The only Sarcocystis species currently known to inhabit the fibers of skeletal and cardiac muscles in horses are S. fayeri, S. bertrami, and S. asinus. We describe herein the invasion of myofibers in a horse by S. gigantea, a sheep-specific species with low virulence in the original host. A hunter gelding was referred to a veterinary surgeon in Newmarket (UK). The anamnestic data reported that the horse had an initial history of swelling of the right forelimb with fluid on the front of the carpus and edema spreading up the forearm. Subsequently, 2 firm lumps were found on the left pectoral muscle adjacent to the axilla of the left forelimb. Histologic examination of biopsies from the lumps revealed multifocal granulomatous eosinophilic myositis associated with intact and degenerate encysted parasites, consistent with Sarcocystis spp. Based on amplification and DNA sequencing of the 18S rRNA gene obtained from formalin-fixed, paraffin-embedded tissue blocks, S. gigantea was identified. The presence of sarcocysts in equine skeletal muscles has been considered an incidental finding, and there are only sporadic associated reports of myositis. Our finding suggests that some Sarcocystis spp. have a wider intermediate host range than believed previously, and that Sarcocystis of other species (not considered horse-associated) can invade the muscle fibers of equids, leading to myositis.

唯一的肉孢子目前已知的栖息在马骨骼肌和心肌的纤维种类是S. fayeri，S. bertrami，和S. asinus。我们描述肌纤维在此所述的入侵由马S.巨，是原宿主中低毒力的绵羊特有物种。在英国纽马克特（Newmarket），一名猎人gel缩被称为兽医。记忆数据表明，该马的最初病史是右前肢肿胀，在腕骨前部有积液，水肿扩散到前臂。随后，在靠近左前肢腋部的左胸肌上发现2个硬块。对肿块进行活检的组织学检查显示，多灶性肉芽肿性嗜酸粒细胞性肌炎与完整和简并的寄生虫相关，与肉囊藻属一致。基于从福尔马林固定，石蜡包埋的组织块体S. gigantea获得的18S rRNA基因的扩增和DNA测序被确定。马骨骼肌中存在囊肿被认为是偶然发现的，并且仅有偶发的肌炎相关报道。我们的发现表明某些肉囊藻属。具有比以前认为的更宽的中间宿主范围，并且肉孢子虫其它种的（不考虑马相关联）可侵入马科动物的肌纤维，导致肌炎。