Multifocal myositis associated with Sarcocystis sp in a horse.

The research presents a case study about a 7-year-old Quarter Horse gelding which suffered from multiple areas of muscle inflammation (myositis) due to an infection with a certain strain of parasite genus “Sarcocystis”.

Overview of the Case

• The study focused on a 7-year-old Quarter Horse gelding diagnosed with multifocal myositis.
• The diagnosis was confirmed using various methods including history, physical examination, serum biochemical analysis, electromyography, and microscopic examination of frozen sections of muscle biopsy specimens.

Findings and Treatment

• The microscopic examination of the muscle tissue showed that there were multiple accumulations of different types of immune cells including histiocytes, lymphocytes, and plasma cells. This was accompanied with muscle fiber degeneration and necrosis.
• Parasitic cysts with morphologic characteristics of a parasite of the genus Sarcocystis were discovered in the areas where there was muscle cell degeneration and death, but also in areas of normal muscle tissue.
• The horse was treated with medications including trimethoprim/sulfamethoxazole and pyrimethamine for 28 days.
• In addition to the medication, the horse was also given phenylbutazone for 5 days along with 30 days of paddock rest.
• At the end of the treatment regimen, it was noted that the horse had gained 35 kg in weight, its appetite had returned to normal, and muscle mass was getting back to normal as well.

Analysis and Conclusion

• Sarcocystis fayeri is the only known species of Sarcocystis reported in equine muscle in the United States, and it rarely causes acute myositis or muscle atrophy.
• However, given the clinical signs observed in this horse, it was suggested that the horse could have been suffering from an underlying immunosuppression, or it may have been infected with a particularly pathogenic strain or a large dose of S fayeri.
• The results of the study highlight the importance of considering Sarcocystis infection in equines presenting with myositis, especially in cases which do not follow the usual presentation associated with Sarcocystis fayeri.