PARASITOLOGY

n 1.5.9 LIFE-CYCLES USING THREE HOSTS

The trematode Paragonimus westermaniadult lives in the lungs of dogs, cats and man. The

eggs are either expelled from the throat or they pass via the trachea and oesophagus into

the intestine where they pass out with the faeces.

From the eggs miracidia hatch out in fresh water. The miracidia penetrate snails of Melania

spp. A multiplicative phase occurs within the ‘liver’ of the snail. First sporocysts are pro-

duced and from these emerges the next phase, the redia. The redia in turn give rise to

cercaria which are released into the water and penetrate into a fresh water crustacean

Astacus japonicus(a crayfish) or into a fresh water crab Eriocheir japonicus. In the muscle of

the crustacean they encyst into metacerceria where they remain until the host is eaten.

PARASITOLOGY

• Figure 1.11A three-
  host life-cycle with two
  free-living dispersal
  phases. Reproduction
  occurs within the first
  intermediate host, which
  may be interpreted as
  asexual or parthenogenetic.
  No reproduction occurs in
  the second intermediate
  host.

DH internal accumulationIH IH

D D

?

DH internal accumulationIH

D

• Figure 1.10An
  egg/larva escapes from
  the definitive host into
  the environment for
  dispersal. Once inside the
  intermediate host the
  parasite undergoes asexual
  reproduction. The definitive
  host eats the intermediate
  host or the larvae escape
  from the intermediate host
  and remain free-living until
  they infect the definitive
  host.

Ligula intestinalisis a pseudophylidean tapeworm, the adults live in the gut of fish-

eating birds such as herons, pelicans, ducks, terns and gulls. Eggs pass out into water and

hatch into a coracidium larva that is eaten by a crustacean, Cyclopsspp, the first intermediate

host. A procercoid develops within the Cyclopswhich is then eaten by a fish, Rutilus

rutilus, the common roach, the second intermediate host in which the plerocercoid

develops. The fish is then eaten by a bird, the definitive host, and develops into an adult

worm.

Other similar examples are the cestodes Schistocephalus solidusand Diphyllobothrium

spp, except that in the latter example fish-eating mammals tend to be the definitive

hosts.

n SUMMARY

The concepts of parasitism, what is a parasite and the different types of parasitism are

discussed and described. The differences between a free-living animal and a parasite are

emphasised. How parasites invade and escape from their host is outlined. The variations in

parasite life-cycles are outlined using appropriate examples. There are parasites that have

a direct life-cycle (ie only one host) and there are those that have more than one host