PARASITOLOGY

PARASITOLOGY

n The merozoites burst out of the host cell and invade fresh epithelial cells. After

several asexual multiplicative cycles, the merozoites transform into male and female

gametes.

n The male gamete divides to form numerous flagellated microgametes.

n The female macrogamete develops from a single gametocyte and is fertilised by a

microgamete. A wall forms around the fertilised zygote which develops into an

oocyst. The nucleus of the oocyst divides into a uninucleate sporocyst (with a shell con-

sisting of four walls) and each sporocyst divides into two sporozoites. The oocyst con-

sists of four sporocysts, each containing two sporozoites which pass out of the body

via the faeces.

n 4.3.2ENTAMOEBA HISTOLYTICA

E. histolyticais a protozoan that lives predominantly in the lumen of the colon and caecum

of humans.

n Resistant cysts are swallowed and an amoeba with four nuclei emerges from the cyst.

Three mitotic divisions follow to form eight uninucleate amoeboid trophozoites. The

trophozoites are capable of invading the cells of the intestinal wall (the epithelial cells)

where most remain. However some enter the blood and then, via the circulation,

invade the liver or the brain. Once inside the latter tissue, they are often the cause of

amoebic abscesses.

n Trophozoites each with four nuclei form into cysts and pass out with the faeces.

n 4.3.3GIARDIA LAMBLIA

A flagellate protozoan that lives on the surface of mammalian gut epithelial cells (see

Figs 2.2 and 4.6).

n The active stage of this parasite is a binucleate, pear-shaped flagellated trophozoite. Each

trophozoite has eight flagellae and a body 7–10 μm in diameter and 10–20 μm long.

n Two blepharoplasts (rigid structures) run along the length of the protozoan.

n The trophozoite attaches itself to the surface of the epithelial cells by a disc-like

depression located at the anterior end.

Trophozoites encyst

in small

intestine

Cyst pass out

in faeces

Trophozoites live on

the surface of the villi

Trophozoites excyst

in the small intestine

Cysts containing four

nuclei are swallowed

• Figure 4.6Giardia
  lambliais a parasitic
  protozoan found in the
  human intestines. Cysts
  are swallowed and
  trophozoites hatch out of
  the cyst into the host’s gut
  and live on the surface of
  the gut villi. After a
  multiplicative phase
  trophozoites encyst and
  pass out with the faeces.