PARASITOLOGY

n 4.7.2TAENIA CRASSICEPS

The adult T. crasscicepslives in the gut of a fox (Vulpes vulpes). It has a scolex with four

suckers and a rostellum with a double row of hooks. The adult is 12–20 cm long.

n The mature proglottids produce eggs which pass out with the faeces and are eaten by

a rodent.

n The oncosphere hatches out of the egg and then penetrates through the epithelial cells

into the peritoneal cavity of the rodent intermediate host.

n In the peritoneal cavity it develops into a cysticercus characteristic of the taeniids.

Each cysticercus (the metacestode stage) has an invaginated scolex with four suckers

and a rostellum and is capable of exogenous budding. Each bud develops into a cyst

containing an invaginated scolex. These cysts detach from the mother cyst and then

repeat the budding process. This can continue for the rest of the life of the inter-

mediate host.

n 4.7.3MESOCESTOIDES CORTI

The adult Mesocestoides cortiparasitises the gut of carnivores. The adult has four suckers

(acetabula) but no rostellum and is about 4–8 cm long. The complete life-cycle of this

parasite is not yet known.

n Eggs pass out via the faeces which are thought to be eaten by an orbatid mite, the first

intermediate host.

n The second intermediate host is a rodent or other small vertebrate including lizards

(Lacertesspp).

n Once the mite is eaten the metacestode, known as a tetrathyridium, develops in the

second intermediate host. The tetrathyridium is normally a solid structure but can

become cystic if conditions change.

n Each tetrathyridium has four acetabula and a rostellum and can produce an exogenous

bud which grows almost to the size of the original before separating and this gives the

appearance of having divided by longitudinal fission.

n Adult worms apparently also have the capability of asexual budding.

n 4.8 HELMINTH PARASITES WITH AN INTERMEDIATE HOST

AND TWO FREE-LIVING STAGES

4.8.1 Schistosoma spp

The Schistosomatidae are the one family of trematodes in which the sexes are separate.

The adult male is 10 mm long and the female 15 mm but much narrower. When both

are mature the female becomes permanently associated with the male which lives within

her gynocophoric groove.

n There are four species of Schistosomathat are infective to man and they are S. mansoni,

S. japonicum, S. haematobiumand S. intercalatum. All of these can cause schistosomiasis,

a disease more commonly known as bilharzia.

n S. mansonihas the most widespread distribution and also can be maintained in laborat-

ory animals.

n The four different species are identified by the external appearance of the eggs.

Except for S. japonicumthe eggs have a single spine but the position of the spine dif-

fers in each of the other three species.

PARASITOLOGY