Generally, opisthorchiasis due to "O. viverrini" is harmless without any clinical symptoms. Mild symptoms may appear such as dyspepsia, abdominal pain, constipation, or diarrhoea. However, under severe infection, enlargement of liver (hepatomegaly) and malnutrition are observed. In rare cases, cholangitis, cholecystitis, and cholangiocarcinoma can also develop. In humans,"O. viverrini" inhabits mainly the bile ducts, and rarely, the gall bladder and pancreatic duct. Heavy infection can produce problems in the liver, gall bladder, and bile ducts. The bile ducts of heavily infected patients are usually dilated and indicate fibrosis. Pathological effects on the bile ducts include inflammation, epithelial desquamation, goblet-cell metaplasia, epithelial and adenomatous hyperplasia, and periductal fibrosis. The collective effects in addition to specific parasite secretion and the host's immune reactions account for the development of cholangiocarcinoma. The infection is not immediately life-threatening; cancer develops after 30–40 years, but death occurs very fast, within 3–6 months of diagnosis.