Carcinogenic parasite

S. haematobium is a digenetic trematode found in Africa and the Middle East. It is the major agent of schistosomiasis, the most prevalent parasitic infection in humans.[10] It is the only blood fluke that infects the urinary tract, causing urinary schistosomiasis, and is the leading cause of bladder cancer (only next to tobacco smoking).[11][12] Its life cycle is transmission between humans and freshwater snail, species of Bulinus. The larvae live in water bodies from where they infect the hosts by penetrating the skin. Adults are found in the venous plexuses around the urinary bladder and the released eggs travels to the wall of the urine bladder causing haematuria and fibrosis of the bladder. The bladder becomes calcified, and there is increased pressure on ureters and kidneys (hydronephrosis). Inflammation of the genitals due to S. haematobium may contribute to the propagation of HIV.[13] Antigens produced by the eggs induce granuloma formation. Granulomata in turn coalesce to form tubercles, nodules or masses that often ulcerate. This creates the pathological lesions found in the bladder wall, ureter and renal; and also tumour, both benign and malignant.[14][15]

O. viverrini is a food-borne liver fluke that mainly attacks the area of the bile duct. Infection with the parasite, called opisthorchiasis is the major cause of cholangiocarcinoma, a cancer of the bile ducts, in northern Thailand, the Lao People's Democratic Republic, Vietnam and Cambodia.[16] O. viverrini has three successive host for its life cycle – the first intermediate hosts are freshwater snails of the genus Bithynia, the second intermediate hosts are different cyprinid fish, and humans are the definitive hosts.[17] Generally opisthorchiasis due to O. viverrini is harmless without any clinical symptoms, but in rare cases, cholangitis, cholecystitis, and cholangiocarcinoma can develop. O. viverrini invades the bile ducts and, rarely, the gall bladder and pancreatic duct. Heavy infection can produce problems such as fibrosis in the liver, gall bladder and bile ducts.[18] Pathological effects on the bile ducts including inflammation, epithelial desquamation, goblet cell metaplasia, epithelial and adenomatous hyperplasia and periductal fibrosis collectively promote cholangiocarcinoma.[19] Though it is not immediately life-threatening, cancer develops after 30–40 years, and the ensuing death is rapid—within 3–6 months of diagnosis.[20]

C. sinensis is also a food-borne liver fluke. It is the most prevalent human trematode in Asia, and is found in Korea, China, Vietnam and also Russia. 85% of the cases are found in China.[21] It is transmitted similarly to O. viverrini, but the species of snails are varied, of which Parafossarulus manchouricus is the most common. The cyprinid fish hosts are also different.[22] General clonorchiasis is indicated with fatigue, abdominal discomfort, anorexia, weight loss, diarrhea, liver cirrhosis and jaundice. The most severe infections cause cholangiocarcinoma and hepatic carcinoma.[23]