The
life cycle of the human malaria species consists of two phases. The sexual phase (Sporogony/Exogenous)
which involves the development and multiplication on the female anopheles
mosquito. The
Asexual phase (Schizogony/Endogenous) which involves multiplication in man
divided into two parts:
THE ARE:
a).
Pre-erythrocytic schizogony in the liver cells.
b).
The erythrocytic-schizogony in the red blood cells.
Endogenous phase in human which is the asexual
cycle schizogony involves exo-erythrocytic multiplication in liver parenchyma
cells to merozites, the merozoites enters red cells inform of ring trophozoite
then matured trophozoites to immature. Schizonts to mature schizonts. The ring
trophozoites differentiate into micro and macro gametocyte, which is the
gametogony cycle of the parasite. The gameto cytes in the human blood enter the
mosquito and fertilization forming the zygote which Ookinate the motile zygote
emerges.
These motile zygotes penetrate the outer lager of stomach wall of
mosquito and encyst. The oocyst grows by multiple divisions. Then the cyst
burst to release sporozoites, this pass through the body cavity of the mosquito
to reach the salivary gland of the mosquito and it is known as the exogenous
phase in mosquito, which is sexual cycle (Sporogony). The sporozoites in saliva
of the mosquito can be re-injected into human host and another cycle begins.
ASEXUAL PHASE IN MAN
Human infection result from the bite of an
infected female anopheles mosquito in which the sporozoites, resulting from the
sexual and subsequent sporogonic cycle of development in the mosquito are
injected into the human blood steam. The sporozoites rapidly (usually within
one hour) enter the parenchymal cells of the liver, where the first stage
development in human takes place (Pre-erythrocytic phase of the life cycle).
They multiply in the liver and are later liberated as merozoites from mature
schizont. The merozoites rupture and leave the liver cells, enter the blood
stream and invade erythrocytes (Erythrocytic phase of the life cycle).
Parasites in the red blood cells multiply in species characteristic fashion,
breaking out of their host cells at the same time. To enter the cells, the
merozoites bind to glycophorine the major erythrocytic glycoprotein that is
specific for a particular species of the parasite. The apex of the merozites
release substances that forms a deep pit on the surface of the red blood cells
and maintain contact by forming a ring. The merozoites are enveloped by Red
blood cells on a vacuole known as parasitophorous vacuole.
In the Red blood cells, the merozoites
develop into rings forms that grow in size to trophozoite absorbing haemoglobin
leaving a pigment known as heamatin or heamazoin (a combination of Hb in
protein which can be seen as dark granules). The trophozites multiply by schizogony
dividing into a number of small merozoites varying with the species to form the
mature schizontes. The erythrocytic cycle is with successive broods of
merozoites appearing at 48 hours intervals for (P. vivax, P. ovale and P.
falciparum) or every 72 hours for p.
malariae.
There is no return of merozoites from
Red blood cells to liver cells without treatment; falciparum infection
ordinarily will terminate spontaneously in less than one year unless it ends fatally. The other three species continue to multiply
in the liver cells long after the initial blood stream invasion or there may be
delayed multiplication in the liver. These pre-erythrocytic cycle coexist with
erythrocytic cycles and in P. vivax
and P. ovale, may persist as non
growing resting forms or hyprozoites, after the parasite have disappeared from
the peripheral blood.
Resurface of an erythrocytic infections
(relapse) occurs when merozoites from hyprozoites in the liver breakout are not
phagocytize in the blood stream and succeeded in re-establishing a red blood
cell infection. During the erythrocytic cycles certain merozoites enter red
cells and become sexually differentiated as male or female gametocyte
(Gametogony). The male is known as micro gametocyte while the female is known
as macro gametocyte which differ in morphology in the species of the parasite.
SEXUAL PHASE IN
MOSQUITO
The
gametocytes are taken up by female anopheline mosquito and undergo further
development. These gametocytes develop into gametes in the gut of he mosquito,
macrogametocyte formd marogamete while the microgametocyte form microgamete.
The microgamete divide to form long
slender thread like structures known asex-flagellations, then macrogametes
developing with nuclei shifted to the periphery where projections are formed.
Fertilization occurs when a microgamete penetrates this projection and the
fertilized macrogamete is called a zygote. The zygote gives rise to the
Ookinate as the ookinate grows, it penetrates the epithelial cell of the mid
gut.
After a period of 1-15 days after injection of gametocytes by mosquito,
the oocyst mature with multiplication of nuclei and transforms which is
temperature dependent. The oocyst burst and sporozoites are released throughout
the insect body. The sporozoites that came in contact with the salivaryd gland
are injected into a healthy man after
blood meal. Their shape vary from narrow, slightly curved as in P. vivax, thick as in P. malariae and sickle
shaped in P. falciparum. Malaria is
always a accompanied by haemmolysis and
in a severe to prolonged attack anaema may be found. The haemolysis is most
severe with P. falciparum, which
invades red blood cells of all ages, including the erythropoietic sterm cells
in bone marrow.
RELATED INFORMATION