Sternocleidomastoid and Pectoralis Major Myositis Due to Streptococcus dysgalactiae Bacteremia in a Healthy Adult: A Rare Case.

Streptococcus dysgalactiae is a β-hemolytic group C/G Streptococcus increasingly recognized as a cause of invasive human disease. Although commonly associated with cellulitis, bacteremia, septic arthritis, and endocarditis, primary infectious myositis due to this organism is rare, particularly in immunocompetent adults. We report a case of a 33-year-old previously healthy male with a history of unexplained recurrent Streptococcus dysgalactiae bacteremia who presented with a five-day history of progressive right-sided neck and upper chest pain accompanied by low-grade fever. Magnetic resonance imaging demonstrated inflammatory changes involving the sternocleidomastoid and pectoralis major muscles without evidence of abscess formation or necrotizing infection. Blood cultures grew Streptococcus dysgalactiae, confirming infectious myositis secondary to bacteremia. The patient was treated with intravenous β-lactam therapy, followed by oral antibiotics, completing a total six-week course, with rapid clinical improvement and complete recovery. This case adds to the limited literature on S. dysgalactiae-associated myositis and underscores the importance of considering deep muscle involvement in patients presenting with focal musculoskeletal pain and concurrent group C/G streptococcal bacteremia. Early imaging and timely antimicrobial therapy are essential to prevent progression to severe or necrotizing disease.