Guest guest Posted June 16, 2006 Report Share Posted June 16, 2006 http://www.merck.com/mrkshared/mmanual/section13/chapter161/161g.jsp Merck Manual Cestodes (Tapeworms) Adult cestodes are typically long, flat, segmented worms that lack a digestive tract and absorb nutrients directly from the host's small intestine. An adult tapeworm has three recognizable portions. The scolex (head) functions as a holdfast organ. The neck is an unsegmented region of high regenerative capacity. If treatment fails to eliminate the neck and scolex, the entire worm may regenerate. The rest of the worm consists of numerous proglottides (segments). Segments closest to the neck are undifferentiated. As proglottides move caudally, the sex organs are formed (tapeworms are hermaphroditic); distal segments are gravid and contain eggs in the uterus. FISH TAPEWORM INFECTION (Diphyllobothriasis) Infection of the intestinal tract with Diphyllobothrium latum, which is often asymptomatic but may cause vitamin B12 deficiency and megaloblastic anemia. Etiology and Pathogenesis The infection occurs worldwide, but especially in cool-lake regions of all continents, where sewage contaminates freshwater fish. Infections in the USA occur in people who eat raw fish. The adult worm inhabits the human intestinal tract. Undeveloped eggs are released from the proglottides in the intestinal lumen and are passed in the stool. A free-swimming ciliated larva (coracidium) hatches from the egg in fresh water, is ingested by microcrustaceans, and develops into a procercoid larva. Infective larvae develop in the flesh of freshwater fish that eat infected microcrustaceans. The worms mature in the human small intestine, begin to lay eggs in about 1 mo (about 1 million per worm/day), and may live months or years. Symptoms, Signs, and Diagnosis Infection is usually asymptomatic, but mild GI symptoms may be noted. Fish tapeworms take up dietary vitamin B12 from the upper small intestine, which causes pernicious anemia in about 1% of infected persons, especially in Scandinavians. Characteristic operculated eggs are easily found in the stool. Prevention and Treatment All freshwater fish should be thoroughly cooked or frozen at -10° C (14° F) for 48 hours to prevent infection. Treatment is with a single oral dose of praziquantel, 5 to 10 mg/kg. Vitamin B12 may be needed to correct the anemia. Certain antiparasitic drugs may have limited availability; manufacturers can be contacted to locate suppliers. BEEF TAPEWORM INFECTION (Taeniasis Saginata) Infection of the intestinal tract with the cestode Taenia saginata, which is usually asymptomatic. Etiology and Pathogenesis The ribbonlike adult worm inhabits the human intestinal tract. Egg-containing proglottides are passed in the stool and ingested by cattle. The eggs hatch in the cattle, liberating embryonic oncospheres that invade the intestinal wall and are carried by the bloodstream to striated muscle, where they develop in 2 mo into a cysticercus, ie, a small cyst that contains a single inverted scolex. Humans are infected by eating the cysts in raw or undercooked beef. The cysticerci attach to intestinal mucosa and mature in about 2 mo. Adult worms (usually only 1 to 2 are present) may live several years. The infection occurs worldwide but especially in cattle-raising regions of the tropics and subtropics in Africa, the Middle East, Eastern Europe, Mexico, and South America. Infection is uncommon in U.S. cattle and is monitored by federal inspection. Symptoms, Signs, and Diagnosis The infection is usually asymptomatic, although epigastric pain, diarrhea, and weight loss may occur. Passage of a motile segment often brings an otherwise asymptomatic patient to medical attention. The diagnosis is usually made by finding typical gravid proglottides or, more rarely, the scolex in stool. Occasionally, eggs from ruptured segments appear in feces. Eggs are indistinguishable from eggs of other Taenia sp. The perianal area may also be examined by pressing the sticky side of cellophane tape against the area, placing the tape on a glass slide, and microscopically examining it for eggs. Eggs may also be present on anal swabs. Prevention and Treatment Infection may be prevented by cooking beef to a minimum of 56° C (133° F) for 5 min. Meat inspection and adequate sanitation help to control infection. Treatment is with a single oral dose of praziquantel, 5 or 10 mg/kg. Alternatively, a single 2-g dose of niclosamide is given as 4 tablets (500 mg each) that are chewed one at a time and swallowed with a small amount of water (0.5 g is the dose for children 2 to 5 yr old, 1 g for older children). Both drugs have cure rates of about 90%. Treatment can be considered successful when no proglottides are passed again within 4 mo. PORK TAPEWORM INFECTION (Taenia Solium Infection; Cysticercosis) Infection of the intestinal tract, often asymptomatic, with the adult cestode Taenia solium; infection with larval stages may lead to neurocysticercosis with seizures. Etiology and Pathogenesis Humans become infected by eating pork containing cysticerci. Humans may also act as intermediate hosts and develop cysticercosis by accidental ingestion of T. solium eggs from human excreta; or, if an adult tapeworm is present in the intestine, regurgitation may bring gravid proglottides from the intestine to the stomach, where oncospheres may hatch and start migration to subcutaneous tissue, muscle, viscera, and CNS. Neurocysticercosis is a very common problem and a major cause of epilepsy in Latin America, South Africa, and India. Infection in the USA is rare, except in immigrants from endemic areas. Symptoms and Signs Infection with the adult worm is usually asymptomatic. Viable cysticerci cause only a mild tissue reaction, but death of the cysts elicits an intense tissue response; thus, symptoms often do not appear for 4 to 5 yr after infection. Infection in the brain may provoke severe symptoms. Symptoms result from mass effect, inflammation after degeneration of a parasite, and obstruction of foramina and ventricles. Patients may present with seizures, signs of increased intracranial pressure, hydrocephalus, focal neurologic signs, altered mental status, or aseptic meningitis. Cysticerci may also infect the spinal cord and eye. Diagnosis The diagnosis is usually made by finding typical gravid proglottides in stool. Eggs from ruptured segments may appear in feces or be found on anal swabs, but they are indistinguishable from eggs of other Taenia species. Eggs are present in <= 30% of stool samples from patients with cysticercosis. X-rays of the brain or muscle may show calcified cysts. CT or MRI is frequently diagnostic, showing many solid nodules or cysts, calcified cysts, ring-enhancing lesions, or hydrocephalus. A highly sensitive and specific immunoblot assay is available. Prevention and Treatment Thoroughly cooking pork prevents infection with adult tapeworms. Prevention of cysticercosis in endemic areas may be difficult: infection may occur despite scrupulous personal hygiene and eating habits. Cysticercosis is also spread by airborne eggs in contaminated dust. A person infected with T. solium should be treated promptly and carefully to eliminate the adult worm(s). Treatment of intestinal infection is the same as for T. saginata. The recommended drugs cause the disintegration of proglottides, thereby releasing eggs that theoretically could cause infection. The treatment of choice for cerebral cysticercosis is praziquantel (50 mg/kg/day po for 15 days). Albendazole (15 mg/kg/day po for 28 days) may be an equally effective and less expensive drug but is not yet available in the USA. Not all patients respond to praziquantel, and not all patients must be treated (cysts may already be dead, or the inflammatory response to treatment may be worse than the disease). Corticosteroids (such as dexamethasone 4 to 16 mg/day) and anticonvulsants may be required to reduce the symptoms due to degenerating cysts. Surgery may be necessary for obstructive hydrocephalus, infection of the 4th ventricle, and spinal and ocular cysticercosis. HYDATID DISEASE (Echinococcus granulosus Infection; Echinococcosis) Infection with larvae of Echinococcus granulosus, which may cause cysts in the liver and other organs. Etiology and Pathogenesis Eggs from the feces of dogs, wolves, and other canines are ingested by herbivore animals (such as sheep or moose) or humans. The oncospheres penetrate the intestinal wall, migrate via the circulation, and lodge in the liver, lungs, and, less frequently, in the brain, bone, and other organs. The larva develops slowly (usually over many years) into a large unilocular, fluid-filled bladder--the hydatid cyst. Brood capsules sprout from these cysts; within these capsules are numerous small infective scolices. Large cysts may contain several liters of highly antigenic hydatid fluid as well as millions of scolices. Daughter cysts sometimes form within or outside primary cysts. The disease is common in sheep-raising areas of the Mediterranean, Middle East, Australia, New Zealand, South Africa, and South America. Foci also exist in regions of Canada, Alaska, and California. Symptoms and Signs Most infections are acquired in childhood, but except when cysts are in vital organs, clinical signs may not appear for decades. Signs and symptoms resemble those of a space-occupying tumor. Most cysts are found in the liver, where they eventually produce abdominal pain or a palpable mass. Jaundice may occur if the bile duct is obstructed. Rupture into the bile duct, abdominal or peritoneal cavity, or lung may produce fever, urticaria, or a serious anaphylactic reaction. Released scolices may produce metastatic infection. Pulmonary cysts are usually discovered on routine chest x-ray. Some rupture, causing cough, chest pain, and hemoptysis. Eosinophilia may be present. Diagnosis CT and ultrasound scans may be pathognomonic if daughter cysts are present, but simple hydatid cysts are difficult to differentiate from a simple epithelial cyst. The presence in cyst fluid of hydatid sand (debris of old scolices and hooklets) is diagnostic. Chest x-ray may show a round, often irregular, pulmonary mass of uniform density. The Casoni skin test is often positive but lacks sensitivity and specificity. Serologic tests are often positive. Detection of antibodies to the echinococcal arc 5 antigen or demonstration of typical bands by immunoblot are highly specific. Prevention and Treatment Infection can be prevented by not feeding infected game to dogs and not allowing dogs to feed on sheep carcasses and offal. Dogs in sheep-raising areas should be dewormed repeatedly. Careful surgery, sometimes via laparoscopy, is the treatment of choice. Some centers are now performing percutaneous aspiration under CT guidance followed by instillation of a scolecoidal agent and reaspiration (the PAIR technique). Albendazole (400 mg bid po for 28 days, 15 mg/kg/day in children) may suppress the growth or kill cysts and is indicated for inoperable cases or for cases of intraoperative cyst spillage to prevent metastatic infections. ALVEOLAR HYDATID DISEASE (Multilocular Echinococcosis) Infection with Echinococcus multilocularis. The adult worms are found in foxes, and the hydatid larvae in small wild rodents. Infected dogs are the main link to occasional human infection. The life cycle of the organism is similar to E. granulosus. The larvae form irregular alveolar cysts that usually contain no scolices. The germinative tissue and brood capsules spread rapidly and produce spongy tumors that are difficult or impossible to treat surgically. The cysts are found mainly in the liver but can metastasize to the lungs, lymph nodes, and other tissues. E. multilocularis is present mainly in Central Europe, Alaska, Canada, and Siberia. The range of natural infection in the USA extends from Wyoming and the Dakotas to Indiana and Ohio. Symptoms, signs, and diagnosis are similar to hydatid disease (see above). Prognosis is poor, unless the entire larval mass can be removed. High-dose mebendazole or albendazole may suppress the growth of the parasite. Concomitant interferon-gamma may enhance the efficacy of these drugs. Liver transplantation has been lifesaving in a small number of patients. Quote Link to comment Share on other sites More sharing options...
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