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

<i>Streptococcus dysgalactiae</i> is a β-hemolytic group C/G <i>Streptococcus</i> 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 <i>Streptococcus dysgalactiae</i> 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 <i>Streptococcus dysgalactiae</i>, 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 <i>S. dysgalactiae</i>-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.