The findings of rash, fever, and arthritis in individuals with a history of rat exposure suggest the diagnosis of Rat-bite fever.

Diagnosis of S. moniliformis is typically made by isolating the organism from blood or synovial fluid. Specific media and incubation conditions should be used. In the absence of a positive culture, identification of pleomorphic gram-negative bacilli in appropriate specimens supports a preliminary diagnosis. Since the organism does not grow in artificial media, diagnosis of S. minus is made by identifying characteristic spirochetes in appropriate specimens using dark field microscopy or differential stains.