Classification

Phylum Platyhelminthes

Class Trematoda

Subclass Digenea

Order Plagiorchiida

Family Opisthorchiidae

Genus Opisthorchis

Introduction

Opisthorchis spp. are liver fluke parasites (trematodes) that humans can get by eating raw or undercooked fish from areas in Asia and Europe where the parasite is found, including Thailand, Laos, Cambodia, Vietnam, Germany, Italy, Belarus, Russia, Kazakhstan, and Ukraine. Opisthorchis viverrini is known as the Southeast Asian liver fluke and O. felineus as the cat liver fluke.

Original Description and Taxonomy

Class Trematoda Rudolphi, 1808

Subclass Digenea Caru, 1863

Order Plagiorchiida La Rue, 1957

Suborder Opistorchiata La Rue, 1957

Superfamily Opistorchioidea Looss, 1899

Family Opisthorchiidae Looss 1899

Subfamily Opisthorchiinae Looss, 1899

Genus Opisthorchis

The original description of the genus may be found in Blanchard (1895). See King and Scholz (2001) for a detailed discussion of the classification and taxonomy of the family Opisthorchiidae, and Scholz (2008) for a presentation of the classification of subfamilies, including Opisthorchiinae Looss, 1899 (which includes the genus Opisthorchis; Gibson et al. 2021), as well as the others in the family: Allogomtiotrematinae, Delphinicolinae, Diasiellinae, Metorchiinae, Microtrematinae, Oesophagicolinae, Pachytrematinae, Plotnikoviinae, Pseudamphistominae, Ratziinae, Tubangorchiinae, and Witenbergiinae. A phylogenetic tree for some of these groups may be found in Waikagul and Thaenkham (2014).

Figure 1. The adult flukes deposit fully developed eggs that are passed in the feces (1). After ingestion by a suitable snail (first intermediate host) (2), the eggs release miracidia (2a), which undergo in the snail several developmental stages (sporocysts (2b), rediae (2c), cercariae (2d)). Cercariae are released from the snail (3) and penetrate freshwater fish (second intermediate host), encysting as metacercariae in the muscles or under the scales (4). The mammalian definitive host (cats, dogs, and various fish-eating mammals including humans) become infected by ingesting undercooked fish containing metacercariae. After ingestion, the metacercariae excyst in the duodenum (5) and ascend through the ampulla of Vater into the biliary ducts, where they attach and develop into adults, which lay eggs after 3 to 4 weeks (6). The adult flukes (Opisthorchis viverrini: 5 mm to 10 mm by 1 mm to 2 mm; O. felineus: 7 mm to 12 mm by 2 mm to 3 mm) reside in the biliary and pancreatic ducts of the mammalian host, where they attach to the mucosa.
(Source: United States Centers for Disease Control and Prevention, Division of Parasitic Diseases and Malaria (DPDx), 2018. Public domain.)

Medical Importance

Members of the family Opisthorchiidae are known parasites of mammals, birds, fish, and reptiles. Liver flukes of the genus Opisthorchis may infect the liver, gallbladder, and bile duct in humans. While most infected persons do not show any symptoms, infections that last a long time can result in severe symptoms and serious illness, including cancers.

Untreated, infections may persist in humans for up to 25–30 years, the lifespan of the parasite. Typical symptoms include indigestion, abdominal pain, diarrhea, or constipation. In severe cases, abdominal pain, nausea, and diarrhea can occur. Opisthorchis felineus, in addition to presenting with the typical symptoms also seen in O. viverrini infections, can present with fever, facial swelling, swollen lymph glands, sore joints, and rash—similar to the signs and symptoms of schistosomiasis. Chronic O. felineus infections may also involve the pancreatic ducts.

Diagnosis of Opisthorchis infection is based on microscopic identification of parasite eggs in stool specimens. Safe and effective medication is available to treat Opisthorchis infections. Adequately freezing or cooking fish will kill the parasite.

Life Cycle

The eggs of Opisthorchis viverrini are ingested by snails in fresh water. After the eggs hatch, infected snails will re-lease microscopic larvae that can enter freshwater fish. Peo-ple become infected when eating raw or undercooked fish that contains the parasite. After ingestion, the liver flukes grow to adult worms that live inside the human bile duct system. The life cycle takes 3 months to complete in humans. Infected people will then pass eggs in their stool or may cough them up (see Figure 1 for a life cycle diagram).

Acknowledgement

This section includes a very brief introduction to the genus Opisthorchis adapted from material in the public domain on the United States Centers for Disease Control and Prevention website as well as the other cited sources.

Use of material from United States federal agencies does not constitute its endorsement or recommendation by the US Government, Department of Health and Human Services, or Centers for Disease Control and Prevention (CDC). The material from the CDC is otherwise available on the agency website for no charge.

Literature Cited

Blanchard, R. A. E. 1895. Animaux parasites. Bulletin de la Societe zoologique de France 20: 217.

DPDx (United States Centers for Disease Control and Prevention, Division of Parasitic Diseases and Malaria). 2018. Opisthorchis.

Gibson, D. I., O. Martínez, and R. A. Bray. 2021. Opisthorchis Blanchard, 1895. WoRMS 108622.

King, S., and T. Scholz. 2001. Trematodes of the family Opisthorchiidae: A minireview. Korean Journal of Parasitology 39: 209–221.

La Rue, G. R. 1957. The classification of digenetic Trematoda: A review and a new system. Experimental Parasitology 6: 306–349.

Waikagul, J., and U. Thaenkham. 2014. Molecular systematics of fish-borne trematodes. In J. Waikagul and U. Thaenkham, eds. Approaches to Research on the Systematics of Fish-borne Trematodes. Academic Press, New York, New York, United States, 130 p.

Supplemental Reading

Chai, J.-Y., K. D. Murrell, and A. J. Lymbery. 2005. Fish-borne parasitic zoonoses: Status and issues. International Journal for Parasitology 35: 1,233–1,254. doi: 10.1016/j.ijpara.2005.07.013

Dao, T. T. H., T. T. G. Nguyen, S. Gabriël, K. L. Bui, et al. 2017. Updated molecular phylogenetic data for Opisthorchis spp. (Trematoda: Opisthorchioidea) from ducks in Vietnam. Parasites and Vectors 10: 575. doi: 10.1186/s13071-017-2514-9

Keiser, J., and J. Utzinger. 2005. Emerging foodborne trematodiasis. Emerging Infectious Diseases 11: 1,507–1,514. doi: 10.3201/eid1110.050614

Lim, J. H., S. Y. Kim, and C. M. Park. 2007. Parasitic diseases of the biliary tract. American Journal of Roentgenology 188: 1,596–1,603. doi: 10.2214/AJR.06.1172

Marcos, L. A., A. Terashima, and E. Gotuzzo. 2008. Update on hepatobiliary flukes: Fascioliasis, opisthorchiasis, and clonorchiasis. Current Opinion in Infectious Disease 21: 523–530. doi: 10.1097/QCO.0b013e32830f9818

Petney, T. N., R. H. Andrews, W. Saijuntha, A. Wenz-Mūcke, et al. 2013. The zoonotic, fish-borne liver flukes Clonorchis sinensis, Opisthorchis felineus, and Opisthorchis viverrini. International Journal for Parasitology 43: 1,031–1,046. doi: 10.1016/j.ijpara.2013.07.007

Rana, S. S., D. K. Bhasin, M. Nanda, and K. Singh. 2007. Parasitic infections of the biliary tract. Current Gastroenterology Reports 9: 156–164. doi: 10.1007/s11894-007-0011-6

Scholz, T. 2008. Family Opisthorchiidae Looss, 1899. In R. A. Bray, D. I. Gibson, and A. Jones, eds. Keys to the Trematoda, Volume 3. CAB International, p. 9–50.