Clonorchis sinensis (Cobbold, 1875) Looss, 1907
ETYMOLOGY:Clon = branched and orchis = testis along with sinensis = representing China.
SYNONYMS:Distoma sinens Cobbold, 1875; Distoma spathulatum Leuckart, 1876; Distoma endemicum Jima, 1886. Also, some have included the members of the genus Clonorchis within the genus Opisthorchis.
HISTORY: This trematode was originally described from the bile passages of a Chinese carpenter working in Calcutta, India. It was described in Japan in 1883 and was recognized as being endemic in south China in 1908 by Heanley. Cats were first noted to be infected in Japan (Ijima, 1887).
GEOGRAPHIC DISTRIBUTION: This fluke is found in Japan, Korea, Taiwan, China, and North Korea. The infection seems to be present in fresh-water fishes surrounding the areas bordering on the China Sea.
LOCATION IN HOST: Gall bladder and bile ducts; occasionally in the pancreatic duct or the small intestine.
PARASITE IDENTIFICATION:Clonorchis sinensis differs from specimens of Opisthorchis in that it has highly branched testes. The highly branched testes separate Clonorchis from the other genera of opisthorchid trematodes. The adults are 10 to 25 mm long and 3 to 5 mm wide. The eggs are yellowish brown with a distinct operculum. There is often a slight protuberance on the end of the egg opposite the operculum. The eggs measure 28 to 35 µm long by 12 to 19 µm wide.
LIFE CYCLE: Cats were first used to show that a fresh-water fish intermediate host was a required part of the life cycle (Kobayashi, 1915). About 80 species of fish have been identified as hosts; most of the piscine hosts are in the family Cyprinidae. A few years later, the snail hosts were identified and are now recognized as species of Parafossarulus, Bulimus, Semisulcospira, Alocinma, and Melanoides. The eggs of the Clonorchis sinensis hatch only if they are ingested by the appropriate snail host.
After ingestion of an infected fish, the young trematodes migrates to the bile duct through the ampulla of Vater. It then takes about one month for the trematodes to reach maturity. The trematodes have been found to live as long as 12 years and 3 months in cats (Miyazaki, 1991).
CLINICAL PRESENTATION AND PATHOGENESIS: Ultimately the host develops cirrhosis of the liver. The changes in the liver have been divided in humans into three basic stages. The first stage consists primarily of proliferation of the biliary-tract epithelium. In the second stage the surrounding liver tissue is compressed by the growing connective tissue around the bile ducts. In the third stage, there is significant cirrhosis and destruction of the liver parenchyma. It is believed that an infection with only a few flukes will seldom induce more than the first stages of the disease. Similar progression of disease is thought to occur in infected cats.
TREATMENT: Praziquantel is likely to prove successful in eliminating these trematodes from many treated cases.
EPIZOOTIOLOGY: Cats are a major host of this pathogen. In some areas of China and Vietnam, up to one-third of the feline population might be shedding eggs of this parasite in their feces. In areas where fish are consumed raw, this can have a major impact on the human population by the maintenance of the parasite in the surrounding fish population.
HAZARD TO OTHER ANIMALS: Dogs and other fish-eating mammals can also be infected with this parasite.
HAZARD TO HUMANS: Large numbers of human beings are infected with this parasite in the geographical regions where it is found. Surveys in Korea and Vietnam have found prevalence rates above 15% in some populations (Chung et al., 1991; Kieu, et al., 1992)
CONTROL/PREVENTION: Prevent the ingestion of infected raw, dried or pickled fish; these latter methods are not necessarily going to kill the metacercarial stage of the trematode.
Chung DI, Kim YI, Lee KR, Choi DW. 1991. Epidemiological studies of digenetic trematodes in Yongyan County, Kyungpok Province. Kisaengchunghap Chapchi 29:325-338.
Ijima I. 1887. Notes on Distoma endemiocum, Baelz. Journal of the College of Science, Imperial University, Tokyo 1:47-59 [Cited in: Grove DI. A History of Human Helminthology. CAB Inter, Wallingford, UK]
Kobayashi H. 1915. On the life history and morphology of Clonorchissinensis. Centralbl Bakt Parasitenk Infekt 75:299-318.
Kieu TL, Bronshtein AM, Sabgaida TP. 1992. Clonorchiasis in the People’s Republic of Vietnam. 2. The clinico-parasitological examination of a focus and a trial of praziquantel treatment. Med Parazitol Mosk 4:7-11.
Miyazaki I. 1991. Helminthic Zoonoses. 494 pages. International Medical Foundation of Japan. Fukuoka, Japan.
Figure 2-38. Clonorchis sinensis from the gall bladder of a cat in China. Note the highly branches testes in the posterior of the body and the restriction of the uterus to between the ventral sucker and the ovary. the vitellaria are not very distinct in this specimen.