A Rare Case of Primary Mycobacterium tuberculosis Myositis of the Thigh in an Immunocompetent Infant

Mycobacterium tuberculosis (MTB) infection is a serious health condition that affects individuals of all age groups. Although MTB infections are more common in immunocompromised patients, they are frequently diagnosed in healthy individuals without apparent risk factors. Extrapulmonary infection is an uncommon manifestation of MTB infection, especially infection of the musculoskeletal system. Here, we present a rare case of a five-month-old immunocompetent infant who presented with a progressively enlarging swelling of the thigh without any other symptoms. After further evaluation, a diagnosis of primary MTB myositis of the thigh was made, which was treated successfully with first-line anti-tuberculosis therapy for nine months. This case report highlights the need to consider MTB infection in infants and children with unusual clinical findings. Due to its nonspecific symptoms and difficulty in diagnosis, clinicians need to maintain a high index of clinical suspicion for MTB infection, even in infants without risk factors for exposure.