Abstract
While Sarcocystis parasites from the muscles of donkey and horse have been characterized as different species, similarities between the parasites in these host raises questions about this assignment (Levine and Tadros, 1980; Matuschka, 1983; Odening et al., 1995b). To resolve this, we examined the tissue cysts of Sarcocystis collected from donkeys and horses were studied by morphological and molecular methods. Morphological studies performed by light microscopy (LM) revealed that each of two types of cysts were present in samples from each host type. Under LM, villar protrusions (VP) were sometimes observed on the larger (Type I) and smaller (Type II) of these cyst types; when present, these were sometimes short and sometimes long. By electron microscopy (EM), VPs from both horse and donkey cysts were found to share similar structures, appearing to be typical of 'type 11a' VPs found on the Sarcocystis wall of Sarcocystis fayeri as described by Dubey et al., 1977. The VP of cysts in both horses and donkeys contained microtubules extending from the villar tips to the ground substance (GS). Ovoid, osmiophilic bodies (OB) were found along the length of the microtubules within the villi, but this feature was not found in all VP. To understand the phylogeny of the parasites, a portion of the coxI gene was sequenced from 22 isolated cysts (9 from donkeys and 13 from horses). Phylogenetic relationships were reconstructed from these sequences and the closest homologues available in GenBank, revealing that all of the samples, regardless of host origin or morphological appearance under LM, grouped in one clade. Ours is the first attempt to combine morphological measurements with coxI sequences in assessing such equine parasites; the results confirm a close relationship of the parasites from horse and donkey with S. fayeri. Further, the data suggest that the cysts in each host likely belong to the same species. As the first named species was Sarcocystis bertrami, we propose S. bertrami (syn. Sarcocystis fayeri) as the descriptor for this parasite of both horses and donkeys. Ultimately, this finding will only be validated by cross-transmission infection experiments that score the ability of parasite isolates from one Equus to infect the other.