Geographic distribution. Endemic in Northeast Thailand, Laos, Kampuchea, etc.
Infection rate. Up to 7 million persons may be infected. The overall prevalence is about 35％; but in some areas, it is over 90％.
Life cycle. The adult worms live in the distal biliary duct or, sometimes, in the gallbladder. Eggs are released into the bile and passed to feces. Within the snails (first intermediate host), the miracidia hatch and develop sporocysts, rediae, and cercariae. Mature cercariae penetrate the muscle of susceptible freshwater fishes (secondary intermediate host) to develop into metacercariae. Consumption of infected fish is the source of infection for the definitive hosts: humans or fish-eating mammals.
Morphology. Adult O. viverrini is 7-12 mm long and 1.5-3 mm wide, transparent, and elongate. The ratio of length to width is approximately 2:1. A pair of testes is located posteriorly. The uterus is a coiled tubule and transverse follicles lie laterally. The eggs are yellowish-brown, oval, and have a tubercle-like knob at the opercular end. The eggs average 28 ㎛ by 16 ㎛ in size and contain a miracidium when laid.
Pathology and clinical symptoms. The pathologic changes induced by the worms are apparently the result of mechanical irritation caused by the worm suckers, toxic metabolic substances, immunologic response of the hosts, and secondary bacterial infection. Severe opisthorchiasis has been associated with cirrhosis, obstructive jaundice, pancreatitis, cholangitis, and cholangiocarcinoma.
Diagnosis. Finding of eggs in feces. Ultrasonography and liver scanning can show lesions compatible with the infection. Immunodignosis by ELISA using crude somatic extracts of O.viverrini is available.
Prevention. Avoid eating raw or undercooked freshwater fishes.
Adult worm of Opisthorchis viverinii.
Tai Soon Yong