BABASAHEB BHIMRAO AMBEDKAR UNIVERSITY Department of Zoology
Lecture Outline /Summary Notes CLASS: M.Sc. Zoology, 2nd Semester
PAPER CODE & NAME: ZL-‐203-‐Parasitology -‐1 Course Teacher: Dr. Suman Mishra
TOPIC-‐ UNIT3: Morphology and Anatomy of Parasites-‐1 CESTODA/ CESTOIDEA
TAXONOMIC POSITION
GENERAL CHARACTERISTICS
Highly specialized class of flatworms of Phylum Platyhelminthes, the members are commonly called tapeworms. It has 3,500 species. They are ribbon-‐shaped, dorsoventrally flattened (tape-‐like) organisms lacking epidermis, body cavity, skeletal and circulatory systems. Digestive system is absent, or when present it is rudimentary and without anus. Nutrition is obtained through body surface from the intestine of host. Nervous system and excretory system are present. Body is segmented except Order Caryophyllidea . All cestodes are exclusively endoparasites and adults live in the vertebrate digestive system while juveniles (larval forms) are found in both vertebrate and invertebrate hosts. The members mostly exhibit a high degree of host specificity. They are predominantly hermaphrodites with well developed reproductive system. They have indirect life cycles with need of atleast one intermediate host. These parasites live in digestive tract of vertebrates as adults, and in bodies of various animals as juveniles (larva).
MORPHOLOGY
The adult body is divided into three main regions.
CLASS CESTODA/ CESTOIDEA Subclass Cestodaria
Order Amphilinidea e.g. Amphilina
Order Gyrocotylidea e.g. Gyrocotyle
Subclass Eucestoda
Order Caryophyllidea: in freshwater fish Order Spathebothriidea: Fresh /marine fish Order Proteocephalata
Order Tetraphyllidea e.g.Acanthobothrium Order Diphyllobothridea e.g. Diphyllobothrium Order Trypanorhyncha e.g. Tetrarhynchus Order Pseudophyllidea e.g.Bothriocephalus Order Cyclophyllidea e.g. Taenia
1. Head (Scolex): It is present at one end and is a hold-‐fast structure. It may have grooves, circular or leaf like suckers, and rostellum armed with hooks and hooklets. The tapeworm firmly anchors itself to the intestinal wall with the Scolex. Mouth is not present in them.
2. Neck: The scolex narrows to form the neck from which strobilization i.e. formation of new proglottid segments occurs in this region. As new proglottids are formed from the neck region, they push the older ones progressively posterior, thus creating a chain of proglottids -‐ the strobila.
The asexual process of forming segments is termed strobilation.
3. Strobila : It is the third body region consisting of a series of linearly arranged proglottids.
Proglottids function primarily as reproductive units. The stroblia can be subdivided into 3 regions:
Immature, Mature and Gravid proglottids. Anterior proglottids are immature, while posterior proglottids are mature and produce eggs.. After fertilization, the proglottids at the posterior end accumulate eggs and are called gravid (ripened).
HOLD FAST ORGANS
The scolex located at anterior end functions as Hold fast organs, serving for attachment to host tissues and consist of suckers, grooves, and rostellum with spine like hooks and hooklets.
Three types of suckers are present on scolex:
1. Bothria-‐ 2, rarely 4/6 shallow sucking grooves (Pseudophyllidea) 2. Bothridia: 4 leaf like flexible structures (Tetraphyllidea)
3. True suckers/acetabula-‐4 muscular cups, with or without hooks (Cyclophyllidea). Hooks are generally grouped at apical end of scolex on a protrusible rostellum
Bothria Acetabula Bothridia NERVOUS SYSTEM
The nervous system consists of “brain” which is a rectangular or circular nervous tissue varying in complexity from a simple ganglion to a combination of several ganglia and commissures. Several pairs of longitudinal nerve cords extend posteriorly from this “brain” along the length of the strobila, lateral to the osmoregulatory canals. The longitudinal nerve cords are connected in each proglottid by cross connectives. Small motor nerves emanating from the cords and cross-‐connectives innervate the reproductive organs and musculature. Small sensory nerves supplying the tegument merge with the cords and connectives
EXCRETORY SYSTEM/OSMOREGULATION
A protonephridial system also runs the length of the tapeworm. It consists of 2 components: the collecting canals and the flame cells. The collecting canals (2 dorsal and 2 ventral) are aligned on lateral side and extend the entire length of the strobila. All 4 canals lie just inside the medullary margin of the parenchyma, and a single transverse canal connects the ventral canals at the posterior end of each proglottid. The ventral canals carry fluid away from the scolex, while the dorsal canals carry towards
scolex thus helping to efficiently remove wastes from body. In some tapeworms, the 4 longitudinal canals are linked within the scolex by either a network of canals or a single ring of vesicles whereas in others, the dorsal and ventral canals on each side are linked by a simple connection in the region of the scolex, with no apparent exchange between the 2 sides. In the terminal proglottid of young worms, there is an excretory vesicle into which the ventral canals empty; In older tapeworms, the posterior ends of the ventral canals open independently to the exterior. Flame cells are associated with the ventral canals; fluid connected by the flame cells passes through secondary tubules into the main canals.
Analysis of fluid within the osmoregulatory system shows that it consists primarily of: glucose, soluble proteins, lactic acid, urea, and ammonia
ULTRASTRUCTURE OF TEGUMENT Body wall
The outer body wall of tapeworms consists of a tegument that bears specialized microvilli known as microthrices (singl. microthrix) projecting from its outer surface. These tips provide resistance to the peristaltic movement of the host’s intestine and agitate intestinal fluids in the immediate microhabitat.
The tapeworms have no digestive system, therefore tegument plays an important role in nutrient absorption. The tegument even absorbs some of the host’s own enzymes which help in digestion.
Nutrition: Lacking a digestive tract, these worms must derive all nutrient molecules from the host or its
microhabitat, and such molecules must cross the tegument. Nutrients cross the tegument by active transport, facilitated diffusion, and simple diffusion. The most important nutrient molecule is glucose, which after polymerization within the parasite, is stored as glycogen usually in the parenchyma and interstitial fluid; the only other major, transported carbohydrate is galactose. The tapeworms reside in environments of very low oxygen tension (small intestine), thus obtain energy anaerobically by substrate phosphorylation via glycolysis. Most adult tapeworms also absorb lipids, probably by simple diffusion. Metabolic rates differ in different parts of the body with neck and immature proglottids having a much higher rate of metabolism than the mature and gravid proglottids.
Tegument consists of following layers:
•A syncytial epithelium, with distal and proximal cytoplasmic regions.
• Tegumental musculature made of 2 layers -‐ an outer circular and an inner longitudinal muscle layer
• Glycocalyx – covering tegument and made up of carbohydrate containing molecules, for protection from host digestive enzymes & enhancing nutrient absorption
FUNCTIONAL ANATOMY OF MALE & FEMALE REPRODUCTIVE ORGANS
Tapeworms are monoecious. They produce large number of eggs. Each proglottid contains a complete set of male and female reproductive organs.
MALE REPRODUCTIVE SYSTEM consists of numerous testes scattered throughout each mature proglottids, vas efferens arising from each testis which unite to form a common vas deferens, a muscular, eversible cirrus modified from distal portion of vas deferens, and usually enclosed in a cirrus pouch. Cirrus pouch opens into common genital atrium . The testes produce sperms which are conducted into the copulatory organ cirrus through a duct system. In most species there is an enlarged area of the vas deferens, the seminal vesicle, for the storage of sperm. The male system of a proglottid matures before the female system (protrandry), therefore, copulation occurs with another mature proglottlid of the same tapeworm or with another tapeworm in the same host. Thus, cross fertilization produces hybrid vigor.
FEMALE REPRODUCTIVE SYSTEM consists of a single, sometimes bilobed ovary, leading into an oviduct which opens into the ootype surrounded by Mehlis’ gland. The ootype receives a single common vitelline duct formed by union of many primary vitelline ducts which arise from the vitelline glands. The vitelline glands (=vitellaria) may form a compact body or consist of numerous follicles scattered throughout the medullary parenchyma in each proglottid. The oviduct continues as the uterus which opens to the outside of the proglottid through a uterine pore for expelling eggs (Order. Pseudophyllidea) or is a blind sac (Order Cyclophyllidea) which accumulates the developing eggs. The vagina carries sperms from genital atrium to the oviduct. Sperms are stored in an enlargement of the vagina called seminal receptacle. The ovary produces eggs which are fertilized in the region where the vagina and oviduct join. Then eggs pass into the ootype where capsules are formed around the eggs through secretions from the vitelline gland. The ootype is also surrounded by the Mehlis’ gland whose secretion helps in the formation of the egg capsule.
The reproductive organs degenerate after storage of egg. Therefore, gravid proglottids are called as “bags of eggs”. In species where uterus opens to the outside of the proglottid through a uterine pore; eggs are expelled through this opening and are discharged into the host’s intestine. The eggs are
released but proglottids are not continuously lost, therefore, the adult tapeworms become very long.
However, in species where the uterus is a blind sac, the developing eggs accumulate in the uterus which becomes distended with eggs and fills the entire medullary region of the proglottid. In this case, this gravid proglottid later becomes detached from the strobila and is discharged from the host with the host’s feces.
DEVELOPMENT
Egg: The egg contains the larval form -‐oncosphere which bears 3 pair of hooks and is surrounded by an inner envelope that in turn is surrounded by another membranous structure, the embryophore. A cellular zone known as the outer envelope lies between the embryophore and the shell (capsule), It is usually the outer most covering of the egg
There are two types of eggs-‐
1. Operculated, which contain immature embryo when voided to the external environment.
2. Non-‐operculated, which contains a fully developed embryo (embryonated) when voided to the external environment.
Tapeworms eggs exhibit certain variations on this basic theme and can be classified into 3 types:
1. Pseudophyllidean Egg -‐ The fully developed egg has a thick, quinone-‐tanned shell, usually with a lid-‐like operculum at one end. Numerous vitelline cells are associated with the zygote, providing stored food for subsequent development. The zygote develops into an oncosphere, which is covered by a ciliated embryophore that enables it to swim upon hatching. This form of organism is called a coracidium (pl. coracidia). E.g. e.g. Diphyllobothrium
2. Dipylidean Egg-‐ Possesses a thin shell, a thin nonciliated embryophore, and a relatively thick outer envelope. e.g. Dipylidium , Hymenolepis
3. Taenioid/Cyclophyllidean Egg-‐ The shell and outer envelope are lacking, and the thick, nonciliated embryophore forms the outermost covering e.g. Taenia , Echinococcus
LARVAL FORMS First larval stages:
Coracidium (Order Pseudophyllidea):
It is the ciliated, first stage Free swimming larva of Pseudophyllidea and other cestodes with aquatic life cycles. The hexacanth larva, bearing six hooks is enclosed within a ciliated covering called embryophore. The small, ciliated coracidium larvae are hatched from mature eggs, and swim about until
Pseudophyllidean Dipylidean Taenoid/Cyclophyllidean
ingested by crustaceans. It is ingested by the first intermediate host, usually an aquatic crustacean (copepod). In the intestine of this host, the coracidia shed their ciliated coats and penetrate the gut wall by using hooks and develop into next larval stage, the procercoid.
Oncosphere (Order Cyclophyllidea):
Also known as the hexacanth larva, the onchosphere bears six hooks, and remains passive in the egg shell or the unciliated embryophore(a flexible membrane) until the embryo is ingested by a vertebrate or invertebrate intermediate host. When ingested by invertebrate intermediate host, usually an arthropod , the onchosphere hatches and penetrates through the hosts gut wall into the haemocoel where it develops into a solid metacestode known as a cysticercoids. When ingested by the vertebrate intermediate host, the onchosphere penetrates the hosts intestinal lining, enters a venule & develops into a metacestode known as bladderworm or cysticercus
Procercoid (Orders Pseudophyllidea & Diphyllidea)
It is the first larval stage of some tapeworms, which usually develops inside the body cavity of copepods. Develops from coracidia in the intestine of first intermediate host (copepod e.g.Cyclops.) has elongated, oval, solid bodied, Retain the six larval hooks on a caudal protuberance known as cercomer.
It is the intermediate stage of members of Pseudophyllidea e,g. Diphyllobothrium latum Second larval stages:
Metacestodes are the pre-‐adult stage that occurs in the intermediate host. Metacestodes are of following types-‐
1. solid-‐ plerocercoid, cysticercoids
Plerocercoid-‐It is the Second metacestodal stage in Cotyloda. It develops from procercoids and is formed when the first intermediate host is ingested by a second intermediate host. It has an Elongated, solid body and bears an adult scolex. e.g. Diphyllobothrium latum
Some plerocercoids (e.g. Schistocephalus) may show advanced development of the genitalia.
Cysticercoid-‐ has a single non-‐invaginated (evaginated) scolex withdrawn into a small vesicle with practically no cavity and fluid but filled with calcareous corpuscles. bladder is slightly developed and it has a solid posterior portion. The Scolex and neck bud from bladder. Caudal appendage-‐ sometimes is presented with oncosphere hooks. E.g. Moniezia, Dipylidium, Anoplocephala & poultry tapeworms. Found in invertebrates such as fleas, oribatid mites, insects, lice etc.
2. Cystic-‐with true bladder
a. Cysticercus-‐ it is a small sac like vesicle which contains a single armed scolex invaginated in a large fluid filled vesicle/bladder. Also called bladderworm. The vesicular wall is a complex structure made up of three distinct layers. The outermost is a smooth and undifferentiated layer called cuticular mantle. The middle is composed of cells that resemble epithelial cells. The innermost is made up of muscle and other fibres. Inside the vesicular wall is an invaginated (facing inward) scolex. The scolex contains suckers and hooks, and a neck attached to a rudimentary body segment .. Its surrounded by fibrous capsule. Inside host tissue they settle in muscles. When humans eat raw or undercooked pork or beef that is contaminated with cysticerci, the larvae grow into adult worms inside the intestine. They are also found in muscles of tongue, diaphragm,
spleen, liver, mesentry. Larval form of Taenia sp. E.g. Cysticercus cellulosae in pigs (larval stage of Taenia solium), Cysticercus tenuicollis in sheep (larval stage of Taenia hydatigena).
b. Coenurus-‐ numerous invaginated scolices in clusters present on internal surface of a large fluid-‐filled cavity. Viable cysts have many , often hundreds of protoscolices. The bladder of coenurus has budding off daughter bladders, either internally, floating in cystic fluid, or externally, attached by stalks. In sub cutaneous tissues cysts are often unilocular-‐having single compartment or cavity. In central nervous systems, cysts are frequently multilocular with many cavities. Sometimes cysts have multiple irregular vesicles with a grape-‐like appearance. e.g. Coenurus cerebralis in the brain and spinal cord of sheep. It is the larval Stage of Taenia multiceps
c. Hydatid-‐ Large sized cyst, with outer thick cuticular layer and inner embryonic or germinal layer, which contains fluid. It possesses numerous daughter cysts and brood capsules each containing 10-‐30 protoscolices. Germinal layer also has number of scolices developing on it either exogenously or endogenously. If a cyst ruptures within host, each liberated protoscolex can produce a daughter cyst. Daughter cysts and individual scolices may be seen floating in the fluid (Hydatid sand). It is generally found in visceral organs of vertebrates including man. E.g. Echinococcus granulosus
