Rat-bite fever septic arthritis: illustrative case and literature review

Rat-bite fever is a rare zoonotic infection caused by Streptobacillus moniliformis or Spirillum minus, which is characterised by fever, rash and arthritis. The arthritis has previously been described as non-suppurative and isolation of the organism from synovial fluid as very uncommon. This article reports a case of septic arthritis diagnosed as rat-bite fever when the organism was cultured from synovial fluid and reviews another 15 cases of S. moniliformis septic arthritis reported in the worldwide literature since 1985. Articles were included in this review if S. moniliformis was cultured from synovial fluid. Of the published cases, 88% presented with polyarthritis, affecting small and large joints although two had monoarticular hip sepsis. Fever was present in 88%, rash in 25% and 56% had extra-articular features. Synovial fluid analysis revealed high cell counts in all cases (mean 51,000 × 10⁹/l) with a predominance of polymorphonuclear leucocytes, and organisms were found on Gram stain in only 50%. Penicillin was used for treatment in 56% of cases and surgery was required in 30%. All patients recovered. Rat-bite fever arthritis can be suppurative and attempts should be made to isolate the organism from synovial fluid. The diagnosis should be considered when there is arthritis and a high synovial fluid cell count but no apparent organism, especially when the patient has had contact with rats.