Erosive polyarthritis caused by sepsis due to a novel species of Streptobacillus notomytis.

Rat-bite fever (RBF) is characterized by fever, rash and arthritis, mainly caused by Streptobacillus moniliformis. We present a case of inflammatory erosive polyarthritis with sepsis caused by Streptobacillus notomytis, a novel species isolated from S. moniliformis. A 67-year-old man presented with fever, purpura and peripheral arthritis. After blood cultures were performed, loxoprofen administration was initiated. Arthritis partially improved, but left first metatarsophalangeal (MTP) arthritis persisted. A gram-negative rod was detected in the blood culture, and meropenem administration was started that improved arthritis. Ultrasonography and computed tomography revealed bone erosion in the left first MTP joint, leading to the diagnosis of septic arthritis. 16S rRNA gene sequence analysis revealed that this strain was S. notomytis. Further questioning revealed the patient may have had contact with rats. It is essential to consider Streptobacillus infection in the differential diagnosis of erosive polyarthritis.