Native Aortic Valve Infective Endocarditis Involving the Mitral-Aortic Intervalvular Fibrosa Caused by Lacticaseibacillus paracasei Presenting With a Prolonged Constitutional Syndrome and Non-diagnostic FDG/PET-CT: A Case Report and Literature Review

Lacticaseibacillus paracasei is a rare cause of infective endocarditis (IE) and is frequently regarded as a blood culture contaminant, which may delay diagnosis.A 75-year-old man presented with a six-month history of progressive weight loss, drenching night sweats, and chills without fever.Initial investigations revealed an inflammatory syndrome, while FDG/PET-CT showed only nonspecific findings, prompting evaluation for inflammatory, infectious, and hematologic disorders.Transthoracic echocardiography (TTE) raised suspicion for IE, and transesophageal echocardiography (TEE) demonstrated a 9 mm vegetation on the mitral-aortic intervalvular fibrosa (MAIVF), without aortic leaflet involvement seen on imaging and with mild aortic regurgitation.Four pairs of blood cultures came back positive for Lacticaseibacillus paracasei, confirming persistent bacteremia.No conventional predisposing factors, including immunosuppression, probiotic use, dental disease, or significant gastrointestinal pathology, were identified.Intravenous antimicrobial therapy achieved microbiological clearance; however, repeat transesophageal echocardiography demonstrated persistent vegetation with MAIVF thinning and cavity formation, suspicious for pseudoaneurysm, prompting referral for cardiac surgery at another institution.The postoperative course was favorable, and the patient was discharged 10 days after surgery.This case illustrates that L. paracasei should not automatically be dismissed as a contaminant and that native aortic valve infective endocarditis involving the MAIVF may present without fever, while FDG/PET-CT may be falsely reassuring.Repeated blood cultures and serial echocardiography remain essential for establishing the diagnosis of endocarditis and guiding management.