We present a case report of an extremely rare infected abdominal aortic aneurysm (AAA) caused by group B streptococcus (GBS). A 66 years old, afebrile male patient with hypertension and history of chronic smoking presented with mild abdominal pain and discomfort. Physical examination revealed a pulsatile mass of 5 cm in diameter located at the periumbilical region. A computed tomography (CT) scan of the whole abdomen exhibited an outpouching lesion with intramural hematoma located at the infrarenal abdominal aorta, sized 5.1x3.7 cm in diameter, suggesting an abdominal aortic aneurysm. A transthoracic echocardiography demonstrated normal endocardium without vegetation. Surgical abdominal
aortic aneurysm repair (aneurysmorrhaphy) was successfully done and intravenous amoxicillin/clavulanic acid was given.
Tissue culture obtained from the aneurysm sac and para-aortic lymph nodes revealed group B streptococcus infection.
Intravenous amoxicillin/clavulanic acid was continued for 14 days, and was transitioned to oral amoxicillin 3 grams/day.
Currently, the patient has been followed-up for 20 months with neither recurrent infection nor post-operative complication.
This is the first case report of infected abdominal aortic aneurysm due to GBS in Thailand.

Keywords: Group B streptococcus (GBS), Mycotic aneurysm, Infected aortic aneurysm, Abdominal aortic aneurysm