Non-invasive methods for identifying oocysts of Sarcocystis spp. from definitive hosts

Because the excreted sporocysts and/or oocysts of various species of Sarcocystis may not be discriminated morphologically, we sought to validate a diagnostic technique based on variation in the 18S rDNA sequence. Oocysts and/or sporocysts from three taxa of Sarcocystis were collected from human, feline, and canine definitive hosts that had fed upon meats infected with the muscle cysts of Sarcocystis hominis, Sarcocystis fusiformis and a species of Sarcocystis from water buffalo that could not be distinguished from Sarcocystis cruzi. Using a new collection method employing filter paper, these excreted oocysts and sporocysts were subjected to DNA extraction, as were the corresponding muscle cysts. Methods employing PCR-RFLP and DNA sequencing of a partial 18S rDNA gene (ssrRNA) sequence were then used to successfully distinguish among the three taxa. The same, unique restriction digestion pattern characterizes the tissue cysts and oocysts and/or sporocysts of each parasite taxon. The technique makes possible amplification and identification of species specific gene sequences based on DNA extracted from as few as 7 excreted sporocysts (the equivalent of 3 and 1/2 oocysts) from freshly prepared material, or as few as 50 sporocysts from feces samples that had been stored in potassium dichromate (K2Cr2O7) for as long as 6 years. This represents the first report using molecular diagnostic procedures to diagnose oocysts of Sarcocystis in faecal samples, describing a valuable new tool for studying the epidemiology of various Sarcocystis species.