Case Presentation: A 40-year-old male with a history of Psoriatic Arthritis (PsA), managed on weekly Etanercept, presented to our academic medical center with fevers and polyarthralgia. Two weeks prior, he presented to urgent care for fever, chills, arthralgias, and myalgias, where he was diagnosed with an acute viral syndrome. Two days later, he re-presented for ongoing symptoms and revealed he was bitten by a pet rat three weeks ago on his right metacarpophalangeal joint, which was swollen and tender for one week and had since resolved. Respiratory viral testing was negative. Two days later, he presented to rheumatology clinic for ongoing fever, chills, and worsening polyarthralgia. He was instructed to restart Etanercept and started on a Medrol dose pack with concern for PsA versus post-viral arthralgias. He re-presented to rheumatology a week later with worsening arthralgias and continued fevers. Autoimmune serologies and infectious work up were obtained. A right knee arthrocentesis was performed for persistent pain and edema, revealing WBC 41,000 and gram-negative rods (GNRs) on gram stain with no growth on culture. The next day he presented to the hospital for new upper extremity weakness. Repeat right knee arthrocentesis revealed WBC 21,000, 94% neutrophils, and no growth on cultures. Due to concern for rat bite fever (RBF) and right knee septic arthritis, broad antimicrobial coverage with Cefepime was started and a right knee arthroscopic incision and drainage was performed. Three days later, blood cultures grew GNRs and speciation revealed Streptobacillus moniliformis. He had resolution of his fevers with significant improvement in polyarthralgia. He was discharged with four weeks of outpatient parenteral antimicrobial therapy with Ceftriaxone.
Discussion: This case highlights a rarely diagnosed disease, RBF, which was complicated by right knee septic arthritis and Streptobacillus moniliformis bacteremia. RBF is caused by infection with Streptobacillus moniliformis, Streptobacillus notomytis, or Spirillum minus. Risk of infection after a rat bite is reported to be up to 10 percent with a seven-day incubation period following exposure. Symptoms include fever, myalgias, and migratory arthralgias. At symptom onset, the bite wound has usually resolved. The actual incidence of RBF is unknown as many cases go undiagnosed as these bacteria are difficult to identify. S. moniliformis and S. notomytis are fastidious organisms that are difficult to grow in culture and S. minus cannot be cultured. As a result, RBF is commonly diagnosed empirically; however, in this case, S. moniliformis was grown and identified on blood cultures. Given the rarity of RBF, antibiotic selection is guided by literature instead of susceptibilities given the fastidious nature of these organisms. Penicillin is the traditional treatment choice based on published experience. However, Ceftriaxone is a reasonable alternative, and was selected in our case, given ease of administration. The morality rate of untreated RBF due to S. moniliformis is approximately 13 percent. The case was convoluted by overlapping symptoms that anchored to a possible PsA flare and illustrates a delay in diagnosis through multiple outpatient providers even with knowledge of a recent rat bite.
Conclusions: RBF is a rarely diagnosed, systemic illness, usually caused by infection from S. moniliformis. RBF is under-recognized and under-reported as many cases go undiagnosed due to the fastidious nature of the organisms making them difficult to grow in culture.