History and admission findings: A 53-year-old man with known HIV infection and AIDS was admitted because of painful swelling at the right knee for 6 weeks. The cause was thought to be osteomyelitis and surgical treatment was planned.
Investigations: No causative pathogen was found at curettage and lavage of an abscess at the right medical head of the tibia, but at a subsequent operative revision acid-fast rods were seen and identified as Mycobacterium haemophilum.
Treatment and course: A systemic antibiotic, 1 g levofloxacin daily, had been started at the initial abscess operation. 2 weeks later, because swelling of the right knee had recurred with marked local and systemic signs of infection, a second surgical intervention was performed. Afterwards, in view of the histological finding of acid-fast bacteria suggesting tubercular osteomyelitis, the patient was put on combined treatment with 300 mg/d of isoniazid, 1600 mg/d of ethambutol, 2 g/d of pyrazinamide, and 1 g of streptomycin i.m. every other day. After molecular microbiological identification of M. haemophilum the antibiotic treatment was changed to 1600 mg/d of ethambutol, 300 mg/d of rifabutin and 1 g/d of clarithromycin. The operation wound healed well.
Conclusion: M. haemophilum infection can be lethal in immunodeficient patients if untreated. Although there is no standard treatment, this rare infectious disease responds relatively well to a modified combined tuberculostatic regimen. Special laboratory techniques to identify the specific causative pathogen are therefore of great importance.