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Septic arthritis caused by Mycoplasma hominis is rarely diagnosed.1 We present a case of M hominis septic arthritis in a renal transplant recipient.
A 36 years old white man receiving haemodialysis was admitted to hospital for renal transplantation. Two weeks later, while taking cyclosporin A, azathioprine, prednisolone, vancomycin, aztreonam, and itraconazole, he developed inflammation of the right knee. Serological examinations for cytomegalovirus (Ig G and M), hepatitis B and C, HIV, and Epstein-Barr virus were negative. A Mantoux test was positive. On physical examination he was not febrile and had arthritis of the right knee. Laboratory findings included a whole blood cell count of 9570/mm3, packed cell volume 22%, platelet count 224 000/mm3, creatinine 5 mg/dl, and uric acid 9 mg/dl. A chest radiograph was normal. A right knee roentgenogram showed soft tissue swelling without erosions or bone lesions. Arthrocentesis yielded 60 cc of cloudy yellow synovial fluid containing white blood cell count 50 000/mm3 (80% polymorphonuclear neutrophils), glucose 114 mg/dl, and lactate dehydrogenase 341 IU/l; no crystals were seen. A direct Gram stain of the aspirate showed no microorganisms. There was no bacterial, fungal, and mycobacterial growth on cultures. Arthritis recurred and two further arthrocenteses were performed. The last aspiration revealed 120 cc of cloudy yellow synovial fluid with white blood cell count 68 000/mm3(82% polymorphonuclear neutrophils), glucose 30 mg/dl, and lactate dehydrogenase 728 IU/l. Deep venous thrombosis in the right leg was diagnosed and anticoagulant treatment was started. Twenty four hours later he developed a spontaneous haemarthrosis in the knee. Because there was no improvement of the haemarthrosis and the suspicion of septic arthritis by M tuberculosis was high, open synovial biopsy with synovectomy was performed. Histological examination showed synovial hyperplasia and infiltration with polymorphonuclear cells. No granulomatous reaction nor amyloid were seen. Direct Gram and Ziehl-Nielsen stains were negative. Normal aerobic and anaerobic cultures were also negative. Urethra, pharynx, and rectal cultures were negative. Serological tests for Salmonella, Brucella, Lyme, Q fever, Rubella, cytomegalovirus, and Epstein-Barr virus were negative. C reactive protein, rheumatoid factor, antinuclear antibody, complement, and immunoglobulin values were normal. After 48 hours incubation, the microbiology laboratory reported a growth in blood culture bottles (Bactec Plus, BACTEC 9240, BBL) from the second and third synovial fluid culture. The isolate grew as minute colonies on anaerobic agar medium and was Gram stain negative. On A7 agar (bioMérieux), it showed a typical ‘fried egg’ appearance and hydrolysed arginine and was identified as M hominis. Treatment with doxycycline 100 mg orally twice daily was given. The patient had a gradual response and there was no evidence of recurrent infection after 12 months of follow up.
Mycoplasma sp have been associated with reactive arthritis that is sexually acquired2-4 and with septic arthritis.1 They do not grow satisfactorily on bacteriological media routinely used for joint aspirates, and anaerobic or special mycoplasmal media must be used. On the other hand, many mycoplasmas and bacterial L-forms are ubiquitous and not generally recognised as important pathogens. They may be present in commercial bovine serum and in tap water and are frequent contaminants of tissue culture.5 Our patient presented with acute monarticular arthritis with clinical features that did not differentiate it from other bacterial joint infections. Synovial fluid leucocyte counts were high with a low glucose concentration. Only 17 cases of M hominis septic arthritis have been reported in the medical literature.1 6 There are no unifying predisposing factors of these cases,1 6-8 although a review of the medical literature shows that eight of these patients were immunocompromised (47%) (table 1), and only one case after renal transplantation. With only one exception,9 all the reported cases presented with monarticular or oligarticular arthritis involving large joints. Our case occurred in a renal transplant recipient and although, antibody deficiency is particularly prone to chronic mycoplasmal infections,10 serum immunoglobulin concentrations were normal. We only performed one measurement and we do not exclude transient hypogammaglobulinaemia. Optimal treatment for M hominis joints infections is unknown. Doxycycline apparently only suppresses and does not eradicate the infection. However, our patient had a good clinical response without recurrent arthritis. If bacterial antigens in the joint are critical in the persistence of arthritis, it is possible that the removal of the synovium contributed to the resolution of the arthritis.
We illustrate the importance of considering infection with unusual organisms in immunocompromised hosts. Correct diagnosis will be made only if mycoplasma infection is considered, and appropriate investigations performed.
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