This exercises uses descriptions from Invertebrate Anatomy OnLine, der.edu/rsfox/310taeniaLab.html. It also relies heavily on images obtained from sci.ohio-state.edu/~parasite/images.html and life cycles images from .cdc.gov/DPDx/default.htm
The parasitic flatworms.
Introduction
Neodermata is a subgrouping that includes the parasitic flatworms, most of which are flukes (Clade or Class Trematoda) or tapeworms (Clade or Class Cestoda). Most parasitic flatworms are endoparasites with complex life cycles requiring multiple hosts including a definitive host inhabited by the adult worm and one or more intermediate hosts inhabited by juvenile stages of the worm. The defining characteristic is the neodermis, or tegument. Neodermis is an epidermis specialized for living in a potentially hostile environment from which it must absorb food but reject toxins. The neodermis is a syncytium with its cell nuclei s ubmerged below the basal lamina .
Trematoda
Trematoda consists mostly of flukes belonging to Digenea. Digeneans are compact, bilaterally symmetrical, endoparasitic flatworms known as flukes. A blind gut with mouth and pharynx is present but there is no anus. Osmoregulation is accomplished via protonephridia. The integument is an elaborate syncytial neodermis without a cuticle. There are no respiratory or hemal systems and hermaphroditism is the rule. About 11,000 species are known making this the second largest taxon, after nematodes, of parasitic worms. Size ranges from less than 1 mm to 6 cm. Flukes are important animal parasites and they cause several important human and livestock diseases.
At least two hosts are required to complete the life cycle and this fact is the basis for the name “digenea”. The definitive host is always a vertebrate and the intermediate host is usually a gastropod mollusk. An additional intermediate host, if present, is an arthropod or fish. In the definitive host the parasite typically inhabits either the hemal system or the gut and its derivatives.
The adult Chinese liver fluke inhabits the bile ducts of the liver of any of several mammals including humans, cats, and dogs. It is an important human parasite in the orient. It is estimated that this species infects more than 30,000,000 humans in these areas. The parasite also infects a number of other animals, including dogs, cats, pigs, and rodents, and these animals serve as reservoirs of infection.
The adult worms measure between 10 and 25 mm in length. The do not actually live in the host's liver, but are found in the bile ducts inside of the liver. Eggs are passed in the host's feces and the first intermediate host is a snail. Cercariae emerge from the first intermediate host and infect the second intermediate host. The second intermediate host is a fish, and over 100 species of fish are susceptible to infection. The definitive host is infected when it eats raw or undercooked fish.
The sheep liver fluke causes fewer problems for its human host. The parasite resides in the bile ducts inside the liver rather than the liver itself. This species is a common parasite of sheep and cattle and, therefore, relatively easy to obtain. Infection in sheep and cattle results in animals that show low productivity (low weight gain, low milk production, etc.). The adult parasites reside in the intrahepatic bile ducts, produces eggs, and the eggs are passed in the host's feces. After passing through the first intermediate host (a snail), cercariae encyst on vegetation. The definitive host is infected when it eats the contaminated vegetation. The metacercaria encysts in the definitive host's small intestine, and the immature worm penetrates the small intestine and migrates through the abdominal cavity to the host's liver. The juvenile worm penetrates and migrates through the host's liver and finally ends up in the bile ducts. The migration of the worms through the host's liver and the presence of the worms in the bile ducts, are responsible for the pathology associated with infection.
The schistosomes are unusual trematodes in that the sexes are separate (they are dioecious), they reside in the blood vessels of the definitive host, and there are no second intermediate hosts in their life cycles. There are a number of species of schistosomes that can infect humans, but most human infections are caused by one of the three following species: Schistosoma mansoni ; S. haematobium ; S. japonicum. Considering the distributions of all three species, schistosomiasis is distributed throughout almost all of Africa, parts of Southeast Asia, parts of northwest South America, and some islands in the Caribbean Sea. It is estimated that approximately 200,000,000 million people are infected with schistosomes, resulting in 1,000,000 deaths each year. The life cycles of the three primary species of human schistosomes are similar. The male and female worms average about 10 mm in length and live in the veins of the abdominal cavity. Here they mate and the females produce eggs. The adult worms can live 20-30 years and, depending on the species, and each female can produce several hundred eggs each day. The eggs escape from the body by penetrating the walls of the veins and small intestine or urinary bladder, and they are passed in the feces or urine. The eggs hatch in water, the first intermediate host (a snail) is infected, and cercariae are liberated from the snails. When humans come in contact with water containing cercariae, the cercariae penetrate their skin and they become infected. This occurs when the humans swim, bath, wash clothes, etc., in rivers and streams. After the cercariae penetrate the skin the immature worms enter the circulatory system and migrate to the veins of the abdominal cavity, and in about six weeks they reach sexual maturity
1. Anatomy of flukes
Adult flukes. The anatomy of digenetic trematodes is usually studied in introductory laboratories using commercially prepared whole mount slides. The Chinese liver fluke, Opisthorchis (= Clonorchis) sinensis, or the sheep liver fluke, Fasciola hepatica, are the most frequently used species. We are fortunate to have slides of Schistosoma mansoni; a very important human disease causing species and an unusual fluke because the adults are dioecious or the sexes are separate.
a. Your assignment is to obtain and label as best you can, one photograph of Fasciola or an adult of some other species of “typical fluke. When you look at your specimen be sure that you can distinguish the difference between the diverticula of the intestine, the vitelline glands, and the testes. The appearance of the diverticula is best seen near the mouth where it is the only structure visible. b. Then obtain and examine one slide of Schistosoma mansoni . c. In your notebook, compare the structures most eviden t in these specimens to those found in the free-living flatworms.
You can find labeled diagrams and descriptions for flukes at the end of this lab. These are not to be memorized, but used to help you identify stages and structures in the adult.
2. Life cycles:
a . If living cercaria e are available in laboratory, examine these. Snails containing rediae or sporocyts may also be available for dissection. Obtain photographs and movies of all stages that you can find.
The only stage that can be usually supplied easily living is the cecaria. Usually a species is chosen that infects beetles or mice as adults. Some care should be taken whenever handling parasites. Wear gloves when you dissect the snails that are carrying cecariae.
Prior to the lab, Snails will probably be placed in the dark for two hours to stimulate them to release cercariae and then placed in a bowl of water.
You will be able to examine the water from the bowl under the microscope. If cecariae are present, snails may be cooled and dissected to see rediae or sporocyts. We will have to wait for specific instructions, which will come with the snails, as these snails can be river or pond snails and infected with a number of trematodes
If cecaria e from snails are not available:
b. Examine the prepared slides of the various stages of the fluke. Each pair should contribute to their notebook and a folder that will be available to the class, one photograph of one stage of the life cycle. Be careful and label soon after taking photographs because some stages in the life cycle vary more in morphology from species to species (such as rediae and sporocysts ) than others (such as cecaria ). Can you determine why this may be? Each photograph should be labeled with the name of the state and species. After examining the photographs , please take the following self test and record your answers in your journal.
Self-test: Identify the stage in the following photographs
2a______ 2b_____ 2c_________ 2d_________
2e____________ 2f________________
A description of the typica l life cycle and diagrams of specific life cycles are included to help you associate stage with proper host.
Most life cycles include two intermediate hosts (the hosts in which the parasite undergoes asexual reproduction) and a definitive host (the host in which the parasite undergoes sexual reproduction). Most adult digenetic trematodes are monoecious (a single organism contains both male and female reproductive organs), and most live in the gastrointestinal tract of the definitive host. The adult parasites produce eggs, and the eggs are passed in the definitive host's feces. The egg either hatches so the larva inside escapes and infects the first intermediate host, or the first intermediate host eats the egg. The first intermediate host is always a snail, most often an aquatic or marine snail. Inside of the snail the parasite goes through several developmental stages, and a larval stage called a "cercaria" is finally produced. The cercaria leaves the snail and swims around until it comes in contact with a second intermediate host. The cercaria penetrates the skin of the second intermediate host and encysts. This encysted stage is called a "metacercaria." The definitive host is infected when it eats an infected second intermediate host. Fish commonly serve as second intermediate hosts for digenetic trematodes, and the metacercariae encyst in the fish's muscles. In such a case the definitive host would be infected by eating the fish. Depending on complexity, these various life stages could be recognized.
Egg leaves the adult containing a miricidium. Miricidia through chemoreception finds a snail, penetrates and develops into sporocysts. The sporocyst is an asexual stage with no mouth or digestive system, that absorbs nutrients through tegument and produce rediae, daughter sporocysts or cecariae.
Redia are elongate forms with a muscular pharynx, that may form cercariae, or free swimming forms that leave the snail to seek another host. They encyst eventually as metacercaria to eventually be eaten by a definitive host.
Oriental liver fluke: Clonorchis sinensis
The sheep liver fluke: Fasciola hepatica
Schistosoma
3. CESTODA or tapeworms
Cestodes are compact worms whose interior is filled with unspecialized, mesenchymal connective tissue known as parenchyma, as it is in other platyhelminths. Excretion and osmoregulation are accomplished via protonephridia, which empty into two pairs of excretory canals. The nervous system includes anterior nerve rings, lateral longitudinal nerve cords, and transverse commissures. Cestodes are hermaphroditic and may utilize either self- or cross-fertilization
Use the compound microscope to study whole mount slides of the scale , a mature proglottid , and a gravid proglottid . a. Choose one slide to photograph and label. b. Compare the anatomy of tapeworms with flukes in your notebook.
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Information for labeling tapeworms. Use the following description as well as labeled drawing to identify important characteristics of tapeworms on prepared slides . These descriptions are taken from a online lab manual that stresses taxonomy and so most of the terms used are not to be memorized, but there as the author requests his work not be modified to any great extent. Be grateful our course is basically an appreciation course.
A more colorful diagram
Laboratory Specimens
Commercially prepared slides of Taenia pisiformis (= T. serrata), the dog tapeworm, are frequently used in the laboratory as examples of tapeworm anatomy. Other species can also be used but differ in some respects. The adults of Taenia pisiformis occur in dogs, cats, and other carnivores and the intermediate host is a rabbit.
The tapeworm body consists of an anterior, head-like scolex and the body, or strobila, consisting of a linear series of segments, or proglottids. Look at the whole mount of a scolex with the scanning lens (40X) of the compound microscope. The scolex is wider than the anterior strobila to which it joined by a narrow neck.
The scolex attaches the worm to the gut wall of the host. For this purpose it has a retractable rostellum armed with two rings of hooks. Examine the hooks with higher power. Just posterior to the rostellum is a ring of four suckers, two dorsolateral and two ventrolateral. Scolex morphology varies widely with taxon.
Two lateral nephridial canals may be visible on each side of the scolex. They connect with each other near the rostellum via a set of convoluted nephridial canals and extend posteriorly through the strobila. A pair of nerve rings are present but will not be evident.
The strobila is long and wormlike, sometimes reaching 15 or more meters in length. (There are records of Taeniarhynchus saginatus, a human parasite, reaching lengths of more than 20 meters.) The strobila is strongly flattened dorsoventrally and is a linear series of hundreds or thousands of proglottids.
The strobila includes a series of young immature proglottids at its anterior end, then a region of sexually mature proglottids in the middle, and a length of gravid proglottids at the posterior end. The gravid proglottids contain viable embryonated “eggs” ready to infect a new host should the get the opportunity. Proglottids increase in size posteriorly.
Each proglottid contains its own complete hermaphroditic reproductive system as well as its share of the common excretory and nervous systems. There is no digestive system. The reproductive systems of the youngest, anteriormost proglottids are not yet formed but more posterior proglottids have increasingly better-developed reproductive systems. Most tapeworms are protandric with the male system of each proglottid maturing first.
Examine a mature proglottid using 40X of the compound microscope. Your slide probably contains several proglottids, some of which may be better than others. You will probably want to make use of them all as you look for the best representation of each structure.