Clonorchis sinensis

Clonorchis sinensis, the Chinese liver fluke, is a liver fluke belonging to the class Trematoda, phylum Platyhelminthes. It infects fish-eating mammals including humans. In humans, it infects the common bile duct and gall bladder, feeding on bile. It was discovered by a British physician James McConnell in 1874 the Medical College Hospital in Calcutta ((Kolkata), India. The first description was given by Thomas Spencer Cobbold, who named it Distoma sinense. It passes its life cycle in three different hosts, namely freshwater snail as first intermediate hosts, freshwater fish as second intermediate host, and mammals as definitive hosts. Endemic to Asia up to Russia, C. sinensis is the most prevalent human fluke in Asia and third-most in the world. It is still actively transmitted in Korea, China, Vietnam, and Russia. Most infection (about 85%) is in China. The infection, called clonorchiasis, generally appears as jaundice, indigestion, biliary inflammation, bile duct obstruction, even liver cirrhosis, cholangiocarcinoma, and hepatic carcinoma. It has been established that C. sinensis is the major causative agent of bile duct cancer (cholangiocarcinoma). For this reason, the International Agency for Research on Cancer has classed it as a group 1 biological carcinogen in 2009. The symptoms of C. sinensis infection (clonorchiasis) have been known from ancient times in China. The earliest record is from corpses buried in 278 BCE at Jiangling County of Hubei Province and the Warring States tomb of the western Han Dynasty, but the parasite was discovered only in 1874 by James McConnell, a professor of pathology and resident physician at the Medical College Hospital in Calcutta. He recovered the fluke from a 20-year-old Chinese carpenter who died on 8 September 1874. On autopsy, he observed that the corpse had a swollen liver (hepatomegaly) and distended bile ducts, which he noted were blocked by 'small, dark, vermicular-looking bodies.' He recovered the vermicules (worms) and compared them with known flukes Fasciola hepatica and Distoma lanceolatum. He concluded that the new fluke was significantly different. He published his observations in the 21 August 1875 issue of The Lancet. The formal scientific description was published in 1875 by Thomas Spencer Cobbold, who named it Distoma sinense. In 1876, Rudolf Leuckart, named it Distomum spithulatum. Kenso Ishisaka recorded the first case of clonorchiasis in Japan in 1877. McConnell identified another infection in a Chinese cook from Hong Kong in 1878. Erwin von Baelz reported the presence of similar flukes from an autopsy of a Japanese patient at Tokyo University in 1883. He recorded two different forms, naming the smaller, more pathogenic form as Distoma hepatis endemicum sive perniciosum, and the larger, less pathogenic form as D.h.e.s. innocuum. Isao Ijima correctly redescribed them as the same species, but still wrongly renamed it Distoma endemicum in 1886. When a new genus Opisthorchis was created by Émile Blanchard in 1895, Cobbold's species name D. sinense was moved to the new genus because of close similarities with the other members. Further analyses by Arthur Looss, though, showed significant differences from the general features of Opisthorchis, particularly on the highly branched testes. He created a new genus Clonorchis (from Greek klon meaning 'twig/branch', orkhis meaning 'testis') in 1907. Similar to Baelz, he differentiated the larger species as Clonorchis sinensis, which is mostly found in China, and the smaller species as C. endemicum, found mostly in Japan. In 1912, Harujiro Kobayashi corrected the classification that the differences in sizes were due to the nature of the host and intensity of infection, and had nothing to do with the biology. Hence, he supported only C. sinensis. Kobayashi was also the first to discover fish as the second intermediate host in 1911. Masatomo Muto discovered snails as the first intermediate host in 1918. An adult C. sinensis is a flattened (dorsoventrally) and leaf-shaped fluke. The body is slightly elongated and slender, measuring 15–20 mm in length and 3–4 mm in width. It narrows at the anterior region into a small opening called the oral sucker, which act as the mouth. From the mouth, two tubes called caeca run the length of body. They are the digestive and excretory tracts. The posterior end is broad and blunt. A poorly developed ventral sucker lies behind the oral sucker, at about one-fourth of the body length from the anterior end. A common genital pore opens just in front of it. As a hermaphrodite, it has both male and female reproductive organs. A single rounded ovary is at the centre of the body, and two testes are towards the posterior end. The uterus from the ovary, and seminal ducts from the testes, meet and open at the genital pore. The testes are highly branched. Other highly branched organs called vitellaria (or vitelline glands) are distributed on either side of the body. The eggs are similar to those of other related flukes such as Opisthorchis viverrini and O. felineus, and are often confused during diagnosis. They small and oval in shape, measuring about 30 x 15 μm in diameter. They are sharply curved and with a clear convex operculum towards the narrower end. At the broader end is a stem-shaped knob. The miracidium can be seen inside the fertilised egg. The eggs of a C. sinensis are released through the biliary tract, and excreted out along with the faeces. The eggs are embryonated and contain the larvae called miracidia. Unlike most other flukes in which the miracidia undergo development and swim in water to infect suitable host, the eggs of C. sinensis are simply deposited in water. The eggs are then eaten by snails.