Parasitology Department General Characters of Trematodes By Hala Elwakil, MD
Intended Learning Outcomes By the end of this lecture, the student will be able to know: 1. General morphology of trematodes 2. Development of trematodes 3. Life cycles of trematodes.
Phylum: Platyhelminthes Trematoda Cestoda Phylum: Nemathelminthes Nematoda
Features of digenic trematodes Digenic trematodes are unsegmented leaf-shaped worms that are flattened dorsoventrally. The body is covered with cuticle
They bear 2 suckers, one surrounding the mouth (oral sucker) and another on the ventral surface of the body (ventral sucker). These serve as the organs of attachment. A genital sucker may be present surrounding the genital opening
The sexes of the parasites are not separate (monecious). An exception is schistosomes, which are diecious (unisexual).
The alimentary canal is incomplete, and no anus is present. - mouth -oesophegus -intestinal caeca Nutrition is taken from the surrounding fluids
The excretory system is bilaterally symmetrical. It consists of flame cells and collecting tubes. These flame cells provide the basis for the identification of the species. 6. small excretory tubulus 7. collection tube 8. excretion bladder 9. excretory pore
The reproductive system consists of male and female reproductive organs and is complete in each fluke. Female organs 18-Ovary 19-Seminal receptacle Oviduct 20-Laurer s canal 21-Ootype 22-Mehli s gland 23-uterus 24-yolk gland 25-common yolk channel Male organs 10-Testis 11-Vas efferens 12-Vas deferns 13-Seminal vesicles 14-Prostate glands 15-Cirrus 16-Cirrus sac
Most trematodes are hermaphrodites and thus possess both male and female genitalia. They have a cirrus (penis). The function of the Laurer canal is probably a vestigial vagina. Cross-fertilisation and self-insemination are both possible. There are exceptions, e.g. schistosomes have separate sexes. the ovary the eggs are fertilized and subsequently surrounded by yolk in the ootype Secretions from the Mehlis gland are added to the egg to form eggshells. The eggshells then undergo a chemical reaction, a kind of tanning process, which makes them tough and harder. In this way the egg acquires its typical form, and becomes more resistant to conditions in the outside world, which are often unfavourable.
The flukes are oviparous. They lay operculated eggs. An exception is schistosome eggs, which are not operculated. All have complicated life cycles, with alternating asexual and sexual developments in different hosts
Egg = Ovum - If the embryo is fully differentiated into a miracidium before the egg is released from the adult, an is embryonated produced. - If the embryo consists of a number of undifferentiated cells when the egg is released, an is unembryonated produced. embryonated egg unembryonated egg
Miracidium Ciliated embryo that hatches from the egg Miracidia that hatch in water: In some species, the egg is ingested by the snail and the miracidium hatches within the snail:
Miracidium Morphology: Small, elongate organism covered by cilia At anterior end is the Apical papillae that functions in contacting the snail intermediate host. When a snail is contacted, the penetration glands secrete enzymes to aid in embedding the miracidium within the snail's tissues. are common for photoreception. A mass of Germ cells lies in the posterior end. These cells divide asexually to form the next larval generation.
Miracidium Upon entering a snail, the miracidium loses its cilia and transforms into a sac-like body called the
Sporocyst Upon entering a snail, the miracidium loses its cilia and transforms into a sac-like body called the MOTHER SPOROCYST. Mother sporocysts occur in the snail at the site of penetration. Mother sporocysts consist of a Germ cells surrounding developing Germ balls mouth or digestive tract?
Sporocysts In some digenetic trematodes, the germ cells in the mother sporocyst divide asexually to give rise to another generation of sporocysts called. DAUGHTR SPOROCYST Mother sporocyst bursts to release these daughter sporocysts. In other trematodes, the germ cells of the mother sporocyst form a new larval stage called the Redia
Redia Each redia contains a Pharynx and short Digestive tract in the mother rediae may develop into a second generation of rediae called Daughter rediae or may develop into. cercaria
Cercaria CERCARIAE leave the sporocyst or redia, migrate out of the snail, Morphology: 1. Cercarial body - oral sucker and acetabulum are present - digestive tract is complete - excretory system is developed - many types of gland cells are present - 2 eyespots are common - no reproductive structures Furcocercous cercaria Microcercou s cercaria
Cercaria 2. used for swimming. - may be unbranched with or without fins - tail characteristics important in identification Microcercou s cercaria Type of cercaria is based on characteristics of the body and tail. We will look at 3 types: Furcocercous type forked tail Leptocercus type straight tail Microcercous type short tail Lophocercus type tail covered with membrane Furcocercous cercaria
Cercariae may: (1) penetrate the definitive host (schistosomes) (2) penetrate a second intermediate host and encyst as metacercariae, or (3) attach to aquatic vegetation and encyst as metacercariae
Metacercaria METACERCARIA is formed as a cercaria loses its tail and encloses itself in a Cyst is formed by: 1. 2. Reproductive organs may begin to form but eggs are never produced.
Metacercaria Metacercariae may occur within a second intermediate host or on aquatic vegetation. They are viable for weeks to years. When the second intermediate host or aquatic vegetation containing metacercariae is eaten by a definitive host, the metacercaria in the digestive tract and matures to the.