Malignant syphilis with presumed multifocal osteomyelitis in a patient with human immunodeficiency virus: A case report.

Malignant syphilis is a rare and aggressive form of secondary syphilis that occurs predominantly in individuals living with human immunodeficiency virus. Owing to its rarity, the prevalence of systemic involvement and the optimal therapeutic approach remain incompletely defined. A 37-year-old man with human immunodeficiency virus presented with fever, weight loss, and a generalized nonpruritic rash characterized by ulcerative lesions covered with rupioid crusts. The initial rapid plasma reagin test was negative; however, repeat testing revealed seroconversion of the rapid plasma reagin test, with persistently positive Treponema pallidum antibodies. Histopathological examination revealed lymphoplasmacytic and histiocytic cell infiltration along with spirochetes detected by immunohistochemical staining for Treponema pallidum. Notably, bone scintigraphy and contrast-enhanced magnetic resonance imaging detected multifocal bone lesions suggestive of syphilitic osteomyelitis, despite the absence of bone pain. Due to the unavailability of intramuscular benzathine penicillin G, the patient was treated with prolonged oral amoxicillin in combination with probenecid, resulting in substantial clinical and serological improvement. This case underscores that malignant syphilis may be associated with clinically silent bone involvement detectable only through imaging studies. It also suggests that oral amoxicillin plus probenecid may represent an alternative treatment option when standard intramuscular penicillin therapy is not available.