Molecular Screening of Sarcocystis spp. in Grazing Sheep (Ovis aries) and Shepherd Dogs (Canis lupus familiaris) from Central Portugal

<i>Sarcocystis</i> spp. are cyst-forming protozoan parasites with a global distribution that infect a wide range of domestic and wild animals, impacting both animal health and livestock productivity. In sheep, infections can cause clinical disease, reproductive losses, and economic damage, particularly when pathogenic species such as <i>Sarcocystis tenella</i> are involved. Grazing sheep, including breeds such as the Serra da Estrela from central Portugal, are at increased risk due to frequent contact with shepherd dogs, which serve as definitive hosts. Despite their significance, data on the occurrence and distribution of <i>Sarcocystis</i> spp. in Portuguese sheep remain limited. This study analyzed 179 samples collected in central Portugal during 2024, including 41 brain tissues and 88 blood samples from sheep, and 50 stool samples from shepherd dogs, using conventional PCR and bidirectional Sanger sequencing. <i>Sarcocystis</i> sp. closely related to <i>S. tenella</i> was detected exclusively in sheep brain tissue, with a prevalence of 4.9% (2/41; 95% CI: 0.60-16.53), while no parasite DNA was found in blood or dog samples. These results provide the first molecular confirmation of <i>Sarcocystis</i> spp. closely related to <i>S. tenella</i> in Portuguese sheep raised for human consumption and establish baseline data for future epidemiological surveillance and control strategies.