Tissue cysts from formalin-fixed, paraffin-embedded brains of sea otters #2 (SO2) and #3 (SO3). A) A single 38 μm diameter tissue cyst was observed in the pyriform lobe of SO2. Fine villous protrusions were barely discernable at the top center of the thick, pink to amphophilic, 2.5 μm thick outer cyst wall. (H&E stain, bar = 35 μm). B) Tissue cysts from another area of the cerebrum of SO2, showing strong labeling with polyclonal antiserum to Toxoplasma gondii (immunoperoxidase reaction, bar = 35 μm). C) Putative immature Sarcocystis neurona tissue cyst in cerebrum of SO3. Compared with those from SO1 and SO2, this cyst is smaller (21 μm diameter) with a thin cyst wall and no visible surface villi, similar to immature S. neurona tissue cysts that have been described in other studies (Table 3). (H&E stain, bar = 30 μm).