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microbiologyparasitic InfectionsParasitology

Echinococcus Granulosus-Life cycle-Habitat and Transmission

echinococcus granulosus signs and symptoms image reload

Echinococcus Granulosus:

Echinococcus granulosus is one of the smallest tape worms. it causes unilocular hydatid cyst in man.


Shape: small, segmented and tape like.
Scolex (head): Globular in shape, bears 4 suckers, and a protrusible rostellum with 2 circular rows of hooks,
Neck: Short and thick.
Stobila (body):
Echinococcus granulosus is consist of 3 segments. The 1st segment is immature and contains no genital organs. 2nd segment is mature and contains testes and ovaries, and the sacculated uterus lies laterally. The 3rd segment is mature and the sacculated uterus is filled with eggs hence it is the gravid segment.

shape: spherical
size: 31 - 43 micrometre in diameter
colour: brown
coverings: outer covering is consist of a transparent shell, it may be absent but when present, it causes the eggs to clump together. the inner covering is known as embryophore, it is brown in colour, thick-walled and radially striated.
The egg contains an oncosphere (14-20 micrometre in diameter) with 3 pairs of hooklets.

Larval Form:
it represents the structure of scolex of future adult worm that remains invaginated within a vehicular body. this is found within hydatid cyst which develops inside the intermediate host.

Life Cycle:
eggs are passed out with the faeces of the definitive host. these are swallowed by intermediate hosts during grazing. in the small intestine of the intermediate host, oncospheres (embryo) are liberated with the rupture of embryophore (inner covering of egg). about 8 hours after ingestion, oncosphere bore their way through the intestinal wall and enter the portal circulation and reach the liver, where they may settle and form hydatid cyst or they may enter into to systemic circulation and lodge in various organs. From the inner surface of the cyst number of brood-capsule develop which contain thousands of protoscolices. Hydatid cyst may rupture releasing protoscolices with invaginated heads into the surrounding tissue. A fully developed protoscolex is infective to the definitive host. when a definitive host (dog, fox, jackal, wolf etc) eats an infected liver of a dead sheep it becomes infected with the organism. in the GIT scolex head evaginates and attach to gut wall by means of its suckers, it grows into an adult worm in about 6-7 weeks and starts producing eggs. Eggs are passed in faeces, thereby repeating the cycle.

Human is dead end host because dogs have no access to the meat of infected human.
A: eggs are released with faeces
B: eggs are eaten by domestic animals while grazing
C: larvae infect various organs and form hydatid cysts
D: hydatid cyst contains infective protoscolices
E., when an animal dies dog, eats infected organ
F: protoscolices reach the dog's intestine and develop into the adult worm
and lay eggs which pass out with faeces.


Definitive Hosts:
dog, wolf, fox and jackal
Intermediate host, 
sheep, horse and other domestic animals
Dead end hosts:
Human beings.

via the faecal-oral route.

Read More
Hydatid cyst, pathogenesis, diagnosis and treatment.

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