Fig. 21.7 Clonorchis sinensis adults in the liver. The parasites are in the bile ducts where they have caused fibrosis. The parasite at the right is cut through the plane of the anterior sucker.(Courtesy of Dr. Herman Zaiman).

Fig. 21.8 Clonorchis sinensis adult in the liver. This is a higher magnification plane of one of the adults seen in Fig. 21.7. The section passes through the oral sucker and the pharynx. The two more or less elliptical rings are the digestive cecae.

Fig. 21.9 C. sinensis adult in a bile duct. The epithelium is markedly hyperplastic. (Courtesy of Dr. Herman Zaiman).

Fig. 21.10 C. sinensis adult liver fluke (bottom center) crawling from a bile duct of a transected cirrhotic liver.(Courtesy of Dr. Herman Zaiman).

After being acted upon by peptic acid within the stomach, the metacercariae excyst in the duodenum under the effects of trypsin. They then pass through the Vaterian segment into the common bile duct, where the metacercariae use their suckers to move against the flow of bile and migrate to the smaller biliary radicles, especially in the left lobe of the liver, where they mature to adult flukes. The entire life cycle requires about 3 months. The flukes may begin laying eggs 3-4 weeks after infection, but they continue to grow for several months from 1 mm to 10-20 mm in length. The flukes use their suckers to attach themselves to the mucosa of the bile ducts or they imbed themselves in sticky mucus without producing ulceration of the duct walls. They may at times be dislodged and swept down with the flow of bile to the common bile duct, where they are often found at autopsy, during surgical exploration, or during cholangiography. Secondary bacterial infection of the ducts probably kills or dislodges many flukes.

Human infection occurs only when man ingests infected raw or inadequately cooked (or even dried, salted or pickled) fresh-water fish containing encysted metacercariae. In Hong Kong, South China, and many parts of East Asia, various species of carp are considered delicacies when eaten raw in thin slices (as sashimi or sushi) or, after dipping in hot rice soup ("yue-shan chuk"), as a raw fish congee. Only adequate cooking will render these fish safe for human consumption.

The lesions produced by C. sinensis in the hepatobiliary system are not as severe as either the frequency of infection or worm burden would suggest. Presently in Japan, autopsies reveal mild to moderate infections, but in Hong Kong most infections (77% in one series) are considered heavy.

Clonorchiasis is easily diagnosed on gross examination of the liver because of the size of the flukes and the thickening of the intrahepatic bile ducts in which they reside. The adult flukes tend to localize in the distal bile passages, especially in the left lobe of the liver. In severe infections, the entire liver is usually involved, and all of the large and medium bile ducts are prominent on the cut surface, appearing as expanded tubes dilated two to three times their usual diameter. The duct walls are several times thicker than normal, yet there is little fibrosis in the portal region and the liver is not appreciably enlarged. Jaundice is not present except in severe or complicated cases, and the liver parenchyma is usually unaffected.

In Hou's classic description of clonorchiasis there is an early stage of proliferation and sometimes desquamation of the biliary epithelium. This is followed by a stage of progressive dilatation of the ducts, thickening and fibrosis of their walls, and crypt and cavity formation, in which many flukes may be found (Figs. 21.7, 21.8, 21.9, 21.10). The vessels around the ducts are altered, but unlike schistosomiasis, there is little fibrosis in the portal tracts so that portal venous hypertension does not occur and cirrhosis is unusual. Some investigators believe that toxic secretions of the parasite may cause some of the pathological changes seen in ducts too small for the worm to penetrate.