Veterinary Medicine: Flatworms (Platyhelminthes) PDF

Study programme: VETERINARY MEDICINE 1 Year, 2d Semester VF/AF/VM-U31 LIFE SCIENCES II ZOOLOGY AND LABORATORY ANIMALS Flatworms (Platyhelminthes)  Plant Biology Zoology and Ecology and Prof. Modestas Ružauskas Lab...

Study programme: VETERINARY MEDICINE 1 Year, 2d Semester VF/AF/VM-U31 LIFE SCIENCES II ZOOLOGY AND LABORATORY ANIMALS Flatworms (Platyhelminthes)  Plant Biology Zoology and Ecology and Prof. Modestas Ružauskas Laboratory Animals Conservation Nature Assoc. prof. Jurgita Assoc. prof. Agila Daukšienė dr. Vytautas Ribikauskas Dailidavičienė Assoc. prof. Lina Merkevičienė LIFE SCIENCES II  The place of Zoology in system of Nature Sciences Zoology – the branch of biology which relates to the animal kingdom, including the structure, embryology, evolution, classification, habits and distribution of all animals, both living and extinct. The term is derived from Ancient Greek (zōon, “animal”) + (logos, “knowledge”). Helminthology Entomology Ichthyology Ornithology Theriology Malacology Mammalogy 3  Taxon – systematic unit Kingdom Animals Chordata Phylum Class Mammalia Superorder Eutheria Order Primates Suborder Antropoidea Family Pongidea Genus Gorilla Species Gorilla gorilla The hierarchy of biological classification's eight major taxonomic ranks, which is an example of definition by genus and differentia. 4  Major/Component group Described Global estimate (described + undescribed) ~6,755,830 Invertebrates ~1,359,365 Insecta ~1,000,000 (965,431–1,015,897) ~5,000,000 Arachnida 102,248 ~600,000 Mollusca ~85,000 ~200,000 Annelida 16,763 ~30,000 Nematoda 10,000 Reptiles 8,734 ~10,000 Amphibia 6,515 ~15,000 Fishes 31,153 ~40,000 Agnatha 116 unknown Cephalochordata 33 unknown Tunicata 2,760 unknown Based on Chapman's (2009) report,the estimated numbers of described extant species as of 2009 can be broken down as follows. 5 Relationships of the medically important parasites. Citation: Parasitology: Introduction, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=187698491&bookid=2381&jumpsectionid=208188084 &Resultclick=2 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 6  Phylum: Flatworms (Platyhelminthes) The name of the taxon derives from the Greek platy meaning "flat" and helminth meaning "worm." Phylum Platyhelminthes comprises 20,000 known species which are categorized under four classes: Class. Turbellaria comprises flatworms that are ciliated and free living, whereas members of class monogea are ectoparasitic (live outside the host body) and constitute monogenetic flukes. Class. Trematoda comprises common flukes. Class. Cestoda comprises endoparasitic (live within the body of the host) worms, which are also known as tapeworms. Class. Monogea. Parasitism - an association where one of the partners is harmed and the other lives at the expense of the other. E.g. Worms like Ascaris lumbricoides reside in the gastrointestinal tract of man, and feed on important items of intestinal food causing various illnesses. Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g. lice, ticks, mites etc. Endoparasites – parasites that live inside the body of their host, e.g. Liver fluke (Fasciola hepatica). 7 http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html Phylum: Flatworms (Platyhelminthes) Trematoda - Flukes (all species parasitic) Cestoda - Tapeworms (all species parasitic) Flatworms are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers) worms. Body flattened dorsoventrally; oral and genital apertures mostly on ventral surface. Flatworms have no body cavity other than the gut (and the smallest free-living forms may even lack that!) and lack an anus; the same pharyngeal opening both takes in food and expels waste. Because of the lack of any other body cavity, in larger flatworms the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains flatworms to be flat; they must respire by diffusion, and no cell can be too far from the outside, making a flattened shape necessary. 8  Phylum: Flatworms (Platyhelminthes) Body Cavities One of the primary ways zoologists group animals has to do with the presence or absence of a coelom, and how the coelom is formed. A coelom is a fluid-filled cavity between the alimentary canal and the body wall. The peritoneal cavity in our abdomen is one part of our coelom, and there are similar spaces around our heart and lungs. However, the type of coelom (or even its existence) differs among groups of animals – both in its structure (such as what types of tissues surround it) and its mode of development. There are three structural types of body plans related to the coelom. 9 http://userwww.sfsu.edu/biol240/labs/lab_16animalbodyplan/pages/bodyplan.html http://universe-review.ca/F10-multicell.htm  Three body cavity styles in modern animals Acoelomate, in which no coelomic cavity exists. Pseudocoelomate, in which a coelom exists, but it is lined by mesoderm only on the body wall, not around the gut. Coelomate (or Eucoelmate, or “True” Coelom), in which the coelom is lined both on the inside of the body wall and around the gut by mesoderm. Animals with a true coelom also have mesenteries, which suspend the body organs within the coelom. 10 http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookdiversity_7.htl  http://www.lookfordiagnosis.com/mesh_info.php?term=Germ+Layers&lang=1 Tree germ layers (triploblastic) The three germ layers (endoderm, mesoderm and ectoderm) encompass the precursors of all structures and organs of the entire body, and are generated by a process called gastrulation (occurs at the early stage of the embryonic development). 11 Symmetry Animals show different patterns of body symmetry. Some groups, such as the phylum Porifera, show no particular pattern of symmetry (asymmetry). That is, no line of bisection exists that could divide the organism into similar-looking halves. Other groups, including the Cnidaria and Echinodermata show radial symmetry, where more than one hypothetical bisection can be visualized. A third pattern, seen in most phyla of animals, is bilateral symmetry, where only one hypothetical bisection can be visualized. While these patterns can be seen readily, the implications of the different patterns of symmetry are important. 12 http://accessscience.com/search.aspx?rootID=800253 http://evolution.berkeley.edu/evolibrary/article/arthropods_04 http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookdiversity_7.htl  Phylum: Flatworms (Platyhelminthes) These worm like animals are bilaterally symmetrical, which means that their right and left sides are the mirror images of each other. This also indicates that these animals have distinct head and tail ends. They are triploblastic, that is they possess three main cell layers: outer ectoderm, middle mesoderm and inner endoderm just like the other bilateral animals. The middle layer in these animals is made up of spongy mesoderm cells and is also called parenchyma. 13 https://classconnection.s3.amazonaws.com/1613/flashcards/698929/jpg/planaria1.jpg https://micoleinahh.files.wordpress.com/2013/01/f-d_e42c770b998bd34900aca0b1a1fa9cf2ea9269d3d1fa6e6c35d7b53bimage_thumb_postcardimage_thumb_postcard.png  http://www.bio.miami.edu/dana/160/160S11_14.html Phylum: Flatworms (Platyhelminthes) Epidermis may be cellular or syncytial (ciliated in some); rhabdities in epidermis of most Turbellaria; epidermis a syncytial tegument in Monogenea, Trematoda (flukes), and Cestoda (tapeworms). Muscular system primarily of a sheath form and of mesodermal origin; layers of circular, longitudinal, and sometimes oblique fibers, beneath the epidermis No internal body space other than digestive tube (acoelomate); speces between organs filled with parenchyma, a form of connective tissue or mesenchyme Absence of Circulatory and Respiratory Organs The animals of platyhelminthes (flukes and tapeworms) lack circulatory and respiratory organs, which is why their bodies are so flat. The flat body shape permits the flow of oxygen and nutrients to reach all parts of the body by simple diffusion process. Carbon dioxide leaves the body by the same diffusion process. The gut of these animals is branched profusely in order to facilitate adequate diffusion of nutrients to all parts of the body. 14  Phylum: Flatworms (Platyhelminthes) Digestion The digestive system is incomplete with a single opening serving as the mouth and anus. A single layer of endodermal cells line the gut, whose role is to absorb and digest the food materials. Some species also feature the secretion of enzymes in the gut or pharynx to soften and break up the food. The undigested food material is regurgitated through the mouth as an anal opening is absent. However, there are exceptions to this. Large species do possess an anal opening and some with exceptionally profusely branched guts possess more than one anal opening. This is because solely excreting from the mouth would prove difficult for them. 15 http://image.slidesharecdn.com/flatwormscycycy-130318192346-phpapp02/95/phylum-platyhelminthes-55-638.jpg?cb=1363634683  Phylum: Flatworms (Platyhelminthes) Excretory System The excretory system of these animals is well- developed and is called protonephredia. Protonephredia comprises a network of tubules within the animal's body tissues. One end of the tubule extends into an exterior pore on the surface of the body, while the other end leads into spherical structure called flame cells. These flame cells possess long cilia which carry out a beating function. When the cilia beats, it gives the flame cells an appearance of a flickering candle and this is where the cells get their name. Excess water and body wastes enter the flame cells, are pushed into the tubules by the movement of cilia and thrown out of the body from the pore on the surface. 16 http://o.quizlet.com/i/hIx-pazWyK86SdM9GaA0lg_m.jpg http://sunny.moorparkcollege.edu/~econnolly/wormsX3_files/image010.gif  Phylum: Flatworms (Platyhelminthes) Nervous System Nervous System marked at the head region is of the primitive type. It comprises pair of anterior ganglia or a nerve ring connected to 1-3 pairs of longitudinal nerve chords with transverse commissures. These ganglia serve as the brain. 17  Phylum: Flatworms (Platyhelminthes) Reproductive System These animals can reproduce asexually and as well as sexually. They can reproduce asexually by transversal bipartition due to their wonderful ability to regenerate their tissues. They attach themselves to a substrate and induce a constriction in the body's mid region and get divided into two parts. The two divided body parts can regenerate into two different animal individuals. These animals are mostly hermaphrodite, that is both female and male reproductive organs are situated in one animal itself. Rarely animals with separate sex are found. Reproductive organs are highly developed. Self-fertilization (fusion of male and female gametes from the same animal) as well as cross fertilization (fusion of male and female gametes from different animals of the same species) is seen. 18 http://www.buzzle.com/articles/characteristics-of-platyhelminthes.html  Class. Flukes (Trematoda) Species. Liver fluke (Fasciola hepatica) The common liver fluke (Fasciola hepatica), also known as the sheep liver fluke, is a parasitic flatworm in the Trematoda class. This species can infect sheep, cattle, humans, and other animals across the world. This species is one of the largest of its kind, reaching an average body length of 1.1 inches, with a width of up to 5 inches. This species is shaped like a worm and is typically wider at the front end, although some individuals have wider back ends. The front end holds a cone like extension, under which the oral sucker can be found. The acetabulum of this species is larger than its oral sucker, but is also located near the front of its body. Its body, known as a tegument, is covered with large spines that resemble spines. 19 https://www.cdc.gov/parasites/fasciola/  Class. Flukes (Trematoda) Species. Liver fluke (Fasciola hepatica) The alimentary canal of F. hepatica has a single mouth which leads into the blind gut; it has no anus. The mouth is located within the anterior sucker on the ventral side of the fluke. This mouth leads to the pharynx, which is then followed by a narrow oesophagus. The oesophagus, which is lined with a thin layer of epithelial cells, then opens up into the large intestine. As no anus is present, the intestine branches, with each branch ending blindly near the posterior end of the body. F. hepatica has no respiratory organs: the adult flukes respire anaerobically (without oxygen). F. hepatica's excretory system contains a network of tubules surrounding one main excretory canal. Each tubule within the excretory system is connected to a flame cell, otherwise known as protonephridia. These cells are modified parenchyme cells. The nerve system of F. hepatica consists of a pair of nerve ganglia, each one is located on either side of the oesophagus. Around the oesophagus is a nerve ring, which connects the two nerve ganglia together. The nerves stem from this ring, reaching the posterior end of the body. 20 http://www.impe-qn.org.vn/impe-qn/en/portal/InfoDetail.jsp?area=58&cat=1046&ID=1081  Liver Fluke (Fasciola hepatica) F. hepatica adult flukes are hermaphrodite. The male and female reproductive organs open up into the same chamber within the body, which is called the genital atrium. The genital atrium is an ectodermal sac which opens up to the outside of the fluke via a genital pore. The testes are formed of two branched tubules, these are located in the middle and posterior regions of the body. From the epithelium lining of the tubules, sperm is produced. The sperm then passes into the vas deferens and then into the seminal vesicle. From the seminal vesicle projects the ejaculatory duct, and this opens into the genital atrium, and many prostate glands surround this opening. The right side of the anterior testis has a branched, tubular ovary. From here, a short oviduct passes to the vitelline duct. This duct connects, via a junction, the ovaries, the uterus, and the yolk reservoir. From this junction, the uterus opens into the genital atrium; this opening is surrounded by Mehlis glands. In some flukes, the terminal end of the uterus is strengthened with muscles and spines. F. hepatica reproduces both sexually. 21 http://www.ulac.lt/ligos/F/fasciolioze  DIFINITIVE AND INTERMEDIATE HOST Definitive host – a host that harbors a parasite in the adult stage or where the parasite undergoes a sexual method of reproduction. Intermediate host - harbors the larval stages of the parasite or an asexual cycle of development takes place. In some cases, larval development is completed in two different intermediate hosts, referred to as first and second intermediate hosts. 22  Life cycle Fascioliasis is a parasitic infection typically caused by Fasciola hepatica, which is also known as “the common liver fluke” or “the sheep liver fluke.” 23  Life Cycle Eggs are passed in the feces of the definitive host; In water, the free-swimming miracidium hatches from the egg and penetrates a suitable snail intermediate host; After penetration of the snail, the miracidium undergoes developmental stages (sporocysts, rediae), resulting in development of many cercaria; Cercaria exit the snail and swim until they contact aquatic vegetation and encyst, developing into metacercaria; Humans ingest the metacercaria on plants such as watercress; Metacercaria hatch and penetrate the intestinal wall and migrate through peritoneal cavity until coming in contact with liver. They penetrate the liver and migrate to bile ducts where they grow to adult worms. 24 https://www.softchalkcloud.com/lesson/files/EpB3D5Obst6GU0/MLAB1331_Trematodes_print.html  Phylum: Flatworms (Platyhelminthes) Class. Flukes (Trematoda) External structure Digestive system Circulation system Respiratory system Nervous system Excretion system Reproduction system Life cycle 25  Phylum. Flatworms (Platyhelminthes) Class. Tapeworms (Cestoda) Species of parasitic cestodal flatworm called tapeworms There are four medically important cestodes: Taenia solium, Taenia saginata, Diphyllobothrium latum, and Echinococcus granulosus. 26  Many helminths have a life cycle that progresses from egg to larva to adult. The egg contains an embryo that, upon hatching, differentiates into a larval form, which then matures into the adult form that produces the eggs. There are special terms applied to the host of certain parasites as they proceed through their life cycle. A definitive host is one in which the sexual cycle occurs or the adult is present, and the intermediate host is one in which the asexual cycle occurs or the larva is present. In some helminthic infections, humans are dead-end hosts—that is, the larval form in the human is not transmitted to other humans or animals. Humans are dead-end hosts for Taenia solium (cysticercosis), Echinococcus (hydatid cyst disease). 27  Platyhelminthes (platy means flat; helminth means worm) are divided into two classes: Cestoda (tapeworms) and Trematoda (flukes). Tapeworms consist of two main parts: a rounded head called a scolex and a flat body consisting of multiple segments. Each segment is called a proglottid. 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. Taenia solium—scolex and several proglottids. Long arrow points to one of the four suckers on the scolex of T. solium. Short arrow points to the circle of hooklets. Proglottids can be seen extending from the scolex toward the left side of the image. (Source: Dr. M. Melvin, Public Health Image Library, Centers for Disease Control and Prevention.) Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 28 Copyright © 2020 McGraw-Hill Education. All rights reserved Body systems 29  A: Taenia solium scolex with suckers and hooks (10×). B: Taenia solium gravid proglottid. This has fewer uterine branches than does the proglottid of Taenia saginata (see panel D) (2×). C: T. saginata scolex with suckers (10×). D: T. saginata gravid proglottid (2×). E: Diphyllobothrium latum scolex with sucking grooves (7×). F: Entire adult worm of Echinococcus granulosus (7×). G: E. granulosus adult scolex (70×). A: Taenia solium scolex with suckers and hooks (10×). B: Taenia solium gravid proglottid. This has fewer uterine branches than does the proglottid of Taenia saginata (see panel D) (2×). C: T. saginata scolex with suckers (10×). D: T. saginata gravid proglottid (2×). E: Diphyllobothrium latum scolex with sucking grooves (7×). F: Entire adult worm of Echinococcus granulosus (7×). G: E. granulosus adult scolex (70×). Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 30  Class. Tapeworms (Cestoda) There is a complete lack of digestive system. The animals of tapeworms lack circulatory and respiratory organs, which is why their bodies are so flat. Tapeworms have well developed muscles, and their excretory system and nervous system are some what similar to those of other flatworms. The excretory system of these animals is well-developed and is called protonephredia. Protonephredia comprises a network of tubules within the animal's body tissues. One end of the tubule extends into an exterior pore on the surface of the body, while the other end leads into spherical structure called flame cells. These flame cells possess long cilia which carry out a beating function. When the cilia beats, it gives the flame cells an appearance of a flickering candle and this is where the cells get their name. Excess water and body wastes enter the flame cells, are pushed into the tubules by the movement of cilia and thrown out of the body from the pore on the surface. Flame cells function like a kidney, removing waste materials. Bundles of flame cells are called protonephridia. 31 The entire body is covered by a special covering called tegument, which is an absorptive layer consisting of a mat of minute hair-like microtriches. 32 33  The main nerve centre of a cestode is a cerebral ganglion in its scolex. Motor and sensory innervation depends on the number of nerves in and complexity of the scolex. Smaller nerves emanate from the ganglion to supply the general body muscular and sensory ending. The cirrus and vagina are innervated, and sensory endings around the genital pore are more plentiful than other areas. Sensory function includes both tactoreception (touch) and chemoreception (smell or taste). Some nerves are only temporary. Tapeworms are exclusively hermaphrodites, with both male and female reproductive systems in each body. The reproductive system includes one or more testes, cirri, vas deferens, and seminal vesicles as male organs, and a single lobed or unlobed ovary with the connecting oviduct and uterus as female organs. The common external opening for both male and female reproductive systems is known as the genital pore, which is situated at the surface opening of the cup-shaped atrium. Though they are sexually hermaphroditic and cross-fertilization is the norm, self-fertilization sometimes occurs and makes possible the reproduction of a worm when it is the only individual in its host's gut. During copulation, the cirri of one individual connect with those of the other through the genital pore, and then spermatozoa are exchanged. 34 35 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. Taenia solium larvae cause cysticercosis. Important Properties Taenia solium can be identified by its scolex, which has four suckers and a circle of hooks, and by its gravid proglottids, which have 5 to 10 primary uterine branches. Taenia solium and Taenia saginata. Humans are infected when they ingest undercooked pork (T. solium) or beef (T. saginata) containing cysticerci (larval stage). Adult tapeworms form in intestine and lay eggs. Pigs and cattle are infected when they ingest either the eggs or proglottids in human stool. 36  Beef tapeworm (Taenia saginata) Disease Taenia saginata causes taeniasis. Taenia saginata larvae do not cause cysticercosis. Important Properties Taenia 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. The life cycle of T. saginata. 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. Humans are the definitive hosts and cattle the intermediate hosts. Unlike T. solium, T. saginata does not cause cysticercosis in humans. 37  Beef tapeworm (Taenia saginata) Taenia saginata—adult tapeworm. Note the tiny scolex on the right side of the image and the gravid proglottids on the left side of the image. White arrow points to the scolex. Ruler is 12 inches long. (Source: Public Health Image Library, Centers for Disease Control and Prevention.) Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 38  Taenia solium and Taenia saginata. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are infected at step 1 when they ingest undercooked pork (T. solium) or beef (T. saginata) containing cysticerci (larval stage). Adult tapeworms form in intestine and lay eggs. Pigs and cattle are infected when they ingest either the eggs or proglottids in human stool. Left side of figure describes the stages within the pigs and cattle (red arrows). (Source: Public Health Image Library, Centers for Disease Control and Prevention.) Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 39 Pork Tapeworm (Taenia Solium) 40  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. (A cysticercus consists of a pea-sized fluid-filled bladder with an invaginated scolex.) 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. Note that pigs are infected by the worm eggs; therefore, it is the larvae (cysticerci) that are found in the pig. A six-hooked 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. Humans are the definitive hosts, and pigs are the intermediate hosts. 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, skin, and brain, where they encyst to form cysticerci. Each cysticercus contains a larva. Taenia solium. Life cycle including cysticercosis stage. 41  Taenia solium. Life cycle including cysticercosis stage. Center and left side of figure describes the cycle of T. solium within the human and the pig similar to Figure 54–1. Note, however, that there are now blue arrows between the egg at the bottom that go up the left side of the figure to the person at the top right. In cysticercosis, humans are infected when they ingest the eggs of T. solium in food contaminated with human feces. The eggs differentiate into cysticerci primarily in brain, eyes, and skin. (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.) Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 42  Cysticercus of Taenia solium in brain—long arrow points to a larva of T. solium. Short arrow points to the wall of the cysticercus (sac) that surrounds the larva. (Used with permission from Rhodes B. Holliman, PhD, Professor Emeritus, Virgina Tech.) Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved A: Taenia solium egg containing oncosphere embryo. Four hooklets are visible. Taenia saginata and Echinococcus granulosus eggs are very similar to the T. solium egg but do not have hooklets. B: Diphyllobothrium latum egg with an operculum on the top (300×). 43 Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved  Echinococcus granulosus Echinococcus granulosus (dog tapeworm) causes echinococcosis. The larva of E. granulosus causes unilocular hydatid cyst disease. Important Properties Echinococcus granulosus is composed of a scolex and only three proglottids, making it one of the smallest tapeworms. The scolex has a circle of hooks and four suckers similar to T. solium. Dogs are the most important definitive hosts. Echinococcus granulosus—adult worm showing the scolex with hooks and suckers and three proglottids. The terminal proglottid shows many uterine The intermediate hosts are usually sheep. branches with eggs. (Source: Dr. Peter Schantz, Centers for Disease Control and Prevention.) Humans are almost always dead-end intermediate hosts. Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 44 The life cycle of E. granulosus 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 (larval form) within Echinococcus granulosus. Life cycle. Dogs are the definitive hosts and contain the adult tapeworm in the intestines. Sheep is an important intermediate host and ingest the eggs in dog feces. Hydatid cysts containing larvae form in the sheep. Humans are accidental intermediate “dead-end” hosts when they ingest food contaminated with dog feces containing the eggs. Eggs hatch oncosphere embryos in human intestine. Hydatid cysts containing protoscolices (larvae) form primarily in the liver, lung, brain, and bone. The life cycle is completed when the entrails (e.g., liver containing hydatid cysts) of slaughtered sheep are eaten by dogs. 45  Echinococcus granulosus Echinococcus granulosus. Life cycle. Center and left side of figure describes the natural cycle of E. granulosus within dogs (top half) and sheep (bottom half). Dogs are the definitive hosts and contain the adult tapeworm in the intestines. Sheep is an important intermediate host and ingest the eggs in dog feces. Hydatid cysts containing larvae form in the sheep. Humans are accidental intermediate “dead-end” hosts when they ingest food contaminated with dog feces containing the eggs (#2 at blue arrow at right). Eggs hatch oncosphere embryos in human intestine (#3 in human figure). Hydatid cysts containing protoscolices (larvae) form primarily in the liver, lung, brain, and bone (#4 in human figure). (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.) Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 46  Echinococcus granulosus—protoscolex of Echinococcus. Protoscolices are the “hydatid sand” within the hydatid cyst. Arrow points to area where several hooklets can be seen. (Source: Dr. L. Moore, Jr., Centers for Disease Control and Prevention.) Citation: Cestodes, Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2381&sectionid=187694961 Accessed: February 11, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved Echinococcosis. A. Echinococcal cyst in lung with white lining membrane. B. Echinococcal cyst wall with lung parenchyma below and five scolices above. (Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford, CT: Appleton & Lange, 1997.) Citation: Cestodes, Ryan KJ. Sherris Medical Microbiology, 7e; 2017. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=176090080&bookid=2268&jumpsectionid=176090148&Resultclick=2 Acces sed: February 12, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved 47  Fish Tapeworm (Diphyllobothrium Latum) The adult D. latum attaches to the human ileal mucosa with the aid of two sucking grooves (bothria) located in an elongated fusiform scolex. In life span and overall length, it resembles the Taenia species discussed previously. The 3000 to 4000 proglottids, however, are uniformly wider than they are long, accounting for this cestode’s species designation as well as one of its common names, the “broad tapeworm.” Diphyllobothrium latum. A. Structure of scolex. B. Structure of egg. C. Scolex from a human case. D. Ova in stool stained with iodine. (Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford, CT: Appleton & Lange, 1997.) The gravid segments contain a centrally positioned, rosette-shaped uterus unique among the tapeworms of humans. Over 1 million oval (55 by 75 μm) Citation: Cestodes, Ryan KJ. Sherris Medical Microbiology, 7e; 2017. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=176090080&bookid=2268&jumpsectionid=176090148&Resultclick=2 Acces sed: February 12, 2020 Copyright © 2020 McGraw-Hill Education. All rights reserved operculated eggs are released daily into the stool. 48 Life cycle of Diphyllobothrium latum Life cycle of Diphyllobothrium latum. (Reproduced with permission from Roberts RL, Janovy J, Nadler S: Foundations of Parasitology, 9th edition. McGraw- Hill, 2013.) Citation: Cestodes, Ryan KJ. Sherris Medical Microbiology, 7e; 2017. Available at: https://accessmedicine.mhmedical.com/content.aspx?sectionid=176090080&bookid=2268&jumpsectionid=176090148&Resultclick=2 Acces sed: February 12, 2020 49 Copyright © 2020 McGraw-Hill Education. All rights reserved Life cycle of Diphyllobothrium latum On reaching fresh water the eggs hatch, releasing ciliated, free-swimming larvae called coracidia. If ingested within a few days by small freshwater crustaceans of the genera Cyclops or Diaptomus, they develop into procercoid larvae. When the parasitized crustacean is then ingested by a freshwater or anadromous marine fish, the larvae migrate into the musculature of the fish and develop into infectious plerocercoid larvae. Humans are infected when they eat improperly prepared freshwater fish containing such forms. Diphyllobothrium latum (fish tapeworm) is encountered in those consuming raw fish (e.g., sushi and sashimi) or gefilte fish. Diphyllobothrium can compete with the host for vitamin B12, and patients can develop pernicious anemia. Most patients are asymptomatic, but some develop intestinal or biliary obstruction. 50 AccessMedicine > 51 Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 15e > Cestodes Warren Levinson, Peter Chin-Hong, Elizabeth A. Joyce, Jesse Nussbaum, Brian Schwartz  Phylum. Flatworms (Platyhelminthes) Beef tapeworm Pork tapeworm (Taenia Fish tapeworm Dog tapeworm (Taeniarhynchus solium) (Diphyllobothrium latum) (Echinococcus granulosus) saginatus) External structure Digestive system Circulation system Respiratory system Nervous system Excretion system Reproduction system Life cycle 52 Review questions: 1. Describe the internal and external structure of Flatworms (Platyhelminthes). 2. Describe the structure and the life cycle of Liver fluke (Fasciola hepatica). 3. Describe the structure and the life cycle of Beef tapeworm (Taeniarhynchus saginatus). 4. Describe the structure and the life cycle of Pork tapeworm (Taenia solium). 5. Describe the structure and the life cycle Fish tapeworm (Diphyllobothrium latum). 6. Describe the structure and the life cycle Dog tapeworm (Echinococcus granulosus). 7. What are the differences of the external structure of various tapeworms? 8. What is the difference between mature and gravid proglottids? 9. What diseases are caused by Tapeworms (Cestoda)? 53 53