Rectal Syphilis Mimics Malignant Tumor, Causing Bleeding in a Male Patient

Rectal bleeding is a common symptom, often benign, but can indicate serious conditions like colorectal cancer (CRC). Clinicians must differentiate between benign causes like hemorrhoids and potentially malignant lesions, even in younger patients. Syphilis, caused by Treponema pallidum, is a sexually transmitted infection known as "the great mimicker" due to its varied presentations. While rare in high-income countries, its incidence is rising, particularly among men who have sex with men. Rectal syphilis can present as a bleeding mass, often mistaken for a malignant tumor, highlighting a critical diagnostic challenge.

A male patient in his 40s was referred for rectal bleeding, constipation, fatigue, weight loss, and perianal itching. He was concurrently treated with semaglutide, which could contribute to weight loss. Initial proctoscopy revealed a bleeding, ulcerated rectal mass, highly suspicious for malignancy. Biopsy samples were taken for histological examination to determine the nature of the lesion and rule out colorectal cancer. Further investigations included immunohistochemistry and serology to identify specific pathogens.

However, biopsy results showed severe inflammation with plasma cells and neutrophils, but no signs of malignancy. Subsequent immunohistochemistry staining for Treponema pallidum was positive, confirming the presence of the spirochete. Serological tests further supported the diagnosis, with a positive Treponema pallidum particle agglutination assay (TPPA) and a rapid plasma reagin (RPR) titer of 1:64, indicating active syphilis. The patient was diagnosed with rectal syphilis and successfully treated with penicillin, leading to complete resolution of symptoms.