PARASITOLOGY

divide by fission. After a period of time some of the daughter parasites encyst and are
then expelled from the host via the faeces for dispersal.

n 1.5. 3LIFE-CYCLES WITH BOTH SEXUAL AND ASEXUAL
REPRODUCTION WITHOUT CHANGE OF HOST AND WITH
INTERNAL ACCUMULATION
Eimeriaspp and Isosporaspp (the Coccidia): the oocysts are swallowed and sporozoites
are liberated into the gut lumen. The sporozoites then invade the cells of the intestinal
epithelium and reproduce asexually by a process of schizogony which produces and
then releases numerous merozoites. Released intercellular merozoites invade healthy
epithelial cells and the process of schizogony is repeated. After at least one generation of
asexual reproduction some of the merozoites transform into male and female gamonts.
The nucleus of the male gamonts divide to produce numerous uninucleate biflagel-
late microgametes. The female gamont does not divide and remains as a uninucleate
macrogamete. The male microgamete penetrates the female macrogamete and the two
nuclei fuse to form a zygote. The zygote develops into an oocyst containing the next gen-
eration of infective sporozoites and is then expelled from the host.

INTRODUCTION TO PARASITOLOGY

Entry internal accumulationHost Dispersal

• Figure 1.4A direct
  life-cycle in which there
  is no apparent sexual
  reproduction. There is an
  accumulation of individuals
  within the host due to
  asexual reproduction
  before dispersal to the
  next host. (D =dispersal)

Entry Host Dispersal

Oocyst Oocyst

internal accumulation

• Figure 1.5A direct life-
  cycle. The infective stage,
  the oocyst, invades the
  definitive host. There may
  be an asexual increase in
  somatic forms that gives
  rise to the male and
  female gamonts. Fusion
  between the male and
  female gametes produces
  an oocyst.

n 1.5.4 LIFE-CYCLES WITH BOTH SEXUAL AND ASEXUAL
REPRODUCTION WITHOUT CHANGE OF HOST BUT WITHOUT
INTERNAL ACCUMULATION
The parasitic generation (endogenous) is represented by a parthenogenic female and the
free-living generation (exogenous) consists of ‘unisexual’ individuals (see Fig. 1.6). The
parasitic individual produces thin-shelled eggs which are either passed out via the faeces
(Strongyloides westeri) or hatch in the gut mucosa into larvae (S. stercoralisor Rhabditisspp)
which pass out of the host’s gut. In the soil two types development occurs: homogonic
(direct), or heterogonic (indirect):

n Homogonic. The larvae (rhabditoid type) develop directly into infective filariform

larvae ready to reinvade a new host.
n Heterogonic. The larvae within 2–5 days develop into either mature male or female

forms. After mating the females release rhabditoid eggs and the larvae hatching from

them molt into infective filariform larvae.