a certain level, the process of schizogony and the subsequent liberation of merozoites becomes
synchronised and periodic.
P. malariaehas a 72 h periodic cycle whereas P. ovale, P. vivaxand P. facilparumhave a
48 h cycle. At the end of each cycle when the erythrocytes burst and release the mero-
zoites into the bloodstream, ‘toxins’ are released into the host. Each time there is the
liberation of parasites into the bloodstream it has a pyrogenic effect upon the host, that
is the host suffers from fever.n Once the merozoite population reaches a certain density some of the merozoites
develop into male and female gametocytes. These tend to accumulate in the periph-
eral circulation and are taken up by the mosquito as it feeds.
n In the gut of the mosquito the male gametocyte produces numerous microgametes which
are released to fertilise the larger female macrogamete. Once fertilised the female
oocyte is known as a zygote.
n The zygote transforms into a motile ookinete which penetrates the gut lining of the
mosquito and is associated with the outer wall of the gut. The ookinete now settles
and develops into an oocyst. The nucleus of the oocyst divides and gives rise to
numerous sporozoites which eventually make their way toward the mosquito’s saliv-
ary gland.4.4.1.2Trypanosomaspp
The trypanosomes are parasitic flagellated protozoa referred to as kinetoplastid flagellates
and are transmitted by insect vectors. Each individual has a single flagellum attached to
a kinetoplast located within the cytoplasm. A kinetoplast is an organelle associated with
part of the mitochondria and has its own DNA. The typical trypanosome form is elong-
ate with a single flagellum.
The most well known genera are Trypanosoma, Leishmaniaand Endotrypanum. These para-
sites infect both man and domestic animals and many are zoonotic. A feature of this group
of trypanosomes is that no sexual reproduction has yet been observed. Asexual multi-
plication occurs in both hosts.
There are two groups of trypanosomes that infect man: the Salivarian; and the
Stercorarian trypanosomes.n Salivarian eg Trypanosoma brucei. The vector host is Glossina(tsetse-fly) and while
within the fly, the parasite develops in the midgut and then migrates to the salivary
gland and is injected into the mammalian host when the fly feeds on blood (an inocu-
lative infection).
n Stercorarian eg T. cruzi. These develop in the hindgut of the arthropod vector and the
infective stages — the epimastigotes — pass out via the faeces onto the host skin
while the insect extracts blood. The epimastigotes are ‘rubbed’ into the wound made
by the fly’s mouthparts (a contaminative infection).4.4.1.3Trypanosoma brucei
There are several subspecies of T. bruceisuch as T. brucei gambienseand T. brucei rhodesiense
which are the main causes of African sleeping sickness (African trypanosomiasis).n The fly injects the metacyclic (or metatrypanosome) form through the host’s skin
where it rapidly enters a blood vessel. Once in the bloodstream they transform into
a trypanomastigote. These forms move by using their flagellum and an undulatingPARASITOLOGY