False-positive dengue IgM in Streptobacillus moniliformis rat-bite fever: the need to look beyond the rapid test-a case report.

Background: This case report explores a unique instance of rat-bite fever (RBF) caused by Streptobacillus moniliformis, with an unexpected false-positive result on dengue serology, making the initial diagnosis challenging. Case Summary: A 47-year-old male presented with acute onset of fever, sore throat, and migratory polyarthritis in a dengue-endemic region. His laboratory investigations revealed a normal white blood cell count, thrombocytopenia, a mildly raised C-reactive protein, and positive dengue IgM serology. He did not report any high-risk sexual exposures or recent diarrheal illness. Initial differential diagnoses included viral arthritis, disseminated gonococcal infection, and reactive arthritis. On the fourth day of admission, his blood cultures returned positive for Streptobacillus moniliformis, and the same organism was isolated through 16S rRNA sequencing of his right knee synovial fluid. He denied any history of rat bites and was unaware of any rodent contact. Treatment was commenced with intravenous ceftriaxone, later transitioning to benzylpenicillin and oral amoxicillin, resulting in significant symptom resolution and full recovery of joint function. Conclusion: This case highlights the diagnostic challenge posed by cross-reactive dengue serology in RBF, emphasizing the need to avoid relying solely on IgM positivity for diagnosing dengue fever. Although comparatively less common, clinicians should be aware of non-specific clinical presentations of RBF, particularly in dengue-endemic areas, and that molecular diagnostic techniques may aid in correct identification and management.