Cestodes PDF

# Cestodes ## Introduction - Tapeworms consist of two main parts: a rounded head called a scolex and a flat body of multiple segments called proglottids - The scolex has specialized means of attaching to the intestinal wall, namely, suckers, hooks, or sucking grooves - The worm grows by adding n...

# Cestodes ## Introduction - Tapeworms consist of two main parts: a rounded head called a scolex and a flat body of multiple segments called proglottids - The scolex has specialized means of attaching to the intestinal wall, namely, suckers, hooks, or sucking grooves - The worm grows by adding new proglottids from its germinal center next to the scolex. The oldest proglottids at the distal end are gravid and produce many eggs, which are excreted in the feces and transmitted to various intermediate hosts such as cattle, pigs, and fish. - Humans usually acquire the infection when undercooked flesh containing the larvae is ingested - However, in two important human diseases, cysticercosis and hydatid disease, it is the eggs that are ingested and the resulting larvae cause the disease. ## Taenia - There are two important human pathogens in the genus Taenia: _T. solium_ (the pork tapeworm) and _T. saginata_ (the beef tapeworm). ## Taenia solium ### Disease - The adult form of _T. solium_ causes taeniasis - _T. solium_ larvae cause cysticercosis ### Important Properties - _T. solium_ can be identified by its scolex, which has four suckers and circle of hooks, and by its gravid proglottids, which have 5 to 10 primary uterine branches - The eggs appear the same microscopically as those of _T. saginata_ and _Echinococcus_ species ### Life Cycle - In taeniasis, the adult tapeworm is located in the human intestine - This occurs when humans are infected by eating raw or undercooked pork containing the larvae, called cysticerci - In the small intestine, the larvae attach to the gut wall and take about 3 months to grow into adult worms measuring up to 5 m. - The gravid terminal proglottids containing many eggs detach daily, are passed in the feces, and are accidentally eaten by pigs. - An embryo (oncosphere) emerges from each egg in the pig's intestine. - The embryos burrow into a blood vessel and are carried to skeletal muscle. - They develop into cysticerci in the muscle, where they remain until eaten by a human and the cycle became completed. - Humans are the definitive hosts, and pigs are the intermediate hosts. ## Taenia saginata ### Disease - _T. saginata_ causes taeniasis. _T. saginata_ larvae do not cause cysticercosis ### Important Properties - _T. saginata_ has a scolex with four suckers but, in contrast to _T. solium_, no hooklets. Its gravid proglottids have 15 to 25 primary uterine branches, in contrast to _T. solium_ proglottids, which have 5 to 10. - The eggs are morphologically indistinguishable from those of _T. solium_. - Humans are the definitive hosts and cattle the intermediate hosts. - Unlike _T. solium_, _T. saginata_ does not cause cysticercosis in humans. ### Life Cycle - Humans are infected by eating raw or undercooked beef containing larvae (cysticerci) - In the small intestine, the larvae attach to the gut wall and take about 3 months to grow into adult worms measuring up to 10 m - The gravid proglottids detach, are passed in the feces, and are eaten by cattle. - The embryos (oncospheres) emerge from the eggs in the cow's intestine and burrow into a blood vessel, where they are carried to skeletal muscle. - In the muscle, they develop into cysticerci. - The cycle is completed when the cysticerci are ingested. ## Diphyllobothrium ### Disease - _Diphyllobothrium latum_, the fish tapeworm, causes diphyllobothriasis. ### Important Properties - In contrast to the other cestodes, which have suckers, the scolex of _D. latum_ has two elongated sucking grooves by which the worm attaches to the intestinal wall. - Unlike other tapeworm eggs, which are round, _D. latum_ eggs are oval and have a lidlike opening (operculum) at one end. - _D. latum_ is the longest of the tapeworms, measuring up to 13 m. ### Life Cycle - Humans are infected by ingesting raw or undercooked fish containing larvae (called plerocercoid or sparganum larvae). - In the small intestine, the larvae attach to the gut wall and develop into adult worms. - Gravid proglottids release fertilized eggs through a genital pore, and the eggs are then passed in the stools. - The immature eggs must be deposited in fresh water for the life cycle to continue. - The embryos emerge from the eggs and are eaten by tiny copepod crustacea (first intermediate hosts). - There, the embryos differentiate and form procercoid larvae in the body cavity. - When the copepod is eaten by freshwater fish, e.g., pike, trout, and perch, the larvae differentiate into plerocercoids in the muscle of the fish (second intermediate host). - The cycle is completed when raw or undercooked fish is eaten by humans (definitive hosts). ## Echinococcus ### Disease - _Echinococcus granulosus_ (dog tapeworm) causes echinococcosis. - The larva of _E. granulosus_ causes unilocular hydatid cyst disease. - Multilocular hydatid disease is caused by _E. multilocularis_, which is a minor pathogen. ### Important Properties - _E. granulosus_ is composed of a scolex and only three proglottids, making it one of the smallest tapeworms. - Dogs are the most important definitive hosts. - The intermediate hosts are usually sheep. - Humans are almost always dead-end intermediate hosts. ### Life Cycle - In the typical life cycle, worms in the dog's intestine liberate thousands of eggs, which are ingested by sheep (or humans). - The oncosphere embryos emerge in the small intestine and migrate primarily to the liver but also to the lungs, bones, and brain. - The embryos develop into large fluid-filled hydatid cysts, the inner germinal layer of which generates many protoscoleces within "brood capsules." - The life cycle is completed when the entrails (e.g., liver containing hydatid cysts) of slaughtered sheep are eaten by dogs. ## Hymenolepis nana - _H. nana_ (dwarf tapeworm) - It is only 3 to 5 cm long and is different from other tapeworms because its eggs are directly infectious for humans, i.e., ingested eggs can develop into adult worms without an intermediate host. - Infection causes little damage, and most patients are asymptomatic. - Diagnosis is based on finding eggs in stools. The characteristic feature of _H. nana_ eggs is the 8 to 10 polar filaments lying between the membrane of the six-hooked larva and the outer shell. ### Life Cycle - Within the duodenum, the eggs hatch and differentiate into cysticercoid larvae and then into adult worms. - Gravid proglottids detach, disintegrate, and release fertilized eggs. - The eggs either pass in the stool or can reinfect the small intestine (autoinfection). - In contrast to infection by other tapeworms, where only one adult worm is present, many _H. nana_ worms (sometimes hundreds) are found. This is a summary of the text provided. The content could be further organized in different ways, depending on your particular needs and the purpose you have for the document.