Southeast Asian Liver Fluke, Opisthorchis viverrini

The Southeast Asian liver fluke (Opisthorchis viverrini) is a parasite that is classified within the Platyhelminthes phylum. It is native to Vietnam, Cambodia, Thailand, and the Lao People’s Democratic Republic. It occurs in higher numbers in northern areas of Thailand when compared to central areas of Thailand, and the disease the worm causes, known as opisthorchiasis, does not occur in southern areas of Thailand.

The Southeast Asian liver fluke is so small that it can only be seen under a microscope. The larvae of this species are brown in color and have two suckers that are located on the front and back ends. All members of the species are hermaphroditic, and the testes are lobe shaped, unlike the testes of the related species, Clonorchis sinensis.

The lifecycle of the Southeast Asian liver fluke is similar to that of Clonorchis sinensis, because it requires two intermediate hosts and one definitive host. The first intermediate host is always a freshwater snail from the Bithynia genus, specifically Bithynia siamensis and its three subspecies. The snail ingests the eggs, which develop from a miracidia to a larval form within the snail. These larvae, called cercaria, are released into the water where they will swim to the surface. The larvae will seek their second intermediate hosts at this stage. These hosts can be a number of cyprinoid fish types like Puntius brevis, Cyclocheilichthys repasson, and Puntioplites falcifer, among many others. The larvae will encyst in the skin, muscle tissue, and fins of the fish host and develop into metacercaria.

Once the larvae have reached the metacercarial stage, they can infect a definitive host like a dog, cat, or human when the host consumes an infected, raw fish. The larvae will detach from the metacercarial cyst within the small intestine of the host, travel through the ampulla of Vater, and rest in the biliary tree and bile duct. It will mature into adult flukes that are able to reproduce in this area, after a period of four to six weeks. Adults will lay up to two hundred eggs per day and can live in the liver for decades.

Humans and other fish eating animals are more likely to encounter this worm between the months of September and February, before the dry season occurs. In Thailand and Laos, raw fish dishes are a common food type, so humans in these areas are even more likely to become infected. Once infected, the human host can contract opisthorchiasis, also known as clonorchiasis, a disease that could lead to a gall bladder cancer called cholangiocarcinoma. This disease is so prevalent in Thailand and Laos that it costs $120 million annually in loss of wages and medical treatments. Infections of this worm and other types of fluke worms typically affect people that are destitute, who cannot afford to prevent an infection or pay for treatment.

Image Caption: Photo of an adult Opisthorchis viverrini. Credit: Droxiang/Wikipedia (CC BY-SA 3.0)