Cattle (Bos taurus) are intermediate hosts for four species of Sarcocystis, S. cruzi, S.
hirsuta, S. hominis, and S. rommeli. Of these four species, mature sarcocysts of S. cruzi are
thin-walled (< 1μm) whereas S. hirsuta, S. hominis, and S. rommeli have thick walls (4 μm or
more). Here we describe a new species of Sarcocystis with thin-walled sarcocysts in cattle. Two
newborn calves were fed sporocysts from the feces of a human volunteer who had ingested raw
beef. The calves were killed 111 and 222 days later. In addition to thick-walled sarcocysts of
Sarcocystis hominis, both calves were coinfected with a Sarcocystis species that had a thinwalled
sarcocysts, distinct from Sarcocystis cruzi. The sarcocysts were mature, microscopic, up
to 80 μm wide and up to 1060 μm long. By light microscopy, the sarcocyst wall was thin (< 1 2
μm thick) and had minute protrusions. By transmission electron microscopy, the sarcocyst wall
had short, conical villar protrusions (vp), that were up to 0.5 μm long, up to 0.5 μm wide, similar
to type 29. The vp on the sarcocyst wall lacked microtubules but had six or more disc-shaped
plaques. The ground substance layer was smooth, approximately 0.5 μm thick, and without
microtubules. The bradyzoites were 8-11 μm long. The structure of the sarcocyst wall was
distinct from any species of Sarcocystis reported from livestock. This unique species is named in
honor of Dr. Alfred Otto Heydorn who provided the sporocysts.