Lab 4: Cestodes Phylum: Platyhelminthes The simplest animals that are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers) are the Platyhelminthes, the flatworms. 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. The greatest number of platyhelminthes are hermaphroditic or monoecious. The sexes are separate in a few cases, such as blood flukes and a small number of tapeworms. The reproductive structures are used more than any other structures for identification and classification of parasitic flatworms. There are currently four classes within the Phylum Platyhelminthes; Tubellarians (Tubellarian flat worms), Digeneans (parasitic flukes), Monogeneans (parasitic flukes), and the Cestodes (tapeworms). The tapeworms: Class: Cestoda Tapeworms (Class Cestoda) are all endoparasitic in nearly every species of vertebrate. This adaptation to parasitism has resulted in the complete loss of the intestine, with only a vestigial oral sucker and pharynx remaining and a tremendous increase in the capacity of the reproductive system. In other words, tapeworms lack a mouth and digestive system, and absorb nutrients through the tegument. Subclass Cestodaria The subclass Cestodaria consists of monozoic (unsegmented) tapeworms, with a single set of reproductive organs. No scolex is present and the shelled embryo contains ten hooks. All are parasites of fish. The Cestodaria will not be studied in this course. Subclass: Eucestoda General tapeworm form and function The true tapeworms are usually polyzoic (segmented), comprised of a scolex which functions as a holdfast and a strobila that is divided into many proglottids. The scolex, or head is the anterior end of the tapeworm and can be equipped with a variety of attachment structures. The scolex may bear suckers, grooves, hooks, spines or any combination of the above. Become familiar with the different attachment organs associated with the tapeworm scolex. In the strobila, each proglottid potentially contains one or more sets of both male and female reproductive organs, making the tapeworms hermaphroditic. Proglottids are added on in the “neck” region (by a process called strobillation) between the scolex and strobila, and mature as they move down the body. The male system develops first and anterior proglottids will often contain only male organs. Older proglottids will contain both reproductive systems. The anatomy of gravid proglottids is obscured by developing eggs. Eggs may be released through the uterine pore or the entire gravid proglottid may break off (apolysis) releasing eggs as it breaks up. True tapeworms usually produce a hexacanth embryo (6-hooked). You have slides and whole body preparations representing 2 orders of tapeworms: Pseudophyllidae and Cyclophyllidea. Order Cyclophyllidea Members of this order are common parasites in the intestines of amphibians, reptiles, birds and mammals. They are characterized by: • a scolex that usually contains four suckers. • a rostellum that is present or absent, and may or may not be armed. • a lateral genital pore • a uterine pore is absent. • vitelline gland is single, compact, and usually posterior to the ovary. • proglottids are often wider than long Gravid segments generally leave the host’s body individually, but may also be released in groups. Non-operculate eggs escape by rupture or disintegration of gravid proglottids and develop into some form of bladderworm in an invertebrate or vertebrate intermediate host. Infective eggs contain an oncosphere larva that bears 6 hooks. They have a variety of intermediate host types, both invertebrate and vertebrate. Depending on the cestode species, the larval stage in the vertebrate intermediate host may be a cysticercus, strobilocercus, coenurus (multiceps), or hydatid. Know these! • a cysticercus has a bladder-like form with an invaginated scolex located at one end • a coenerus is similar but bears multiple scoleces. • In a hydatid, brood pouches develop within the bladder and gives rise to multiple scolices (20 or so) and each hydatid cyst may contain thousands of scolices. Larvae in invertebrate intermediate hosts (usually arthropods) are cystercercoids; these have no bladder but a simple invaginated scolex. The scolex has 4 prominent suckers and a terminal rostellum, which may or may not be armed with hooks. 2 Family Taeniidae The largest cyclophyllideans are in the family Taeniidae, and are the most medically important tapeworms of humans. Most tapeworms in this family are large parasites of mammals, although some species are found in birds. These parasites are characterized by: • a scolex consisting of 4 large suckers and a non-retractable rostellum that may be armed with hooks. • The genital pores are irregularly alternating and the compact vitelline gland is located posterior to the ovary. • The gravid uterus is median with lateral branches. • The eggshell is thick with radial striations. • The larval stage may be either a cysticercus, coenurus, strobilocercus, or hydatid. Genus Taenia There are several species of Taenia that humans are likely to encounter. These include two species for which humans serve as the definitive host: Taenia saginata, the beef tapeworm; and T. solium, the pork tapeworm. Several species of Taenia also infect dogs and cats (e.g.,T. pisiformis), and humans are likely to encounter these when they note the presence of these tapeworms' proglottids in their pets' feces. All species of Taenia have similar life cycles. The adult tapeworm lives in the definitive host's small intestine. Proglottids, which contain eggs, break off the posterior end of the tapeworm, and these proglottids are either passed intact in the host's feces or they dissolve in the host's intestine and eggs are passed in the feces. When a suitable intermediate host ingests the eggs, the oncosphere larva is released and, with the aid of 3 the embryonic hooks, penetrates the intestinal wall and enters the bloodstream. Upon reaching the liver the oncosphere begins to develop into a cysticercus. Bladderworms break out of the liver and attach to the mesenteries throughout the abdominal cavity. The definitive host is infected when it eats an intermediate host infected with cysticerci. Upon ingestion the scolex evaginates, attaches to the intestinal lining, the bladder disintegrates, and the strobila is formed by the budding of the neck region. As adults in the definitive host's small intestine, tapeworms rarely cause problems; in exceptional cases the tapeworms might physically block the intestinal tract, due to their large size, or proglottids might become lodged in the appendix and result in appendicitis. The proglottids of Taenia are large and muscular. Occasionally single proglottids or long chains of proglottids might crawl out of the anus of an infected human. Understandably, most humans would find this quite disturbing, and it surely would not be an appropriate topic for "polite conversation." Slide: Taenia pisiformis is a cosmopolitan parasite of the dog and other canids. Study the whole mount of this species and note the scolex with rostellum, four suckers, and double row of hooks. Note the difference in the general shape of the immature, mature, and gravid proglottids. Study a mature segment and note the following: excretory vessels, ovary, vitellaria, testes, genital pore, cirrus pouch, vas deferens, uterus, and vagina. All structures may not be visible in your specimen. Study the immature segments and identify as many structures as possible. Note the branched uterus in gravid proglottids. Slide - Taenia eggs. These are non-operculate so they have no little door for the larvae to exit through. They rupture to release eggs. Since tapeworms are parasites of terrestrial hosts, a thick shell surrounds the eggs. Note the radial lines. Slide: Taenia solium cysticercus in muscle tissue (cross-section). Taenia solium, the human pork tapeworm, has humans as its only natural definitive host. Humans can be the host of the cysticercus as well as the adult. Humans become infected by ingesting poorly cooked pork containing cysticerci. Once ingested, the scolex evaginates in the small intestine 4 and attaches to the wall of the gut where it strobilates. Gravid proglottids develop within about 5 weeks. T. solium is the most dangerous tapeworms of humans because of the possibility of self-infection with cysticerci. Slide: Taenia sp. Cysticercus Tapeworm (Taenia pisiformis) 5 Genus Echinococcus The genus Echinococcus contains the smallest tapeworms in the Taeniidae. Echinococcus granulous is a parasite of carnivores, particularly canids, with the adult living in the small intestine of the definitive host. It is one of the smallest tapeworms, measuring less than 10 mm in total length. Many mammals may serve as intermediate hosts, but herbivores, such as sheep, cattle, and man, are most likely to become infected by eating the eggs on contaminated foliage. Echinococcus eggs contain an oncosphere. Oncosphere larvae are released from the eggs once in the stomach of an herbivore. Larval stages of this parasite represent the most unusual of asexual types of reproduction among cestodes. Once the eggs hatch, the oncosphere larvae is released and goes on to penetrate the gut wall, enter the host blood stream and migrate to other organs (the liver and lungs being the usual sites for development). By a very slow process, the oncosphere develops into a type of bladder worm called a hydatid. The hydatid is a large tumor-like bladder filled with developing scolices. Over the course of infection, the hydatid will develop a thick outer non-cellular layer and a thin inner germinal layer and become a hydatid cyst. The inner layer eventually produces the protoscolices that are infective to the definitive host.