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Blood Fluke: A Dangerous Parasite

Blood fluke or Schistosoma is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms that infect the blood vessels of humans and other animals, causing a disease called schistosomiasis. Schistosomiasis is considered by the World Health Organization as the second-most socioeconomically devastating parasitic disease (after malaria), with hundreds of millions infected worldwide.

Blood flukes have a complex life cycle that involves two hosts: a snail and a mammal. The eggs of the blood fluke are released from the infected mammal through urine or feces and hatch in fresh water, releasing miracidia. The miracidia swim and penetrate specific snail intermediate hosts, where they undergo several developmental stages and produce cercariae. The cercariae emerge from the snail and swim in the water until they find and penetrate the skin of a suitable mammalian host, such as a human. The cercariae lose their tails and become schistosomulae, which migrate through the blood vessels to the lungs, heart, and liver. There they mature into adult worms, which pair up and reside in the mesenteric or vesical veins, depending on the species. The female worm lays eggs that either pass out of the body or get trapped in the tissues, causing inflammation and damage.

There are several species of blood flukes that infect humans, each with a different geographic distribution and clinical manifestation. The main species are:

• Schistosoma haematobium: This species causes urinary schistosomiasis, which affects the bladder and ureters. It is found mainly in Africa and the Middle East. The symptoms include hematuria (blood in urine), dysuria (painful urination), bladder obstruction, kidney damage, and increased risk of bladder cancer.
• Schistosoma mansoni: This species causes intestinal schistosomiasis, which affects the large intestine and liver. It is found mainly in Africa, South America, and the Caribbean. The symptoms include abdominal pain, diarrhea, blood in stool, hepatomegaly (enlarged liver), splenomegaly (enlarged spleen), portal hypertension (high blood pressure in the portal vein), and esophageal varices (enlarged veins in the esophagus that can rupture and bleed).
• Schistosoma japonicum: This species also causes intestinal schistosomiasis, but with more severe liver involvement. It is found mainly in China, Southeast Asia, and the Philippines. The symptoms are similar to those of S. mansoni infection, but with more frequent and serious complications such as liver fibrosis (scarring), cirrhosis (liver failure), ascites (fluid accumulation in the abdomen), and hepatic encephalopathy (brain dysfunction due to liver toxins).
• Schistosoma mekongi: This species is closely related to S. japonicum and causes a similar disease. It is found mainly in Cambodia and Laos.
• Schistosoma intercalatum/guineensis: These species cause mild intestinal schistosomiasis that affects the lower bowel. They are found mainly in Central and West Africa.

The diagnosis of schistosomiasis is based on the detection of eggs in urine or stool samples, or in tissue biopsies. Serological tests can also be used to detect antibodies or antigens in blood samples. The treatment of schistosomiasis is usually done with a single oral dose of praziquantel, an anthelmintic drug that kills the adult worms. However, praziquantel does not prevent reinfection, so preventive measures such as improving sanitation, providing safe water sources, controlling snail populations, and educating people about the risk of exposure are also important.