Rat-bite fever diagnosed by polymerase chain reaction of a bite-site crust specimen three weeks after exposure: a case report.

Rat-bite fever (RBF), a zoonotic infection caused mainly by Streptobacillus moniliformis (S. moniliformis), is challenging to diagnose due to the difficulty of isolating the causative pathogen, which can delay the diagnosis or lead to inappropriate treatment. We report herein a case of RBF in which polymerase chain reaction (PCR) testing of a crust specimen from the bite site diagnosed the condition as late as three weeks after the exposure. A previously healthy, 66-year-old, female patient presented with a fever and migratory, asymmetric polyarthritis three weeks after being bitten on her left finger by a rat. Physical examination revealed polyarthritis and a dry, non-purulent crust at the bite site. While blood cultures returned negative, S. moniliformis was isolated from a synovial fluid culture after seven days. Notably, polymerase chain reaction (PCR) testing detected S. moniliformis DNA in both synovial fluid and crust from the bite site, thereby definitively diagnosing RBF despite negative PCR findings for plasma, serum, and urine. This case highlights the diagnostic value of bite-site crust specimens for PCR testing even weeks after exposure and emphasizes the importance of appropriate specimen selection.