PARASITOLOGY

PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE

or lungs, where it settles and develops into a hydatid cyst (the metacestode stage, see
Fig. 4.9).
n As soon as the oncosphere has become attached to tissue, it secretes a hyaline mem-

brane around itself. The membrane thickens and differentiates into an inner germinal
layer and an outer acellular layer.
n The germinal layer produces protoscoleces as well as more of the laminate layer. A

tissue layer surrounds the protoscolices to form a brood capsule. The brood cap-
sules, each containing several protoscoleces, detach from the germinal membrane
and remain with the fluid-filled hydatid cyst. The hydatid cyst continues to grow and
reaches a size of up to 10 cm in diameter. Each brood capsule if released from the cyst
is capable of forming secondary cysts.
n If the intermediate host plus the hydatid cyst is eaten by the definitive host the proto-

scoleces are released and each has the potential to develop into the adult tapeworm.

Scolex

Suckers

Immature proglottid

Proglottid with reproductive organs

Ripe proglottid with mature eggs

Egg

Host tissue

Cyst wall. Laminated layer

Germinal layer

Brood capsule with daughter

brood capsules

Protoscolex

Young brood capsule with

protoscoleces

• Figure 4.8The adult
  stage of the tapeworm
  Echinoccocus granulosus
  is the smallest of the
  tapeworms and consists
  of a scolex and three
  proglottids. The adult
  worms are parasites of
  domestic dogs and other
  canine species as well
  as some of the larger
  carnivores such as lions.


• Figure 4.9The hydatid
  cyst of E. granulosus
  is the largest of the
  tapeworm metacestodes.
  It is a large fluid-filled cyst
  and can measure up to
  10 cm in diameter. The
  innermost layer of the cyst
  is the fertile membrane
  from which arises the
  brood capsule containing
  protoscoleces.