PARASITOLOGY

PARASITOLOGY

swallowed, end up in the gut where the larvae hatch out and then proceed to undergo

the migratory phase. All ascarid larvae have a compulsory migratory route which starts

where the adult parasite eventually ends up, ie the lumen of the ileum. It is interesting

to speculate as to why the larvae appear to be compelled to undergo the migratory

phase. If Ancylostomalarvae are accidentally swallowed instead of having to penetrate

through the skin they remain in the gut and do not migrate round the body.

An explanation for the migration of ascarid larvae could be that each larval stage

requires a different physiological and biochemical environment; and the different organs

encountered en route represent intermediate hosts. Alternatively, migration could be an

escape from the host’s immune system.

n 4.1.4TRICHINELLA SPIRALIS

The adult stages of this nematode are found in the gut and larval stages in striated

muscle. The host of T. spiralisis first the definitive host and then the same host serves

as an intermediate host (see Fig. 4.3).

n The adults live in the small intestine. The females are 3 mm long and the males

1.5 mm. The worms mature rapidly and copulation takes place after about 30 h.

Female worms have been found to be inseminated after 36 h. Each female gives birth

to about 1,000 live larvae.

n The newborn larvae burrow through the gut mucosa into the lamina propria and are

then distributed around the body via the blood circulation. Further development of

the larvae only occurs within striated muscle.

n After reproduction and the birth of live larvae the adult worms are eventually actively

expelled from the gut.

Gut Male and female worms

Striated muscle

Encapsulated larvae

nA. Life-cycle of Trichinella spiralis

Nurse cells

Fibrous sheath

Larva

Larva encapsulated in striated muscle

Connective tissue

Striated muscle

nB. Trichinella spiralis

• Figure 4.3A.
  Trichinella spiralisis a
  widely distributed
  nematode that uses the
  same host as both an
  intermediate and definitive
  host. The mature male and
  female adults are found
  within the gut. The female
  gives birth to live larvae
  which then burrow through
  the gut wall and migrate
  to and encyst in striated
  muscle. The next larval
  phase develops when the
  muscles are eaten.
  B. A larva of T. spiralis
  encysted within host
  striated muscle. The larva
  adopts a characteristic
  spiral shape and is
  surrounded by nurse cells
  enclosed within a fibrous
  sheath.