Multi-Pathogen Knee Infection, Including Treponema pallidum and Brucella: A Case Report

BACKGROUND Infectious arthritis is an infection of the joint and joint cavities, with Staphylococcus aureus being the most common causative agent. Most cases of infectious arthritis are caused by a single pathogen, with polymicrobial infections being comparatively rare. This article describes a patient with a knee infection caused by Treponema pallidum, Brucella spp., and possibly co-infected with Mycobacterium tuberculosis and Escherichia coli (E. coli). The aim of this paper is to discuss the characteristics of atypical joint diseases caused by multi-pathogenic infections and to provide a reference for the diagnosis and treatment of related diseases. CASE REPORT A 58-year-old male veterinarian from western China presented with pain, swelling, and restricted movement in his left knee after arthroscopic surgery. The imaging results showed extensive and severe damage to the knee joint structure, but the laboratory results showed that the neutrophil level was normal and he did not experience severe pain. His blood and cerebrospinal fluid were positive for Treponema pallidum antibodies and his blood was positive for IgG antibodies against Brucella abortus. Because the knee joint was severely damaged, knee arthrodesis surgery was performed, with a satisfactory outcome. CONCLUSIONS This case was a multi-pathogenic joint infection with a complex course. By comparing the patient's test results and clinical condition with the common features and symptoms of Charcot arthropathy, we concluded that syphilis had invaded the central nervous system, and that Charcot arthropathy, likely secondary to neurosyphilis, masked the severe pain typically associated with extensive joint destruction. This case highlights the diagnostic challenges associated with such conditions.