PARASITOLOGY

All the members of this phylum are endoparasites. At certain stages in the life-cycle

all individuals possess an apical complex with a group of microtubules and organelles at

one end of the cell and they all have flagellated gametes.

There are two main groups:

n The gregarines, parasites of invertebrates.

n The coccidia, parasites of both vertebrates and invertebrates, the latter usually serv-

ing as the vector or intermediate host. Many members of this group live in the red blood

cells of vertebrates eg Plasmodiumspp, see Fig. 2.4.

n 2.5 BIOLOGY OF PARASITIC PROTOZOA

The parasitic protozoa have like all endoparasites become adapted to a very specialised

environment. However there are only a few morphological and anatomical adaptations

that are unique to parasitic protozoa. The endoparasitic forms are either intracellular or

extracellular.

The host cells that tend to be the most commonly parasitised are those rich in nutri-

ents and with a high metabolic rate:

n Epithelial cells which absorb digested nutrients from the gut.

n Erythrocytes with their potentially high oxygen content and haemaglobin and hepatic

cells rich in stored food.

PARASITOLOGY

Sporozoites invade gut mucosa

Merogony

Gamete formation

Male gametocyte

Male gametes

Female gametocyte

Repeat merogony

Oocyst hatches

in duodenum

Fertilisation

Oocyst released

into lumen

Resistant oocyst

shed in faeces

nSexual cycle

nAsexual cycle

• Figure 2.3Eimeria
  tennela, a parasitic
  protozoan that is
  commonly found in the
  gut of domestic chickens
  causing the disease
  coccidiosis. The digested
  oocysts release
  sporozoites that then
  invade the epithelial cells
  of the gut wall and
  undergo asexual
  reproduction (merogony).
  After more than one
  asexual cycle male and
  female gametes are
  formed.