Infection with Sarcocystis species is common in herbivores but is rare in bears. Histological sections of 374 black bears (Ursus americanus) from Pennsylvania were examined for sarcocysts. In total, 3 sarcocysts were found in 3 bears, with 1 sarcocyst per section. Sarcocysts from 2 bears were considered a new species, Sarcocystis ursusi. Sarcocysts of S. ursusi n. sp. were microscopic and contained only bradyzoites. By light microscopy, the sarcocyst wall was thin (<0.5 μm thick) and had minute serrations. Ultrastructurally, the serrations on the sarcocyst wall consisted of villar protrusions (Vp) that were mostly 0.5 μm long. The Vp had bundles of electron-dense microtubules that were as wide as long; these microtubules extended deep into the ground substance layer, a feature that distinguished this species from unnamed sarcocysts from black bear. Bradyzoites were 4.8–6.0 μm long. The sarcocyst from the third bear was structurally different from S. ursusi; its sarcocyst wall was approximately 2 μm thick and had finger-like villi on the cyst wall giving the sarcocyst wall a striated appearance.

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