Primary Pyomyositis Due to Resistant Streptococcus Pneumoniae Secondary to Acute Otitis Media in a 7-Month-Old Infant: Case Report

Primary pyomyositis is a bacterial infection of skeletal muscle, often with abscess formation, and the most common etiologic agent is Staphylococcus aureus. It is rare in infants, especially in the absence of a trauma history and when caused by Streptococcus pneumoniae. Early diagnosis can be challenging due to nonspecific clinical features, often mimicking other conditions such as septic arthritis, cellulitis and osteomyelitis. A previously healthy 7-month-old infant was admitted with persistent fever, irritability and refusal to move the right lower limb. She had a previous visit to another health service 4 days earlier for fever, irritability and left-sided otorrhea, and had been treated with oral amoxicillin for 3 days after a presumed diagnosis of acute otitis media (AOM). There was no history of trauma. Due to inflammatory signs in the right thigh, the condition was initially managed as septic arthritis of the hip, but joint lavage yielded no purulent fluid and cultures were negative. Persistence of symptoms prompted additional investigation with blood culture, which isolated S. pneumoniae with susceptibility requiring increased exposure to ceftriaxone (MIC 0.75) and levofloxacin, and susceptibility only to vancomycin. Contrast-enhanced CT of the thigh was then performed based on the new findings, revealing a collection compatible with pyomyositis in the right vastus lateralis muscle. Antibiotic therapy was escalated from oxacillin to ceftriaxone plus vancomycin for 14 days, and then de-escalated at discharge to oral levofloxacin for an additional 10 days. Surgical drainage was not performed, and the patient evolved with progressive regression of the collection and complete clinical resolution. This case is notable for the rare occurrence of pyomyositis in an infant without classic risk factors such as a history of trauma, and for the likely mechanism of hematogenous dissemination of S. pneumoniae following an initial AOM, which is an unusual behavior for pneumococcus. Furthermore, the isolation of a pneumococcal strain susceptible only to vancomycin required treatment with broad-spectrum antibiotics. The decisive role of microbiological testing in diagnosis is also highlighted. This report reinforces the need to consider pyomyositis in the differential diagnosis of fever and limited limb movement in infants, even with atypical presentation and an uncommon agent, given the serious consequences that delayed diagnosis can cause.