page_title: Opisthorchis viverrini
section_title: First intermediate host
hierarchical_section_title: Opisthorchis viverrini / Lifecycle / First intermediate host
caption_attribution_description: English: Fluke-infected fish are plentiful in the local rivers such as the Chi River in Khon Kaen province, Thailand.
context_page_description: Opisthorchis viverrini, common name Southeast Asian liver fluke, is a food-borne trematode parasite from the family Opisthorchiidae that infects the bile duct. People are infected after eating raw or undercooked fish. Infection with the parasite is called opisthorchiasis. O. viverrini infection also increases the risk of cholangiocarcinoma, a cancer of the bile ducts.
  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 by M.J. Poirier in 1886. The first human case was discovered by Robert Thomson Leiper in 1915.
  O. viverrini 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.
context_section_description: The first intermediate hosts include freshwater snails of the genus Bithynia. The only known host is Bithynia siamensis (that include all its three subspecies). 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.
caption: Fluke-infected fish are plentiful in  rivers such as the Chi River in Khon Kaen Province, Thailand.
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