PARASITOLOGY

The cestodes are protandrous and it is thought that the cross-fertilisation takes place

between proglottids.

n 3.8.4 CESTODE LIFE-CYCLES

The cestodes require an intermediate host to complete their life-cycle. The eggs, having

passed out into the environment via the faeces of the definitive host, are swallowed by

the intermediate host. One major exception is Hymenolepis nanawhere, if the eggs are

swallowed by the mammalian definitive host (mostly small rodents but man can also

become infected), they can develop directly into adults and reinfect the same host or a

similar host living in close association.

All types of vertebrates (homeothermic and poikilothermic) and arthropods are

used by cestodes as intermediate hosts. There is far less host-specificity with regard to

the intermediate hosts than there is with the definitive hosts. However there is a special

and environmental relationship between the two types of host. The intermediate is

almost invariably the prey of the definitive host (see Fig. 3.5).

The embryo that hatches out of the egg is conventionally referred to as the larva. The

term metacestode was introduced in the early 1950s to describe the stage that the

PARASITOLOGY

Vagina

Ovary

Vitelline duct

Uterus

Seminal receptacle

Mehlis gland

Vitelline gland

Intermediate host eaten by

definitive host

Adult worm in gut of vertebrate

Eggs pass out with

faeces

Eggs swallowed by

intermediate host

Metacestode stage develops in the

tissues or body cavity of

intermediate host

• Figure 3.4The details of
  the female reproductive
  system. The vitelline gland
  and the Mehlis gland are
  thought to be associated
  with the production of the
  egg membranes.


• Figure 3.5All adult
  tapeworms live in the gut
  or associated organs of
  vertebrates. The scolex
  becomes embedded within
  the gut mucosa and the
  rest of the ‘body’ lies
  unattached in the gut
  lumen. Cross-fertilisation
  between proglottids
  probably occurs. Eggs
  pass out via faeces and
  are eaten by the
  intermediate host. The
  larva hatches out of the
  egg and develops into
  a metacestode which
  remains within the
  intermediate host until it
  is eaten by the definitive
  host.