The patient was an 87-year-old male farmer from Yongcheng, Henan. He reported having nausea and decreased appetite without obvious causes on June 25th, 2023, accompanied by diarrhea (4-5 times/d) and low-grade fever (body temperature of 37.5 ℃). He was treated at a local community hospital without detailed examinations and showed no improvement after receiving infusions of unknown drugs. Due to a referral, he was admitted to the Yongcheng Central Hospital on July 5th. The patient has been undertaking farming for a long time, and animals such as cats and dogs are bred in his home. He has no history of breeding pigs or close contact, no history of applying pig manure, and no history of drinking unboiled water. On admission, physical examination showed a thin body type, no apparent abnormalities in heart and lung auscultation, a soft and flat abdomen, no gastrointestinal waves or peristalsis, no varicose veins in the abdomen, no liver or spleen felt under the ribs, tenderness in the lower abdomen, no tenderness or rebound tenderness in other parts, negative mobile cloudy urine, active bowel sounds, and no audible gas-water sound. The blood routine test showed a white blood cell count of 11.9 × 10⁹/L, a red blood cell count of 4.11 × 10¹²/L, a platelet count of 358 × 10⁹/L, a hemoglobin of 119 g/L, a blood potassium of 3.13 mmol/L, and an albumin of 32.5 g/L in the liver function test. The stool routine test showed a dark green loose stool, a positive occult blood test, no red blood cells or white blood cells, no parasitic eggs detected, and no abnormalities in the fecal culture. A plain and enhancement scan of the entire abdomen was performed, which showed an increased volume of the gallbladder, slightly dilated bile ducts, common bile duct, and pancreatic duct in the hepatic portal area, slightly thickened gastric wall near the cardia, and diffuse thickening and edema of the intestinal walls in the rectum and sigmoid colon. The patient was treated for rehydration and correction of electrolyte imbalance. Simultaneously, oral berberine tablets were given (0.1 g each time, 3 times/d), smecta powder (3 g each time, 3 times/d), and bifidobacterium (1 g each time, 3 times/d, taken 2 h apart from smecta powder). The frequency of the patient’s bowel movements gradually decreased and gradually turned to soft and yellow stools. On July 11st and 12th, the stool samples were microscopically observed and both tested positive for suspected Balantidium coli. After consultation with the Yongcheng Center for Disease Control and Prevention, it was confirmed to be B. coli. The patient was adjusted to oral berberine (0.1 g each time, 3 times/d) and metronidazole tablets (0.2 g each time, 2 times/d), and continued to receive nutritional support treatment. On July 13th, the patient’s bowel movements had converted to daily, yellow and soft stools, and no parasites were found under microscopy. On July 18th, the patient was discharged from the hospital after comprehensive treatment, and follow-up observations showed a good prognosis.