PARASITOLOGY

n 1.5.6 ASEXUAL REPRODUCTION WITH INTERNAL ACCUMULATION
BUT NO SEXUAL REPRODUCTION AND NO EXTERNAL PHASE

n Trypanosomidae (extracellular blood parasites) are infected via arthropod vectors and

reproduce asexually in both hosts and sometimes twice within one life-cycle.
n Piroplasmidae are intracellular blood parasites transmitted by Ixodidticks. Asexual

reproduction occurs within red blood cells of the vertebrate host and multiple fission

takes place within the tick host, resulting in an accumulation of parasites within the

tissues of the tick. The parasites are passed on to the next generation of ticks via the

tick eggs.

n 1.5.7 LIFE-CYCLES WITH TWO CHANGES OF HOST BUT WITHOUT
ANY INTERNAL ACCUMULATION
This occurs only in few acanthocephalans eg Corynosoma stramosum.

INTRODUCTION TO PARASITOLOGY

Host

asexual multiplication

Host

asexual multiplication

• Figure 1.8A two-host
  indirect life-cycle. Sexual
  reproduction has not been
  detected in either host.
  The parasites are capable
  of asexual reproduction in
  either or both hosts.

DH IH IH

D

• Figure 1.9A three-host
  indirect life-cycle. One
  definitive host and two
  intermediate hosts (IH).
  There is a free-living
  dispersal stage between
  the definitive host and the
  first intermediate host. The
  second intermediate host
  is the prey of the definitive
  host.

n 1.5.8 LIFE-CYCLES THAT HAVE MORE THAN ONE HOST WITH BOTH
SEXUAL AND ASEXUAL PHASES AND INTERNAL ACCUMULATION
This type of reproduction is found among a limited group of cestodes and the digenean
trematodes:

n In the Cestoda, the metacestode stage undergoes asexual reproduction by budding or
fission within the intermediate host eg Taenia crassiceps, Mesocestoides corti, Echinoccocus
granulosus, Multiceps multiceps.
n A large number of metacestodes accumulate within the intermediate host. The main
dispersal phase occurs with eggs passing out of the definitive host.
n Digenean trematodes eg Fasciola hepatica, Schistosomaspp. The adults live within the
definitive host and produce eggs which pass out via the faeces. In fresh water motile
larvae hatch from the eggs and penetrate a mollusc (intermediate host). They undergo
a multiplicative phase and release cercaria from the mollusc which penetrate the
definitive host.