A 30-year-old man was admitted to the hospital with a 4-day fever and a 2-day anorectal pain. He reported having anal intercourse with a new male partner 5 days before the onset of his symptoms. The patient had no recollection of any skin lesions on his partner and was confused about whether the partner had signs of human monkeypox (mpox). Vesicles were observed in his throat, face, trunk, and limbs following a physical examination. Anorectal examination indicated perianal erosion (Figure 1a). Two samples were collected from the face and wrist using the protocol recommended by the World Health Organization.¹ Real-time polymerase chain reaction was performed at the Tokyo Metropolitan Institute of Public Health using the same procedure described in the reference.² The mpox viral gene was found in both skin lesions. Human immunodeficiency virus, syphilis, Chlamydia trachomatis, Neisseria gonorrhea, and herpes simplex virus 1 and 2 were not detected during sexually transmitted disease screening. Stool tests revealed no ova, parasites, or bacteria. Computed tomography of the abdomen and pelvis revealed circumferential wall thickening of the rectum and sigmoid with reactive lymphadenopathy (Figure 1b). A colonoscopy revealed severe proctitis and circumferential erosion (Figure 1c). Rectal biopsies revealed inflammation but were negative for malignancy. The patient was diagnosed with human mpox proctitis and treated with analgesics. His symptoms gradually improved during hospitalization, and he was discharged on day 24. At the 34-day follow-up, his symptoms disappeared, and colonoscopy revealed mucosal inflammation in some areas; however, inflammation was significantly alleviated in most colonoscopic observations (Figure 1d).

Since the human mpox outbreak in May 2022, majority of the cases have been among men who have sex with men, and several patients have presented atypical symptoms with manifestations in the anogenital area.^{3, 4} A study revealed that approximately 30% of individuals may have concomitant proctitis;⁵ however, the clinical course and endoscopic findings of human mpox proctitis are unknown. Our endoscopic findings imply that rectal inflammation improved along with the remission of rectal discomfort.