Cestode
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This document provides information on the tapeworm Cestode. It discusses several types of tapeworms that can infect humans, including Taenia solium (pork tapeworm), Taenia saginata (beef tapeworm), Diphyllobothrium latum (fish tapeworm), and Echinococcus granulosus (dog tapeworm). For each tapeworm, it describes their life cycles, symptoms and diseases they can cause, identification properties, and methods for laboratory diagnosis. The document contains detailed diagrams illustrating the life cycles of the different tapeworms.
Cestode
- 1. Cestode Aman Ullah B.Sc. MLT M. Phil Microbiology Master in Health Research Certificate in Health and Physical Education
- 2. 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
- 3. Taenia • There are two important human pathogens in the genus Taenia: T. solium (the pork tapeworm) and T. saginata (the beef tapeworm)
- 4. 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
- 5. 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.
- 6. 6 LIFE CYCLE T solium
- 7. Cycle of cysticercosis • In cysticercosis, a more dangerous sequence occurs when a person ingests the worm eggs in food or water that has been contaminated with human feces. • Note that in cysticercosis, humans are infected by eggs excreted in human feces, not by ingesting undercooked pork • Also pigs do not have the adult worm in their intestine, so they are not the source of the eggs that cause human cysticercosis • The eggs hatch in the small intestine, and the oncospheres burrow through the wall into a blood vessel • They can disseminate to many organs, especially the eyes and brain, where they encyst to form cysticerci
- 8. Pathogenesis • The adult tapeworm attached to the intestinal wall causes little damage • The cysticerci, on the other hand, can become very large, especially in the brain, where they manifest as a space-occupying lesion • Living cysticerci do not cause inflammation, but when they die they can release substances that provoke an inflammatory response Clinical Findings • Most patients with adult tapeworms are asymptomatic, but anorexia and diarrhea can occur • Some may notice proglottids in the stools • Cysticercosis in the brain causes headache, vomiting, and seizures • Cysticercosis in the eyes can appear as uveitis or retinitis, or the larvae can be visualized floating in the vitreous Laboratory Diagnosis • Identification of T. solium consists of finding gravid proglottids with 5 to 10 primary uterine branches in the stools • Eggs are found in the stools less often than are proglottids. Diagnosis of cysticercosis depends on demonstrating the presence of the cyst in tissue, usually by surgical removal or computed tomography (CT) scan
- 9. 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
- 10. 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
- 11. 11 Life Cycle
- 12. Pathogenesis & Epidemiology • Little damage results from the presence of the adult worm in the small intestine. The epidemiology of taeniasis caused by T. saginata is related to the access of cattle to human feces and to the consumption of raw or undercooked beef Clinical Findings • Most patients with adult tapeworms are asymptomatic, but malaise and mild cramps can occur. In some, proglottids appear in the stools and may even protrude from the anus Laboratory Diagnosis • Identification of T. saginata consists of finding gravid proglottids with 15 to 20 uterine branches in the stools • Eggs are found in the stools less often than are the proglottids
- 14. Taenia Egg of T. saginata or T. solium • It is round to oval • Embryo is surrounded by a thick, brown, radially striated wall • Hooklets are present in the embryo • Careful focusing is necessary to see the three pairs of hooklets
- 15. 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
- 16. 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).
- 17. 17 Life Cycle
- 18. Pathogenesis • Infection by D. latum causes little damage in the small intestine • In some individuals, megaloblastic anemia occurs as a result of vitamin B12 deficiency caused by preferential uptake of the vitamin by the worm. • The epidemiology of D. latum infection is related to the ingestion of raw or inadequately cooked fish and to contamination of bodies of fresh water with human feces Clinical Findings • Most patients are asymptomatic, but abdominal discomfort and diarrhea can occur Laboratory Diagnosis • Diagnosis depends on finding the typical eggs, i.e., oval, yellow-brown eggs with an operculum at one end, in the stools
- 19. Egg of D. latum • It is yellow-brown and oval in shape • Has an operculum (lid) which is usually difficult to see • Contains a mass of granulated yolk cells surrounding an undeveloped ovum • Sometimes a small projection is visible at the non-operculated end of the egg
- 20. 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
- 21. 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
- 22. 22 Life Cycle
- 23. Pathogenesis • E. granulosus usually forms one large fluid-filled cyst (unilocular) that contains thousands of individual scoleces as well as many daughter cysts within the large cyst • Individual scoleces lying at the bottom of the large cyst are called "hydatid sand." The cyst acts as a space-occupying lesion, putting pressure on adjacent tissue • The outer layer of the cyst is thick, fibrous tissue produced by the host Clinical Findings • Many individuals with hydatid cysts are asymptomatic, but liver cysts may cause hepatic dysfunction • Cysts in the lungs can erode into a bronchus, causing bloody sputum, and cerebral cysts can cause headache and focal neurologic signs Laboratory Diagnosis • Diagnosis is based either on microscopic examination demonstrating the presence of brood capsules containing multiple protoscoleces or on serologic tests
- 24. 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
- 25. 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
- 26. Life Cycle
- 27. Egg of H. nana • It is colourless, oval or round • Hooklets are present in the embryo • Careful focusing is necessary to see the three pairs of hooklets. • At each end of the egg, thread-like structures called polar filaments are usually visible.