PARASITOLOGY

Female forms penetrate the intestinal villi and eventually produce eggs. Parasitic

males are apparently unable to penetrate the mucosa.

n 1.5.5 PARASITES WITH A SEXUAL PHASE IN ONE HOST (THE

DEFINITIVE HOST) AND LARVAL DEVELOPMENT BUT NO ASEXUAL

OR INTERNAL ACCUMULATION IN THE INTERMEDIATE HOST

This type of life history (see Fig. 1.7) involves metamorphosis of the larva in a different

host from that inhabited by the adult parasite:

n Cestodes (the tapeworms) — those that do not have an asexual multiplicative phase

— for example Taenia solium, T. saginata, Dipylidium caninum.

n Nematodes such as Dracunculus medinensis; Wuchereria bancrofti.

n Almost of the Acanthocephala (spiny headed worms).

PARASITOLOGY

Entry Host Exit

• Figure 1.6Only adults
  of one sex (usually
  parthenogenetic females)
  are found within the host.
  Both male and females
  develop during the free-
  living stage.

Entry DH IH

D

IH = intermediate host

DH= definitive host

• Figure 1.7An indirect
  life-cycle involving at least
  two hosts, a definitive host
  and an intermediate host
  (IH). The eggs/larvae are
  released from the
  definitive host for
  dispersal. These stages
  infect the intermediate
  host from which further
  larval stages are released
  to invade the definitive
  host. Adult cestodes are hermaphrodite and live within the gut of a vertebrate. Eggs are
  passed out via the faeces into the external environment — a dispersal phase. The eggs
  are eaten (accidentally) by the intermediate host. They hatch out and migrate into the
  host tissue and develop into the metacestodes stage. They remain within the intermedi-
  ate host until it is eaten by the definitive host. These parasites have become adapted to a
  predator–prey relationship between the definitive host and the intermediate host.
  The nematodes that have life-histories within this category have separate sexes but
  both are within the same definitive host. The fertilised females produce live larvae
  (microfilariae) which circulate round the body via the body fluids or migrate through the
  connective tissues. The larvae are dispersed to new hosts via vectors, usually a blood suck-
  ing arthropod.
  Trichinella spiralis, a nematode — the mature adults live within the gut lumen. The
  female produces live larvae that migrate to striated muscles, the adults are expelled
  from the gut after about 14 days. The larvae encyst within the striated muscles and
  remain there until the host is eaten by the next host. In this example the same host is
  first the definitive host and then becomes the intermediate host.