Opisthorchis viverrini

Opisthorchis viverrini, common name Southeast Asian liver fluke, is a trematode parasite from the family Opisthorchiidae that attacks the area of the bile duct. Infection is acquired when people ingest raw or undercooked fish. It causes the disease opisthorchiasis (also called clonorchiasis). Opisthorchis viverrini infection also predisposes the infected for cholangiocarcinoma, a cancer of the gall bladder and/or its ducts.

Opisthorchis viverrini (together with Clonorchis sinensis and Opisthorchis felineus) is one of the three most medically important species in the family Opisthorchiidae.

Opisthorchis viverrini is endemic throughout Thailand, the Lao People's Democratic Republic, Vietnam and Cambodia. In Northern Thailand, it is widely distributed, with high prevalence in humans, while in Central Thailand there is low rate of prevalence. The disease opisthorchiasis (caused by Opisthorchis viverrini) does not occur in southern Thailand.



Description
The testes of an adult Opisthorchis viverrini are lobed in comparison of dendritic testes of Clonorchis sinensis.

The eggs of Opisthorchis viverrini are 30 × 12 μm in size and they are slightly narrower and more regularly ovoid than in Clonorchis sinensis. But eggs of Opisthorchis viverrini are visually indisgushiable in Kato technique smears from other eggs of flukes from other fluke family Heterophyidae.

The metacercariae of Opisthorchis viverrini are brownish, eliptical with two nearly equal-sized suckers: the oral sucker and the ventral sucker. They are 0.19-0.25 × 0.15-0.22 mm in size.

Life cycle


Opisthorchis viverrini is a hermaphroditic liver fluke. Its life cycle is similar to the life cycle of Clonorchis sinensis. It involves a freshwater snail, in which asexual reproduction takes place, and freshwater cyprinid fishes (family Cyprinidae) as intermediate hosts. Fish–eating (= piscivorous) mammals, including humans, dogs and cats, act as definitive hosts, in which sexual reproduction occurs. As a result of poor sanitation practices and inadequate sewerage infrastructure, Opisthorchis viverrini-infected people pass the trematode's eggs in their feces into bodies of fresh water.

First intermediate host
The first intermediate hosts include freshwater snails of the genus Bithynia. The only known host is Bithynia siamensis (that include all its three subspecies). Aquatic snails, which represent the first intermediate hosts of Opisthorchis viverrini, ingest the eggs from which the miracidia undergo asexual reproduction before a population of the free swimming larval stage, called a cercaria, is shed from the infected snails.

Second intermediate host
The cercaria then locates a cyprinoid fish, encysts in the fins, skin and musculature of the fish, and becomes a metacercaria. Habitats of second intermediate hosts of Opisthorchis viverrini include freshwater habitats with stagnant or slow-moving waters (ponds, river, aquaculture, swamps, rice fields).

In 1965 there were known 9 fish hosts of Opisthorchis viverrini. Up to 2002 there were known 15 species of fishes from 7 genera of the family Cyprinidae, that serves as second intermediate host. Further research by Rim et al. (2008) showed additional five more host species:
 * Puntius brevis
 * Puntius gonionotus - synonym: Barbonymus gonionotus, Java Barb
 * Puntius orphoides
 * Puntius proctozysron - synonym: Puntioplites proctozystron
 * Puntius viehoeveri - synonym: Barbonymus gonionotus
 * Hampala dispar
 * Hampala macrolepidota
 * Cyclocheilichthys armatus - synonym: Cyclocheilichthys siaja
 * Cyclocheilichthys repasson
 * Labiobarbus lineatus
 * Esomus metallicus
 * Mystacoleucus marginatus
 * Puntioplites falcifer
 * Onychostoma elongatum
 * Osteochilus hasseltii
 * Hypsibarbus lagleri
 * Barbodes gonionotus - synonym: Barbonymus gonionotus

Definitive host


The metacercarial stage is infective to humans and other fish-eating mammals including dogs and cats. The natural definitive host is the Leopard Cat (Prionailurus bengalensis). The young adult worm escapes from the metacercarial cyst in the upper small intestine and then migrates through the ampulla of Vater into the biliary tree, where it develops to sexual maturity over four to six weeks, thus completing the life cycle.

Fish contain more metacercaria from September to February, before the dry season and this is the period, when humans are usually infected. Infection is acquired when people ingest raw or undercooked fish. Dishes of raw fish are common in the cuisine of Laos and the cuisine of Thailand: koi-pla, raw fish in spicy salad lab-pla, salted semi-fermented fish dishes called pla-ra (pla ra), pla som and Som fak.

The adult worms, which are hermaphrodites, can live for many years in the liver, even decades, shedding as many as 200 eggs per day which pass out via bile into the chyme and feces. The lifespan of Opisthorchis viverrini is over 10 years.

Opisthorchis viverrini secretes a granulin-like growth protein especially in its gut and integument.

Effect on human health
Medical care and loss of wages caused by Opisthorchis viverrini in Laos and in Thailand costs about $120 million annually or $120 million per year can cost Northeast Thailand only.

Infection of Opisthorchis viverrini and of other liver flukes in Asia affect the poor and poorest people. Opisthorchiasis have received less attention in comparison of other diseases and it is a neglected disease in Asia.

Genetics
Currently, a total of only ~5,000 expressed sequence tags (ESTs) are publicly available for Opisthorchis viverrini, a dataset far too small to give sufficient insights into transcriptomes for the purpose of supporting genomic and other fundamental molecular research.

Although the genome size of Opisthorchis viverrini has not yet been reported, it is known to have six pairs of chromosomes, i.e. 2n = 12.