The life cycle of malarial parasite in two different hosts
like –
1 . In man – Man represents the intermediate
host of the malarial parasite . The parasite resides inside the red cell and liver cell and
reproduce by asexual ,method . Human cycle starts with the introduction of
sporozoites by the bites of the infected female anophiline mosquito .
Sporozoites are found in the salivary gland of the mosquito and it may vary
from 950 to 2.00.000 or more . But a mosquito can inject 3000 sporozoites at
each bite . The sporozoites are thread like curved organs measuring about 12µ in
length and an elongated nucleus is present centrally . There is no pigment .
The sporozoites will come into the blood after the bite of mosquito and wil
enter the liver after 30 to 60 minutes .
Life cycle –
(a)
Pre erythrocytic schizogony – This phase will start
in the liver . The cycle lasts for 8 days in plasmodium vivax , , 6 days in
plasmodium falciparum and 9 days in plasmodium oval . In the liver sporozoites
will form pre erythrocytic schizont and eacbh mature schizont contains about
12.000 merozites in case of P.vivax and 40.000 in case of P.falciparum . in
case of p.vivax , P.ovale and P.malriae , some of the merozites enter the blood
after completion of pre-erythrocytic schizogony and some re-enter the liver
causing relapse . But in case of P.falciparum , all merozites will enter the
blood . So , there is no relapse .
(b)
Erythrocytic schizogony – The parasite resides
in red cells and this cycle last for 48Hrs in P.vivax , P.ovale , and P.falciparum and 72 Hrs in p.malariae . Trophzoite
, Schizont and merozite – thses stages ill occur inside the red cell . .
The newly formed
merozites will attack new R.B.C . In the R/.B.C some will develop into
trophozoites and schizont stage and some will form gametocytes .
(c)
Gametogony – Some of the merozoites develop
into gametocytes ( male or female ) instead of developing into trophozoites and
schizonts . gametocytes will form in the R.B.C of capalaries of internal organ
like spleen and bone marrow . Gametocytes of P.vivax appear in peripheral blood
from the first day of fever . gametocyte will not cause any febrile condition .
The maturation of gametocytes is completed in 96 Hrs .
(d)
Para-erythrocytic schizogony – This phase is
absent in P.falciparum but persist in case of P.vivax , P.ovale, and P.malariae
in the form of a local life cycle .
2. In
Female Anopheline Mosquito – Mosquito represents the definitive host of the
malarial parasite as there is sexual method of reproduction . The stages inside
the mosquito are –
a)
A female anopheles will draw blood of an
infected person and take both sexual and asexual form of parasite . But asexual
form will die immediately in stomach of the mosquito .
b)
A mosquito may be infected after its blood –meal
from an infected person when there is at least 12 gametocytes per cmm .in the
infected person’s blood and also the number of female gmetocytres will be more
than the number of gametocytes .
c)
Inside the stomach of the mosquito , the pigment
of microgmetocytes will shw violentb movement and exflagellation occurs and it
is charecterised by 4 to 8 thread like filamentous structures . They become
detached and form microgametes . . On the other hand , there is no
exflagellation of macrogametocytes . So , from one microgametocyte , one
macrogamete will form .
d)
By the process of chemotaxis , microgmetes are
attracted towards the macrogametes and unite together and form zygote . Zygote
will form in 20 minutes to 2 Hrs afrter the mosquito’s blood meal from an
infected person .
e)
Then the zygote which is an actively moving body
, develops into ookinete or travelling vermicule .
f)
The okinete will penetrate epithelial cells of
the stomach and lie below the outer limiting membrane of the stomach wall .
Then it develops into a rounded mass surrounded by a cyst wall . This rounded
body is called oocyst . The oocyst measures about 6 to 12 µin diameter . This will contain a single vesicular
nucleus and pigments granules of the macrogamete .
g)
The oocyst will mature and it will increase
about 6 to 60 µ in diameter . By the successive nuclear division , there
develops a large number of sickle shaped bodies called sporozoites .
h)
The sporozoites will come in the body cavity of
mosquito . The sporozites will be distributed to various organs and tissues of
mosquito through the circulating fluid . Then they will come to salivary glands
and ducts , These sporozoites infect another person during blood meal of
mosquito .
i)
Development of sexual cycle in the mosquito in
different species –
1.
P. vivax
– 8 to 10 days
2.
P.falciparum – 10 to 12 days
3.
P.malariae -
18 to 21 days
4.
P.ovale – 14 to 18 days .
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