Opisthorchis viverrini

Species of fluke
Speciesbox
| image = Opisthorchis viverrini 3. png
| image upright = 0.7
| image alt = An adult "Opisthorchis viverrini" showing (from top) oral sucker, pharynx, caecum, ventral sucker, vitellaria, uterus, ovary, Mehlis gland, testes, excretory bladder. | image caption = An adult Opisthorchis viverrini showing (from top) oral sucker, pharynx, caecum, ventral sucker, vitellaria, uterus, ovary, Mehlis gland, testes, excretory bladder. | genus = Opisthorchis
| species = viverrini
| authority = (Poirier, 1886) Stiles & Hassal, 1896
| synonyms = *Distoma viverrini Poirier, 1886
| synonyms ref =

A small, leaf like fluke, O. viverrini completes its lifecycle in three different animals. Snails of the species Bithynia are the first intermediate hosts, fish belonging to the family Cyprinidae are the second intermediate host, and the definitive hosts are humans and other mammals such as dogs, cats, rats, and pigs. It was first discovered in the Indian fishing cat (Prionailurus viverrus) by M. J. Poirier in 1886. The first human case was discovered by Robert Thomson Leiper in 1915. O. viverrini (together with Clonorchis sinensis and Opisthorchis felineus) is one of the three most medically important species in the family Opisthorchiidae. In fact O. viverrini and C. sinensis are capable of causing cancer in humans, and are classified by the International Agency for Research on Cancer as a group 1 biological carcinogen in 2009. O. viverrini is found in Thailand, the Laos, Vietnam, and Cambodia.

O. viverrini was first described by a French parasitologist Jules Poirier in 1886, who discovered the parasite in an Indian fishing cat (Prionailurus viverrus), originally from Southeast Asia, that died in the Zoological Gardens attached to the National Museum of Natural History in Paris. He named it Distomum viverrini. American parasitologists Charles Wardell Stiles and Albert Hassall redescribed it and assigned it to the existing genus Opisthorchis (created by a French zoologist Raphaël Blanchard) in 1891. The first human specimen was described by a British parasitologist Robert Thomson Leiper in 1915, but without knowing the exact parasite. (He simply reported it as "Notes of the occurrence of parasites presumably rare in man.") Leiper received the specimens from an Irish medical doctor, Arthur Francis George Kerr, who had collected them from the post mortem examination of two prisoners at a jail in Chiang Mai, northern Thailand. In the next year, Kerr himself reported from investigation of 230 male prisoners that 39 (17 percent) of them had the infection. Kerr initially misidentified the parasite as O. felineus, an already known human parasite, because of their close resemblance. C. Prommas also reported O. felineus in 1927 from an autopsy of a 17 year old Thai male residing in Roi Et, northeast Thailand. It was in 1955 when Elvio H. Sadun from the U. S. Public Health Service analysed the cases of opisthorchiasis in Thailand and concluded that all the infections were due to O. viverrini. A systematic comparison in 1965 confirmed the differences from O. felineus. It is connected to the seminal vesicle, which is a coiled tube running up to the ejaculatory duct, which in turn opens through a small opening called genital pore just in front of the ventral sucker. Two ovaries are situated in front of the testes, and they form several lobes. The uterus runs along the ejaculatory duct and opens at the genital pore. A sac like S shaped tube called excretory bladder is between the two testes. The remaining body spaces are mostly occupied by a highly branched glandular organ called vitellaria (often called vitelline glands). Unlike the anterior end, the posterior end is rounded. The eggs of O. viverrini are 30 × 12 μm in size and they are slightly narrower and more regularly ovoid than in C. sinensis. Snails are infected by the free swimming larvae called miracidia in water bodies where faecal matters of infected mammals are deposited. Inside the snail tissue, the miracidia grow into sposocysts, that contain spore like daughter cells. The daughter cells called rediae multiply and develop into numerous larvae called cercariae. Each cercaria has a large head and a long tail. The cercariae escape from the snail and enter the water body again as free swimming larvae. Their tails act as a propeller for swimming and they actively search for a fish host. In 1965, 9 fish hosts of O. viverrini were known. Up to 2002, 15 species of fishes from seven genera of the family Cyprinidae were known to serve as second intermediate host.

[[File:Koi pla. jpg|thumb|The finished dish of koi pla made of raw fish accompanied by rice and vegetables. This dish is a dietary staple of many northeastern Thai villagers and is a common source of infection with O. viverrini. Fish contain more metacercaria from September to February, before the dry season, However, there is no record of opisthorchiasis due to O. viverrini in southern Thailand. A national survey in Lao PDR (under the project of Korea Laos Collaborative Project for Control of Foodborne Trematode Infections in Lao PDR) between 2007 and 2011 indicates that it is the most prevalent helminth infection, amounting to 55.6% of the infection. It is not highly prevalent in Vietnam, but accurate survey is difficult because it is often co infected with other flukes such as Haplorchis pumilio, H. taichui, and C. sinensis. It is most abundant in the northern provinces. It is least prevalent in Cambodia. A national survey between 2006 and 2011 showed that it is the second most prevalent helminth accounting for 5.7% of the total infection, after hookworm with 9.6% of the infection.

Generally, opisthorchiasis due to O. viverrini is harmless without any clinical symptoms. Mild symptoms may appear such as dyspepsia, abdominal pain, constipation, or diarrhoea. However, under severe infection, enlargement of liver (hepatomegaly) and malnutrition are observed. In rare cases, cholangitis, cholecystitis, and cholangiocarcinoma can also develop. In humans, O. viverrini inhabits mainly the bile ducts, and rarely, the gall bladder and pancreatic duct. Heavy infection can produce problems in the liver, gall bladder, and bile ducts. The bile ducts of heavily infected patients are usually dilated and indicate fibrosis. Pathological effects on the bile ducts include inflammation, epithelial desquamation, goblet cell metaplasia, epithelial and adenomatous hyperplasia, and periductal fibrosis. The collective effects in addition to specific parasite secretion and the host's immune reactions account for the development of cholangiocarcinoma. The infection is not immediately life threatening; cancer develops after 30–40 years, but death occurs very fast, within 3–6 months of diagnosis. There is no approved drug for the infection; however, Swiss researchers have tested tribendimidine and achieved a 70% cure rate. Surgery and supportive treatment are complicated and generally unavailable in the endemic areas. A general trematocide praziquantel is used for the infection, but is not technically recommended. In addition to praziquantel other commonly used anthelmintics such as albendazole, artesunate, and miltefosine are found to be effective on the cercariae but not on the metacercariae. Its ability to cause cancer is worsened by the discovery that its infection is often associated with those of Helicobacter species (including H. pylori, which is primarily associated with ulcers, but also may cause stomach cancers).

O. viverrini has 12 (six pairs) of chromosomes, i. e. 2n = 12.