Pyomyositis in an Immunocompetent Adult Man.

Pyomyositis is an uncommon bacterial infection of skeletal muscle, most frequently affecting large lower-limb muscle groups and often associated with abscess formation. While historically prevalent in tropical regions, cases are increasingly reported in temperate climates. Diagnosis is often delayed due to nonspecific early symptoms and deep muscle involvement. We report a 24-year-old previously healthy male presenting with a one-month history of intermittent dull right lower back pain, antecedent fever with chills, intermittent low-grade fever, night sweats, and 10 kg weight loss over two months. Examination revealed a firm, tender swelling in the right flank. Laboratory studies showed anemia (hemoglobin 10.6 g/dL), markedly elevated CRP (181.9 mg/L) and erythrocyte sedimentation rate (47 mm/hr), and a normal leukocyte count. Contrast-enhanced CT demonstrated a bulky right iliopsoas muscle with multiple abscesses. Initial management included IV ceftriaxone, anticoagulation prophylaxis, and supportive care, with planned image-guided drainage. Follow-up imaging and inflammatory markers at two weeks showed marked abscess regression, prompting step-down to oral therapy. The patient later re-presented with recurrent flank swelling; CT revealed a new posterior abdominal wall intramuscular abscess. Ultrasound-guided aspiration yielded sterile pus, negative for acid-fast bacilli. He completed a 21-day course of oral amoxicillin/clavulanate with clinical improvement. This case highlights the diagnostic challenge of pyomyositis in immunocompetent individuals, in whom constitutional symptoms and deep muscle involvement can obscure early recognition. Cross-sectional imaging is critical for diagnosis, particularly when clinical signs are subtle. Optimal management requires a multidisciplinary approach integrating appropriately dosed antibiotics of adequate duration, timely image-guided drainage, and vigilant follow-up to prevent recurrence. Early recognition and intervention are essential to reduce morbidity in this potentially serious infection.