PARASITOLOGY

gut but also function in absorbing digested soluble nutrients from the gut lumen. These

cells are ideal for an intracellular gut parasite. The sporozoites, once inside the cell,

develop and undergo reproduction by schizogony. The new generation bursts out of the

cell and invades non-infected cells. This rapidly increases the number of individual

parasites.

Parasites that enter the host, whether into the body fluids or the tissues, are vunerable

to the host’s defences and in particular the macrophages. These cells’ prime function is

to phagocytose (digest) non-host material and they are potentially lethal to any invading

organisms. However, there are protozoans, eg Leishmaniaspp, that can invade and sur-

vive within macrophages. These parasites either resist or inhibit the production of destruc-

tive enzymes and inside a macrophage is probably the safest place for the parasites to live.

There are protozoa that are mostly extracellular in the mammalian host, eg Try-

panosoma brucei(the African trypanosomes). These parasites have a single large flagellum

and an undulating membrane. The parasites use both to propel themselves through the

blood and they eventually exit from the blood to live in the cerebrospinal fluid.

Extracellular parasites are continuously exposed to the host’s immune responses and the

African trypanosomes have evolved their own mechanism of avoidance. The host’s

attack is based on attempting to destroy the parasite’s surface membrane but the para-

site is able to regularly change the surface molecular configuration of its membrane in

order not to be immediately recognisable to host immune mechanisms. This process is

known as antigenic variation.

Trypanosoma cruzithe American trypanosome enters a mammalian host, via a vector,

in the amastigote form and then pentetrates the reticulo-endothelial cell system or the

muscles of the heart and commences an intracellular phase. In the muscle tissue the

parasite forms a pseudocyst. After multiplication within the pseudocyst, the parasite

transforms into a trypanomastigote form that escapes into the blood to begin an inter-

cellular phase. Some of the circulating trypanosomes reinvade tissues and others are

taken up by the vector when feeding.

PARASITOLOGY

Sporozoite invading liver cell

Ring stage

trophozoite

Erythrocytic

schizogony

Merozoites

escaping

to reinvade blood cells

Microgamete

Macrogamete

Schizogony within liver cell Merozoites escaping

• Figure 2.5Merozoites
  released from the
  hepatocytes into the
  circulating blood invade
  erythrocytes (red blood
  cells) and undergo
  schizogony. This cycle is
  repeated more than once.
  Male and female gametes
  are formed during the
  blood phase and are taken
  up by a feeding mosquito.
  Fusion between the male
  and female gametes
  occurs within the mosquito
  gut.