Opisthorchiasis viverrini is a common infection of the civet cat and other fish-eating mammals, such as domestic cats and dogs (reservoir final hosts), in Cambodia, the Lao People’s Democratic Republic, Thailand and Viet Nam. The transmission cycle is similar to that of clonorchiasis and opisthorchiasis felinea: the adult flukes live in the bile ducts and lay eggs that are dispersed into the environment through defecation. When they reach freshwater, eggs develop into miracidia that are ingested by various species of aquatic snails, where they further develop and reproduce asexually, giving rise to numerous cercariae (larvae). Cercariae leave the snail and pass into water, where they swim to penetrate beneath the scales of various species of carp-like freshwater fish where, as metacercariae, they encyst the subcutaneous tissue. When carnivores eat raw fish, the metacercarial cysts hatch in the intestine and the young worms migrate to the bile ducts, thus completing the cycle.
Humans may substitute reservoir hosts in the transmission cycle when they eat raw, salted, pickled, smoked, marinated, dried, partially cooked or poorly processed fish, thus ingesting the metacercariae. In humans, acute opisthorchiasis viverrini may be asymptomatic or scarcely symptomatic in light infections, but if the number of worms is significant (up to several thousands), fever and right upper-quadrant pain are not infrequent and may be associated with intermittent colic pain caused by obstruction of the gallbladder by the worms themselves. Chronic opisthorchiasis viverrini, which results from protracted episodes of re-infection over time, may be most severe. The presence of the worms in the smaller bile ducts causes chronic inflammation of their walls, resulting in fibrosis of the ducts and destruction of the adjacent liver parenchyma. Recurrent pyogenic cholangitis may also be frequent. Chronic opisthorchiasis viverrini is strongly associated with cholangiocarcinoma, a severe and often fatal form of bile duct cancer. The International Agency for Research on Cancer (IARC) classifies O. viverrini as a Group 1 agent (carcinogenic to humans).
Diagnosis, treatment and control
Diagnosis of opisthorchiasis viverrini is suspected on the basis of the clinical picture, on the anamnestic recall of consuming raw fish, on the detection of eosinophilia, and on typical findings of ultrasound (US), computed tomography (CT) or magnetic resonance imaging (MRI) scans. Confirmation of diagnosis relies on different types of diagnostic techniques:
- parasitological techniques to detect Opisthorchis eggs in stool samples; their cost and sensitivity may vary according to the type of technique used; they can only be employed in the chronic phase; some techniques, such as the Kato-Katz thick smear, quantify the intensity of infection (allowing estimation of the severity of the infection);
- immunological techniques to detect worm-specific antibodies in serum samples or worm-specific antigens in serum or stool samples; these techniques are usually more sensitive than the commonly used parasitological techniques; detection of antibodies does not distinguish between current, recent and past infections; their ability to quantify intensity of infection is disputed; stool tests are easier to perform and reportedly better accepted by individuals in endemic areas; these techniques are still at an experimental stage;
- molecular techniques such as the polymerase chain reaction are also at an experimental stage.
Praziquantel is the only medicine recommended by WHO for treatment of opisthorchiasis viverrini. It should be administered at the dose of 25 mg/kg 3 times daily for 2–3 consecutive days or of 40 mg/kg, single administration.
For the purposes of public health control, WHO recommends carrying out community diagnosis at the district level, and implementing preventive chemotherapy with praziquantel at a dosage of 40 mg/kg, single administration, as indicated by the attached table:
Complementary interventions such as information, education and communication on safe food practices, improved sanitation and veterinary public health measures should also be implemented in order to decrease transmission rates.