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Rat bite fever

Katherine Yudeh King, MD, PhD
Section Editors
Daniel J Sexton, MD
Morven S Edwards, MD
Deputy Editor
Jennifer Mitty, MD, MPH


Rat bite fever (RBF) is a rarely diagnosed, systemic illness caused by infection with either Streptobacillus moniliformis or Spirillum minus. S. moniliformis causes most cases of the disease in the United States. S. minus causes RBF primarily in Asia, although it is probably present worldwide.


In the United States, rat bite fever (RBF) is typically caused by S. moniliformis. The disease is rare, with only several cases documented each year. As an example, between 2000 and 2012, only 17 cases were identified in California [1]. However, its actual incidence is unknown because RBF is not a nationally notifiable disease, and many cases go undiagnosed since these bacteria are difficult to identify and are likely to respond to empiric antibiotic therapy. (See 'Treatment' below.)

The risk of RBF due to S. moniliformis after a rat bite is reported to be 10 percent [2], and an estimated 20,000 rat bites occur in the United States each year [2]. Historically, over 50 percent of reported cases occurred in children, and RBF was most likely to be seen in those living in poverty [3]. However, the demographics have changed, and now include pet store workers and laboratory technicians as rats have become popular pets and research subjects [4,5].

In Asia, RBF is known as sodoku (so: rat, and doku: poison) and it is primarily caused by S. minus [2].


S. moniliformis — S. moniliformis is a pleomorphic fastidious branching gram-negative bacillus (picture 1). It stains irregularly and can be mistaken for gram-positive pleomorphic rods. The bacteria are microaerophilic; they require specific media for isolation (10 to 20 percent serum) and incubation in a 5 to 10 percent CO2 environment. Sodium polyanethol sulfonate (SPS), an anticoagulant added to most aerobic blood culture bottles, inhibits growth of S. moniliformis. However, anaerobic culture bottles, resin bead culture systems, and trypticase soy agar and broth may demonstrate growth since they do not contain SPS.

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Literature review current through: Nov 2017. | This topic last updated: Jul 18, 2016.
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