Echinococcus granulosus - Introduction, Classification, History, Habitat, Morphology

Introduction of Echinococcus granulosus

The genus Echinococcus has four species that cause infection in humans- Echinococcus granulosus, Echinococcus multilocularis, Echinococcus vogeri, and Echinococcus oligarthrus. Echinococcosis is an infection caused by both the adult and larval stages while hydatid is an infection caused by the larvae (metacestode) of E. granulosus and E. multilocularis.

Echinococcus granulosus, also known as hydatid tapeworm or dog tapeworm, is the parasite that causes echinococcosis in man and other herbivores. Hydatid cyst is the larval stage of this parasite which is highly toxic and antigenic to humans.

Classification of Echinococcus granulosus

Phenotypically, Echinococcus granulosus is classified as:

Kingdom: Animalia

Phylum: Platyhelminthes

Class: Cestoda

Order: Cyclophyllidea

Family: Taeniidae

Genus: Echinococcus

Species: E. granulosus

History of Echinococcus granulosus

Echinococcus granulosus, in its adult form, was first described in 1695 by Hartmann from a dog intestine. Almost a century later, in 1782, the larval stage (hydatid cyst) was demonstrated by Goeze. Von Shiebold first described the life cycle of the parasite in 1853 while Naunyn demonstrated the hydatid cyst in a man’s bladder in 1893 proving it the worm stage of Echinococcus granulosus.

Habitat of Echinococcus granulosus

The adult tapeworms are found in dogs and wild canines where they remain attached to the wall of the intestinal mucosa.

The larval stage (hydatid cyst) of Echinococcus granulosus habitats in humans as well as herbivorous animals.

Morphology of Echinococcus granulosus

The morphological forms of Echinococcus granulosus include the adult form, larvae, eggs, and cysts.

Adult form

• small - measures 3mm to 6mm in length

• Echinococcus granulosus inhabits the small intestine of dogs

• present in large numbers attached to the wall of the intestinal mucosa

• consists of a head (scolex), neck, and body (strobila)

Scolex

• Echinococcus granulosus scolex is pyriform in shape

• measures 300μm in diameter

• has four suckers and two rows of hooks

Neck

• Echinococcus granulosus neck is short and thick

Strobila

• Echinococcus granulosus strobila consists of three segments or proglottids

• first segment: immature

  second segment: mature – narrowest proglottids

  third/terminal segment: gravid – largest and broadest (2mm to 3mm in length and 0.6mm in breadth)

  fourth segment: (if present) always gravid

• the third segment contains several hundreds of eggs, does not have a uterine opening, and thus ruptures to release eggs before or after passing with stool

Larva

The larval form of Echinococcus granulosus is also known as the Hydatid cyst.

Hydatid cyst

• Echinococcus granulosus hydatid cyst found in organs of man and other herbivores

• contains scolex which is invaginated within a vesicular body

• in humans, the cyst is unilocular, subspherical, and filled with fluids

• at the end of one year, it measures around 5cm in diameter

• cyst wall has two layers-

  an outer laminated layer (ectocyst)

  an inner germinal layer (endocyst)

• ectocyst is tough elastic later, hyaline, milky, opaque, non-nucleated, laminated, measures 1mm thick, and is selectively permeable to macromolecules

• the vital layer of the cyst- the endocyst is cellular, nucleated, measures 22nm to 25nm in thickness, gives rise to brood capsules with scolices, secretes hydatid fluid, and gives rise to the outer layer

• some cysts called acephalocysts or sterile cysts lack brood capsules and even if the brood capsules are present- they lack scolices

Hydatid fluid

• Echinococcus granulosus hydatid fluid is a clear and colorless fluid that fills the interior of the cyst

• nutritive as it provides nourishment for the growing brood capsules and scolices

• is slightly acidic (pH 6.7), has a low specific gravity (1 to 1.010), and is rich in electrolytes (NaCl 0.5%, sodium sulfate, and salts of succinic acid

• hydatid sand, a large number of brood capsules and protoscolices, can be seen floating in the Echinococcus granulosus hydatid fluid- resembling sand grains

• fluid is highly toxic and antigenic

• if a cyst ruptures inside the body, an anaphylactic reaction may prove fatal

Growing cysts

The growth rate of Echinococcus granulosus cysts is highly variable with an average growth of 1cm to 1.5cm per year. It is surrounded by three layers:

• the outer layer (pericyst) – consists of only fibrous tissues and does not form any organic part of the cyst

• middle layer – contains numerous fibroblasts, eosinophils, and newly formed blood vessels

• inner later – contains radically arranged giant cells and eosinophils

Infective form

Eggs

• infective form of Echinococcus granulosus

• found in the feces of the dog

• infective to man, sheep, cattle, and other intermediate hosts which are mostly herbivores

• measures 31μm to 43 μm in diameter

• spherical shape and brown in color

• has two layers- an outer thin wall and inner embryophore

• each egg has a hexacanth embryo with three pairs of hooklets

• but the eggs are not morphologically different from that of the Taenia species