Thigh pyomyositis caused by group A streptococcus in an immunocompetent adult without any cause

BACKGROUND: Pyomyositis is typically caused by Staphylococcus aureus, and is rare in temperate climates, although its prevalence has been recently increasing. This infection often involves the thigh, and is associated with immunodeficiency. CASE PRESENTATION: We report the case of a healthy 20-year-old Japanese woman who experienced a fever and continuous pain for several days. She was admitted to our hospital and was diagnosed with pyomyositis after we discovered an abscess between the muscles of her dorsal distal left thigh using computed tomography. This is a rare case of thigh pyomyositis, as it was caused by group A streptococcus and occurred in an immunocompetent adult from a temperate climate. CONCLUSIONS: Our review of the literature revealed that group A streptococcus pyomyositis typically occurs in temperate climates, among young adults without any underlying disease, and is associated with a poorer prognosis, compared to general pyomyositis. We suggest that pyomyositis should be considered when immunocompetent adults present with apparently idiopathic inflammatory muscle lesions.