PARASITOLOGY

In such cells the parasites undergo development, reproduction and escape to invade
similar non-infected cells. Erythrocytes, and to a lesser extent epithelial cells, have a high
turn-over rate. They are regularly replaced and hence it may be in the interest of the
parasite’s survival to produce new individuals to invade fresh cells.
Examples of parasites of the erythrocytes are the Plasmodiumspp (the malaria parasites)
and Babesiaspp. The Plasmodiumspp that infect humans and primates have an exoerythrocytic
stage; immediately after the sporozoite is injected into the host it first invades hepato-
cytes (liver cells). In the liver cells there is rapid growth and development into a schizont
which undergoes schizogony (a multiplicative stage, see Fig. 2.5). The liver cell ruptures
and the newly formed merozoites are released and are temporally non-cellular but as soon
as possible infect red blood cells (erythrocytes). The growth, development and multi-
plicative phase is repeated in the red blood cells.
Babesiaspp — the vector host is a tick (Boophilusspp) and when an infective adult tick
bites sporozoites are inoculated into the mammalian host. The sporozoites invade ery-
throcytes and undergo reproduction by binary fission and the newly formed merozoites
invade non-infected red blood cells. There are no exoerythrocytic stages.
In both of the above examples the parasites have adapted to surviving in short-lived
cells that are both numerous and continuously replaced. From the parasite’s point of view
this provides ample opportunity to increase its population and ensure that individuals sur-
vive to be transmitted to the next host.
Intestinal protozoan parasites such as Eimeria tenellaenter their host via the oral route
and once swallowed pass through the stomach into the small intestine, where they break
out of their protective cyst. The first cells encountered by the infective stage of the para-
site are those lining the lumen — the epithelial cells. Epithelial cells not only protect the

PROTOZOA

Gametocytes develop in red blood cells

Mosquito takes up

gametes while feeding

Mosquito

inoculates

sprozoite

Sporozoite rapidly

reaches liver via

circulation and

invades liver cells

Ookinete

on outer stomach

wall

Mosquito Fertilised oocyte

Male and female
gametes

Sporozoite
ready to be
inoculated

Merozoites exit liver cells
and infect red blood cells
Schizogony repeated
periodically

Asexual multiplication
within liver cells (schizogony)

• Figure 2.4Plasmodium
  spp are parasitic blood-
  dwelling protozoa that
  cause the disease
  malaria. P. falciparum;
  P. malariae; P. ovaleand
  P. vivaxare the four
  species of Plasmodium
  that are transmitted to
  humans by female
  mosquitoes while feeding
  on blood. Sporozoites
  inoculated into the blood
  by a feeding mosquito are
  rapidly transported to the
  liver, invade hepatocytes
  and undergo asexual
  reproduction (schizogony).