Amoebic colitis insufficient to metronidazole monotherapy

Amoebic colitis is a parasitic gastrointestinal disease caused by Entamoeba histolytica (E. histolytica). In Japan, metronidazole (MNZ) monotherapy is often used and most cases are effective. However, we report a case of MNZ-insufficient amoebic colitis caused by residual cysts. A 73-year-old man had been staying in Southeast Asia for over a decade. He had undergone a screening colonoscopy and ulcerative lesions were observed in the cecum, and a biopsy confirmed amoeba parasites. The patient was treated with MNZ monotherapy. However, he forgot to take the medicine for several days, and the ulcerative lesions persisted. The patient was referred to our facility, and we performed a colonoscopy and confirmed trophozoites. Since we considered that previous treatment failure was due to the low oral dosage, we re-prescribed MNZ. A colonoscopy after 6 months showed that the ulcerative lesions persisted. We clinically diagnosed MNZ-insufficient amoebic colitis caused by residual cysts and prescribed MNZ and paromomycin (PRM) each for 10 days. One year later, no ulcerative lesions were observed. MNZ-insufficient amoebic colitis should be considered, when ulcerative lesions remain after MNZ administration and PRM is effective drug against cysts, and we propose a combination therapy of PRM to MNZ.