A case of atypical cat scratch disease with bone and joint infection diagnosed through clinical metagenomics.

Cat scratch disease (CSD) is a common zoonotic infection caused by Bartonella henselae (B. henselae) and typically presents with fever and regional lymphadenopathy. However, skeletal involvement, including osteomyelitis and arthritis, is rare. We report a 28-year-old immunocompetent female who presented with a five‑month history of persistent right knee swelling without fever or lymphadenopathy. She had previously undergone distal femoral tumor resection with prosthetic joint replacement, and this episode of chronic knee swelling together with the imaging findings was highly suggestive of prosthetic joint infection. Approximately one month before the onset of knee swelling, she had sustained a scratch from a cat. Conventional microbiological tests, including joint effusion and drainage fluid cultures, were negative. Metagenomic next‑generation sequencing (mNGS) of joint effusion identified B. henselae with 27 specific sequence reads, 0.1 % genome coverage and an RPM ratio of 1.9. This result was subsequently confirmed by a quantitative PCR assay targeting the nuoG gene. The patient underwent surgical debridement followed by oral minocycline and rifampin for 8 weeks, resulting in marked clinical improvement. This case underscores that B. henselae infection should be considered in culture‑negative bone and joint, particularly prosthetic joint, infections with a history of cat exposure, and that mNGS can provide valuable etiological evidence in atypical CSD.