Abstract

Rat-bite fever (RBF) is a rare systemic illness caused by Streptobacillus moniliformis, Spirillum minus, or Streptobacillus notomytis. As the name implies, the disease is typically transmitted by a rat bite. S.moniliformis, a fastidious gram-negative bacillus, is the most common cause of RBF in North America and has an average incubation period of 5 days. RBF typically presents as a triad of fever, arthritis, and rash, with the morphology of the rash being highly variable. Definitive diagnosis of RBF may prove difficult, as the responsible bacteria are not easily identified with standard testing. We describe a 34-year-old female who presented with a two-day history of malaise, fever, chills, polyarthralgia and a tender/slightly pruritic rash two days after being bitten by a rat she was feeding to her anaconda. Initial vital signs were remarkable for fever (37.8°C) and tachycardia (115 beats/min). She also had a mild thrombocytopenia (143 x 103/mL). Physical examination revealed an erythematous vesicular and papular rash involving her extremities, buttocks, and oral mucosa. The patient was initiated on intravenous ceftriaxone following blood cultures, which were negative. A punch biopsy from the left elbow revealed a prominent superficial and deep dermal leukocytoclastic vasculitis with necrosis and fibrin thrombi. The superficial dermis displayed congestion, edema, erythrocyte extravasation, and neutrophilic inflammation extending into the epidermis, with subepidermal clefting. Gram and GMS stains were negative. The tissue was sent out for further analysis. Immunohistochemical (IHC) stains and PCR were negative for Rickettsia species. Sequence analysis of 16S rRNA gene PCR assay and IHC were both positive for S.moniliformis. A diagnosis of RBF was made, and the patient was successfully treated with a two-week course of doxycycline.

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