Methicillin-Resistant Staphylococcus aureus Septic Pulmonary Embolism Presumed to Originate From Anal Canal Cancer With a Cutaneous Fistula: A Case Report.

Septic pulmonary embolism (SPE) is a severe condition often linked to infective endocarditis (IE) or intravascular catheter infections, with Staphylococcus aureus being a common pathogen. However, SPE originating from a gastrointestinal malignancy, particularly when caused by a non-enteric pathogen like methicillin-resistant Staphylococcus aureus (MRSA) via a malignancy-associated fistula, is rarely reported. We report the case of a 49-year-old man undergoing preoperative chemotherapy for anal canal cancer who developed MRSA-induced SPE complicated by multiple lung abscesses. Initial investigations revealed multiple bilateral pulmonary nodules with cavitation on chest computed tomography, and blood and sputum cultures subsequently grew MRSA. No evidence of IE was found on transthoracic echocardiography, and other common sources of SPE were not identified. A cutaneous fistula adjacent to the anal canal cancer, without overt signs of local infection, suggested that the cutaneous fistula was a plausible portal of entry for MRSA. The patient was successfully treated with an extended course of anti-MRSA therapy, including linezolid, and supportive care, leading to clinical and radiological improvement. This case highlights that anal canal cancer with an associated cutaneous fistula may serve as an occult portal of entry for MRSA, leading to SPE. Clinicians should consider this atypical mechanism in patients with malignancy presenting with SPE, especially when common sources are absent. Prompt recognition and targeted antimicrobial therapy are crucial for managing such complex infections.