n Cysticercus (Commonly known as a bladder worm)This a fluid-filled cyst containing a
single invaginated scolex and normally only develops in vertebrates, in particular
herbivorous mammals. Some taenioid species produce external buds and hence
undergo a form of asexual multiplication within the intermediate host.
n StrobilocercusThis metacestode is found mainly in the liver of rodents. There is small
bladder from which a solid structure extends with a scolex at its end.
n TetrathyridiumA small solid structure with four suckers and a scolex without hooks.
Although normally a solid structure, it can under certain circumstances can become
cystic and fluid-filled. In certain genera the tetrathyridia divide by longitudinal fission
or by a process of budding.
n Hydatid cystA fluid-filled cyst whose inner membrane gives rise to protoscoleces
enclosed within vesicles known as brood capsules. The inner walls of the brood cap-
sule continue to produce more protoscoleces. The formation of the brood capsules can
be regarded as a form of internal budding.
n Coenurus cystA fluid-filled cyst where the scoleces are formed in groups on the cyst wall.
Secondary external cysts develop and usually remain attached to the original cyst to
form a ‘bunch of cysts’.Whatever the form or shape of the larval or metacestode stage, they remain within
the intermediate host until it is eaten by the future definitive host. When a metacestode
is in its usual or most commonly encountered intermediate host, it nearly always
migrates to a predetermined or preferred site. However when the metacestodes are
swallowed by an animal other than the usual host, in many instances they still develop
but may end up in ‘unfamiliar sites’ often causing considerable problems to this host. For
example the cysticercus of Taenia soliumis normally found in pig muscles, but in man it
can migrate to the brain and cause neurocysticercosis. The hydatid cyst of Echinoccocus
granulosusis the cause of hydatid disease in man.n 3.9 CYCLOPHYLLIDEA OR TAENIOIDEA (TAPEWORMS)
These are parasites of birds and mammals. The scolex consists of four acetabula
(suckers); there are no uterine pores; there is a single compact vitellarium posterior to
the ovary.
Examples:
Taenia saginata(the beef tapeworm). Definitive host (the host in which sexual matu-
rity occurs) is man and the intermediate host is cattle. Eggs pass from the gut onto grass
and are eaten by grazing cattle. The larva hatches and develops into a metacestode,
migrates through the gut wall and settles in striated and heart muscle. It develops into a
cysticercus (bladderworm) and remains until eaten by man.
Taenia solium(the pork tapeworm). Definitive host is man and the intermediate host
is pig. Cysticercus develops in striated and heart muscle.
Echinococcus granulosus. Definitive host is a carnivore (mainly domestic dogs) and
the intermediate host can be man, sheep, horses, cattle, camels, etc. The adult is the
smallest of the tapeworms — only three proglottids. Eggs swallowed by the inter-
mediate host hatch in the small intestine. Metacestode migrates through gut wall into
the abdominal cavity where it attaches to the liver or other viscera and develops into
a hydatid cyst.n 3.10 TREMATODES
There are three main orders: Aspidogastrea; Monogenea; Digenea.PARASITOLOGY