Opisthorchiasis — DTM&H MCQ

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HardHelminthic infectionsOpisthorchiasisDTM&H

A 62-year-old man from northern Thailand has recurrent cholangitis and a new hilar cholangiocarcinoma. He has eaten raw freshwater fish since childhood and stool microscopy previously showed small operculated eggs. Which parasite is the most likely underlying risk factor?

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Correct answer: E — Opisthorchis viverrini

The correct answer is E, Opisthorchis viverrini. This liver fluke is endemic in northern Thailand and the Mekong basin and is acquired by eating raw or undercooked freshwater cyprinid fish, exactly the exposure described. Chronic biliary infestation causes recurrent pyogenic cholangitis and periductal fibrosis, and sustained inflammation from long-standing infection is a recognised driver of cholangiocarcinoma, particularly of the hilar/perihilar type. Opisthorchis (and the related Clonorchis sinensis) eggs are small, ovoid and operculated on stool microscopy, matching the description given, and IARC classifies O. viverrini as a definite (Group 1) carcinogen for cholangiocarcinoma. Why the other options are wrong: C. Schistosoma haematobium: acquired by freshwater contact (cercariae penetrating skin), not by eating fish, and its eggs are terminal-spined, non-operculated; it causes urinary schistosomiasis and squamous cell bladder cancer, not biliary tract disease. B. Echinococcus granulosus: transmitted via ingestion of eggs from dog faeces/contaminated food, producing hepatic hydatid cysts, not a fish-borne fluke, and diagnosis relies on imaging and serology rather than operculated ova in stool. A. Strongyloides stercoralis: soil-transmitted via larval skin penetration, produces rhabditiform larvae (not operculated eggs) in stool, and is linked to autoinfection/hyperinfection, not cholangiocarcinoma. D. Enterobius vermicularis: the pinworm causes perianal itching from eggs deposited perianally (detected by adhesive tape test), has no association with freshwater fish ingestion or biliary malignancy. Key point: recurrent cholangitis plus cholangiocarcinoma in someone who eats raw freshwater fish from Southeast Asia and has operculated ova on stool microscopy points to Opisthorchis viverrini (or Clonorchis sinensis) liver fluke infection.

Reference: Manson's Tropical Diseases, 24th edition, Chapter on Liver, Lung and Intestinal Flukes (Opisthorchiasis and Clonorchiasis); see also World Health Organization Foodborne Trematode Infections fact sheet, https://www.who.int/news-room/fact-sheets/detail/foodborne-trematode-infections