Early Release – Osteomyelitis Caused by Metamycoplasma gateae after Cat Bite, Japan, 2026 – Volume 32, Number 12—December 2026 – Emerging Infectious Diseases journal

Metamycoplasma gateae, a member of the family Mycoplasmataceae, was first isolated from cat saliva in 1967 (1) and reclassified from Mycoplasma into the genus Metamycoplasma in 2018 (2). Although M. gateae is known to colonize cats and cause feline infections (3–6), human infections have not been reported. We report a case of osteomyelitis caused by M. gateae in an immunocompromised patient with cat-bite exposure.

A 39-year-old woman was admitted to Oita University Hospital with fever and pain in her right hand, which her pet cat had bitten 42 days before admission. She had systemic lupus erythematosus treated with methylprednisolone (8 mg/d) and mycophenolate mofetil (2,000 mg/d) and type 1 diabetes mellitus managed with continuous subcutaneous insulin infusion. Swelling and pain in the right hand developed 32 days before admission and was diagnosed by a dermatologist at the hospital as cellulitis and treated with intravenous ceftriaxone, followed by oral clindamycin for 2 weeks. Magnetic resonance imaging, performed 31 days before hospital admission, had shown no osteomyelitis. The pain persisted despite treatment. Two days before admission, swelling spread to the wrist and fourth and fifth proximal interphalangeal joints, and fever (38.3°C) developed along with worsening pain.

At admission, physical examination revealed swelling and tenderness of the right hand. Laboratory tests showed elevated leukocytes (9,660 cells/μL, reference range 3,300–8,600 cells/μL), C-reactive protein (15.3 mg/dL, reference range 0.0–0.14 mg/dL), and glycated hemoglobin (7.6%, reference range 4.9%–6.0%) and markedly decreased serum immunoglobulin levels (88 mg/dL IgG [reference range 861–1,747 mg/dL]; 13 mg/dL IgA [reference range 93–393 mg/dL]; and 2 mg/dL IgM [reference range 50–269 mg/dL]). Magnetic resonance imaging revealed subcutaneous fluid collection within the dorsum of the hand and osteomyelitis of the third metacarpal bone, leading to a diagnosis of cellulitis with osteomyelitis. We performed an incision with drainage and collected pus, tissue, and blood culture specimens. We cultured the pus and tissue specimens in aerobic, anaerobic, and enrichment media under standard conditions (Appendix). We initiated empirical antimicrobial drug therapy with piperacillin/tazobactam (4/0.5 g every 8 h). Pus and tissue cultures were negative; serologic testing for Bartonella antibodies was negative. We changed the antimicrobial drug therapy to ampicillin/sulbactam (2/1 g every 6 h) on day 5. However, pain in the right hand worsened; we changed the treatment back to piperacillin/tazobactam only on day 8.

Figure

Figure. Patient’s right hand in case study of osteomyelitis caused by Metamycoplasma gateaeafter cat bite, Japan, 2026. A) Erythema and swelling of the wrist, dorsum of the hand, and…

On day 11, 16S rRNA gene sequencing of DNA from a tissue specimen identified M. gateae as the causative bacterium. The nucleotide sequence was deposited in GenBank (accession no. LC934417). The sequence showed 99.86% (1,450/1,452 nt) sequence similarity to a reference strain (GenBank accession no. CP143578.1) (Appendix Table). Because the swelling persisted on day 12 (Figure, panel A), we switched treatment to minocycline (100 mg every 12 h). The swelling improved by day 19 (Figure, panel B) along with gradual recovery of the range of motion. We discharged the patient on day 29 and advised her to continue oral minocycline for >4 weeks.

M. gateae can cause severe infections in feline hosts, including erosive polyarthritis, tenosynovitis, ulcerative keratitis, and lower respiratory tract infections, particularly in immunocompromised cats (3–6). M. gateae infections have not been reported in humans. Because of the high prevalence of M. gateae in domestic cat populations, which is present in >80% of adult cats (7), cases of human infection might be unrecognized. No standardized antimicrobial drug treatment regimens have been established for M. gateae infections in humans. We selected minocycline because of its high level of bone penetration. The patient’s immunocompromised state, caused by hypogammaglobulinemia, steroid therapy, and type 1 diabetes, might have contributed to infection susceptibility. Hypogammaglobulinemia is a known risk factor for Mycoplasma infections (8).

No pathogens were identified in tissue or blood cultures, and β-lactam/β-lactamase inhibitor therapy was ineffective. However, BLAST (https://blast.ncbi.nlm.nih.gov) analysis of the 16S rRNA gene sequence obtained from a tissue sample revealed the highest sequence identity to M. gateae (99.86% identity to GenBank accession nos. CP143578.1 and NR_029180.1). The next closest match was Mycoplasma arginini (99.31% sequence identity), a common colonizer of feline respiratory tracts (8). The differences in sequence identities support the identification of the isolate as M. gateae. We did not perform whole-genome sequencing, and misclassification cannot be excluded. Clinicians should consider M. gateae in immunocompromised patients with indolent infections after animal contact or bites. If Mycoplasma infection is suspected, specimens should be cultured on specialized media under appropriate conditions (8). Previous antimicrobial drug exposure might have reduced culture yield and masked other pathogens. Although incision and drainage might have contributed to recovery, we did not observe clinical improvement until minocycline was initiated, suggesting that targeted antimicrobial drug therapy played the primary role in clinical improvement.

In conclusion, we report a case of M. gateae–mediated osteomyelitis in an immunocompromised patient. Negative routine tissue and blood cultures, despite cat-bite exposure, together with a poor clinical response to β-lactam/β-lactamase inhibitor therapy prompted 16S rRNA gene sequencing, which identified M. gateae. M. gateae infection should be considered in patients with cat-bite exposure when routine cultures are negative and β-lactam/β-lactamase inhibitor therapy is ineffective.

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