet unclear, are associated with encephalitis of sheep in Britain.