Platyhelminthes (Gr. platys: flat, helmins: worms). The term "worm" is loosely applied to elongated invertebrate animals without appendages.
Key characteristics:
Triploblastic: Three germ layers.
Dorsoventrally flattened body. Oral and genital apertures mostly on the ventral surface.
Acoelomate: No internal space other than the digestive tube; space between organs filled with parenchyma.
Digestive system incomplete (no anus) and may be absent in some species.
Sexes separate or hermaphrodite.
Digestive cavity has one opening for intake of nutrients (ingestion) and removal of undigested wastes (egestion). Food cannot be processed continuously.
A phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates.
Acoelomates: Lacking a body cavity.
No specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion.
Classification of Platyhelminthes
Class Turbellaria
Class Trematoda
Class Cestoda
Class Trematoda: Fasciola hepatica
Reproductive System:
Female:
Female genital opening
Uterus
Ovary
Oviduct
Oötype
Mehlis gland
Laurer's Canal
Transverse vitelline duct
Longitudinal vitelline duct
Vitelline gland
Genital atrium
Male:
Male genital opening
Cirrus pouch
Penis
Vesicula seminalis
Vas deferens
Testes
Morphology of adult Fasciola hepatica:
Large, leaf-shaped fluke, reaching a size of 30 \, mm long and 13 \, mm wide.
Body widens at the base of the oral cone
Oral sucker
Cecum
Ventral sucker
Uterus
Life Cycle of Fasciola hepatica
Unembryonated eggs are passed in feces (diagnostic stage).
Embryonated eggs in water.
Miracidia hatch and penetrate the snail (Lymnaea sp., the intermediate host).
In snail tissue: sporocysts, rediae, and cercariae develop.
Cercariae are released from the snail.
Metacercariae encysted on water plants (infective stage).
Humans, sheep, or cattle ingest metacercariae on water plants (final host).
Excystation occurs in the duodenum.
Young flukes migrate to the liver and then to the hepatic biliary ducts where they mature into adults.
Fascioliasis
Symptoms:
Abdominal pain
Vomiting
Anemia
Fever
Headache
Schistosoma (Bilharzia)
Five species cause human schistosomiasis:
S. haematobium
S. mansoni
S. japonicum
S. intercalatum
S. meckongi
Persons suffering from schistosomiasis are usually anemic and have diarrhea, enlarged spleen, and enlarged liver.
Infective stage: cercaria
Main host: man
Intermediate host: snail
Infected site: mesenteric or pelvic blood vessels
Disease: schistosomiasis
Adult worms mate in small vessels of the portal circulation in the liver.
S. haematobium (urinary schistosomiasis) lays eggs in the urinary bladder.
S. mansoni (intestinal schistosomiasis) lays eggs in the wall of the rectum and colon.
Schistosomiasis Life Cycle
Eggs hatch, releasing miracidia.
Miracidia penetrate snail tissue.
Sporocysts develop in the snail (successive generations).
Cercariae are released by the snail into the water and are free-swimming.
Cercariae penetrate the skin (infective stage).
Cercariae lose tails during penetration and become schistosomulae.
Schistosomulae enter the circulation.
They migrate to the portal blood in the liver and mature into adults.
Paired adult worms migrate to:
Mesenteric venules of bowel/rectum (S. mansoni, S. japonicum) laying eggs that circulate to the liver and are shed in stools (diagnostic stage).
Venous plexus of bladder (S. haematobium) laying eggs that are shed in urine (diagnostic stage).
Life cycle:
Eggs reach canal water and hatch, giving miracidia which invade the snail intermediate host.
After 3-5 weeks, cercariae get off the snail and swim in water, reaching the main host (man) to penetrate the skin.
Cercaria after penetration is called Schistosomula.
They reach the right side of the heart via venous blood vessels & lymphatic's, then to the aorta.
Then will be distributed all over the body.
Only those which reach the liver via the hepatic artery will reach maturity, and all other schistosomula will die.
Acute Schistosomiasis
Occurs 6-9 weeks after infection, i.e., at the onset of egg deposition.
Symptoms of Schistosomiasis
Schistosoma haematobium:
Calcification of the bladder
Dilation of the ureter
Tumors
Carcinoma of the bladder
Schistosoma mansoni:
Colonic polyps
Granuloma in the liver
Visceral bleeding
Block in portal flow
Weight loss
Anemia
Bloody diarrhea
Intestinal obstruction
Growth retardation
Liver fibrosis
Nausea
Fever
Class Cestoda: Tapeworms
Subclass: Eucestoda
Taenia sp. (T. saginata & T. solium)
Tapeworms are endoparasitic in the intestine of man (definitive host).
Cattle and pigs are the intermediate hosts of T. saginata & T. solium respectively.
In the majority of cases, only one worm is present in the intestine of the host.
A disease known as cysticercosis may occur.
Morphology:
A Cestode is usually differentiated into 3 parts:
The scolex or the head, which is found at the anterior end and carries suckers.
The scolex of T. saginata “beef tapeworm” is provided with 4 suckers.
The scolex of T. solium “pork tapeworm” is also provided with four suckers, in addition to a rostellum.
The neck is the region of growth from which all the distal portions are derived.
The strobila consists of segments or proglottids, beginning with immature proglottids, then mature ones (which contain fully developed sex organs), and finally gravid proglottids (which are the reservoir of eggs).
Taenia Species Morphology
Taenia solium (Pig Tapeworm):
Scolex with rostellum and hooks, suckers, neck
Gravid proglottids
Taenia saginata (Beef Tapeworm):
Scolex with suckers and neck
Gravid proglottids
Nerve cord
Excretory canal
Genital pore
Uterus filled with eggs
Mature proglottids of Taenia sp.:
Female:
Uterus
Vagina
Receptaculum seminis
Oviduct
Ovary
Oötype
Mehlis gland
Vitelline duct
Vitelline gland
Male:
Testes
Vas efferens
Vas deferens
Vesicula seminalis
Muscular Cirrus (Penis)
Male genital opening
Genital atrium
Female genital opening
Lateral nerve cord
Longitudinal excretory canal
Transverse excretory canal
Life Cycle of Taenia
Gravid proglottids pass with feces.
Eggs are eaten by the intermediate host (pig for T. solium, cattle for T. saginata).
In the intestine of the intermediate host, the embryophore decays, and the oncosphere migrates to the intestine.
The oncosphere forms a bladder worm (cysticercus).
In the human stomach, digestion of the cyst occurs, and evagination of the scolex happens in the intestine of the host.
Formation of scolex occurs, and the adult worm resides in the intestine.
Life Cycle of Taenia saginata and Taenia solium
Taeniasis (Cysticercosis) is the disease caused to humans by infection with the adult tapeworm Taenia saginata or Taenia solium.
Humans are the only definitive hosts for T. saginata and T. solium.
Eggs or gravid proglottids pass with feces. The eggs can survive for months in the environment.
The intermediate hosts, cattle (T. saginata) and pigs (T. solium), become infected by ingesting vegetation contaminated with eggs or gravid proglottids.
In the animal stomach, the embryophore is digested, and the hexacanth embryo is liberated in the lumen of the intestine.
The embryo invades the intestinal wall and migrates to the striated muscles, where they develop into cysticerci.
A cysticercus can survive for several years in the animal.
Humans become infected by ingesting raw or undercooked infected meat.
In the human intestine, the cysticercus develops over 2 months into an adult tapeworm, which can survive for years.
The adult tapeworms attach to the small intestine by their scolex and reside in the small intestine.
The adults produce proglottids which mature, become gravid, then detach from the tapeworm, and migrate to the anus, passing with the stool (approximately 6 per day).
The eggs contained in the gravid proglottids are released after the proglottids pass with the feces.
T. saginata may produce up to 100,000 eggs, and T. solium may produce 50,000 eggs per proglottid, respectively.