Identification of Toxoplasma gondii and Encephalitozoon cuniculi by immunoperoxidase techniques and electron microscopy, in stored, formalin-fixed, paraffin-embedded tissue.

The peroxidase-anti-peroxidase (PAP), avidin-biotin-complex (ABC) techniques, and electron microscopy, were used to identify protozoan parasites in formalin-fixed material from routine necropsy cases. The material comprised paraffin blocks, that had been stored for up to 18 years, from 18 cases of suspected toxoplasmosis and encephalitozoonosis. With the ABC method a higher dilution of primary antibody could be used than with the PAP method. However, with both methods, a distinct reaction occurred with appropriate dilutions. The age of the material did not seem to effect the result. Frequently, phagocytized and necrotic parasites were also stained. Gystozoites (bradyzoites) of T. gondii were stained faintly compared to the endozoites (tachyzoites). A Toxoplasma-like parasite from a dog did not react with anti-Toxoplasma serum, and ultrastructurally it proved to be consistent with an unidentified cyst-forming sporozoan parasite previously reported in dogs. Electron microscopy based on paraffin-embedded tissue, seems to be a valuable method for identification of protozoan parasites, and thus provide a supplement or alternative to the immunoperoxidase methods.