Mycoplasma hominis septic arthritis in a patient with hypogammaglobinaemia and rheumatoid arthritis.

We describe the case of Mycoplasma hominis septic arthritis in a 58-year-old woman with a history of rheumatoid arthritis and ulcerative colitis on immunosuppressive therapy with rituximab. Treatment with anti-CD20 antibodies (eg, rituximab) leads to an immediate depletion of B cells and hence risk of reductions in immunoglobulins and increased risk of infections. This effect may last long after drug cessation. M. hominis is commensal to the genitourinary tract in sexually active adults. Extragenital M. hominis infections including septic arthritis are rare, but hypogammaglobulinaemia is a predisposing factor. As M. hominis requires extended culture, special media or PCR analysis, it is not tested routinely, which in many cases delays diagnosis and correct treatment. In our case, a diagnosis of M. hominis septic arthritis was made after 9 weeks by PCR analysis and culture of joint fluid. The patient responded well to an 8-week treatment course of moxifloxacin and doxycycline. © BMJ Publishing Group Limited 2021. No commercial re-use. See rights and permissions. Published by BMJ.