Variation in immunohistochemical staining properties of various protozoal structures (merozoites, bradyzoites and the outer tissue cyst wall) from the brain of sea otter #1, an animal with PCR-confirmed Sarcocystis neurona infection. A and B) Application of polyclonal antiserum to S. neurona: strong positive staining of merozoites (A) and the tissue cyst wall (B) is noted but with negative staining of bradyzoites (B). C and D) Application of monoclonal antibody (SN2G5) to S. neurona: strong positive staining of merozoites (C) contrasts with negative staining of the tissue cyst wall (D) and sparse, patchy labeling of the enclosed bradyzoites (D). E and F) Application of polyclonal antiserum to Sarcocystis falcatula: results are similar to those from application of the S. neurona monoclonal antibody, with strong positive staining of merozoites (E) contrasting with negative staining of the tissue cyst wall (F) and sparse, patchy labeling of the enclosed bradyzoites (F) (bar = 50 μm).