Prevalence and Genetic Characterization of Morphologically Indistinguishable Sarcocysts of Sarcocystis cruzi in Cattle and Sarcocystis poephagicanis in Yaks

Sarcocystis cruzi in cattle (Bos taurus) and Sarcocystis poephagicanis in yaks (Bos grunniens) are morphologically indistinguishable. However, the relationship between the two parasites is still unclear. Here, muscular tissues of the two species of domestic animal collected from abattoirs in China were examined for sarcocysts of S. cruzi and S. poephagicanis. The sarcocysts isolated from the samples were analyzed using light microscopy (LM), transmission electron microscopy (TEM), and DNA analysis. Sarcocysts of S. cruzi and S. poephagicanis were found in 405 of 950 (42.6%) cattle and 304 of 320 (95.0%) yaks. LM and TEM showed that the sarcocysts of the two parasites had similar morphological characteristics. The thin-walled sarcocysts had hair-like protrusions on the surface. The ultrastructures were demonstrated to include a primary cyst wall containing irregularly folded, hirsute, or bone-like protrusions. Four genetic markers of the two parasites were sequenced and analyzed, namely, 18S rDNA, 28S rDNA, mitochondrial cox1, and apicoplast rpl6. The sequences of the four loci had an interspecific similarity of 97.9-98.6%, 97.2-98.1%, 89.5-90.4%, and 96.9-97.2% identity, respectively. Phylogenetic analysis using 28S rDNA and cox1 sequences indicated that both S. cruzi and S. poephagicanis were placed into a group encompassing Sarcocystis spp. in ruminants with canid as known or putative definitive hosts. Sarcocystis cruzi and S. poephagicanis represent separate species, and cox1 and rpl6 were suitable for distinguishing between them.