Life Cycle of Plasmodium vivax (Malarial Parasite)
Systematie Position
Kingdom : Protista
Phylum : Protozoa
Class : Sporozoa
Genus : Plasmodium
Habit s and habitat:-
Plasmodium is an intracellular sporozoan parasite causing malaria in man. The
parasite lives in the RBC’s and liver cells of man and alimentary canal
and salivary glands of female Anopheles mosquito.
Structure:-
Structure of plasmodium is different stages of its life cycle. A fully grown malarial
parasite is amoeboid and uninucleated structure known as trophozoite. Trophozoite
is surrounded by double layered plasma lemma. Cytoplasm contains Palade’s
granules, endoplasmic reticulum, ribosome, mitochondria, vesicles and vacuoles
having haemozoin. Cytoplasm contains nucleus having nucleolus and granular nucleoplasm.
Life Cycle of Plasmodium vivax
Hosts:-
Plasmodium completes its lifecycle in two hosts (digenetic): Man and female Anopheles
mosquito.
1. Primary or definitive host:
Female Anopheles mosquito is the primary host of Plasmodium in which it completes
its sexual life cycle.
2. Secondary or Intermediate host:
Man is the secondary host of plasmodium in which it completes its asexual life
cycle.
The lifecycle of Plasmodium can be divided into three phases:
1. Asexual sehizogony
2. Sexual gamogony
3. Asexual sporogony
ASEXUAL CYCLE OF Plasmodium, IN MAN
Infective form of Plasmodium is known as sporozoites. Sporozoites are 11-12µ
long slender, uni-nucleated. Sickle-shaped structure present in the salivary
glands of infected mosquito. When an infected female Anopheles mosquito bites
a healthy man, a large number of sporozoites enter into the blood stream of
man. Within half an hour, sporozoites enter the liver cells and undergo asexual
multiplication called schizogony.
1. Asexual Schizogony:-
Schizogony is the asexual phase of reproduction of Plasmodium. It takes place
in liver cells and RBC’s of man. Schizogony can be divided into following
phases:
a) Pre-erythrocytic schizogony
b) Exo-erythrocytic schizogony
c) Erythrocytic schizogony
d) Post- erythrocytic schizogony
a. Pre-erythrocytic schizogony:
In the liver cells, sporozoites grow to form a large and spherical schizont.
Schizont divides by multiple fission and forms a large number of cryptozoites.
They may either pass into the blood circulation to start erythrocytic schizogony
or enter fresh liver cells to start Exo-erythrocytic schizogony. Pre-erythrocytic
schizogony takes 8 days to complete.
b. Exo-erythrocytic schizogony:
After re-entering fresh liver cell each cryptozoites divides to form a large
number of metacryptozoites similar to pre-erythrocytic schizogony.
Meta-cryptozoites are two types:
Smaller micro-metacryptozoites and larger macro-metacryptozoites. The micro-metacryptozoites
enter the RBC’s to start erythrocytic schizogony, while the macro-metacryptozoites
invade fresh liver cells to continue exo-erythrocytic schizogony. It takes normally
4 days to complete.
c. Erythrocytic schizogony:-
As stated above, the erythrocytic schizogony begins when the RBC’s of
blood are attacked either by pre-erythrocytic cryptozoites or by exo-erythrocytic
micro-metacryptozoites. It takes normally in 8 to 12 days after above 2 phases.
Stages of erythrocytic schizogony are:
i. Trophozoite Stage:-
The merozoites (cryptozoites and micro- metacryptozoites) after entering into
the blood stream, feed on erythrocytes, become rounded and modify into trophozoite.
As the merozoites grow a vacuole appears in the center and the nucleus is pushed
to one side. It gives a ring like appearance and known as signet ring stage.
The parasite ingests haemoglobin and decomposes it into protein and haematin.
Protein is use as food whereas unused haematin forms toxic. Yellowish brown
malarial pigment, haemozoin.
As the signet ring parasite grows, vacuole disappears and the parasite becomes
amoeboid in appearance, thrusting out pseudopodial processes. This stage is
called amoeboid stage. At this stage RBC develops numerous granules, the Schuffner’s
granules.
Parasite grows in size, becomes rounded and almost completely fills the RBC called Schizont.
The nucleus of schizont divides by multiple fission to form 6 to 24 daughter
nuclei. These nuclei arrange at the periphery, while the toxic haemozoin granules
accumulate at the center of RBC. It appears as a flower rose, so called rosette
stage.
Nuclei of rosette stage are surrounded by a little cytoplasm and are develop
into merozoites. With the rupture of the RBC, these merozoites are liberated
into the blood plasma along with toxic haemozoin. These normally attack fresh
RBC’s to repeat the erythrocytic cycle or may change into gametocytes.
One complete erythrocytic cycle takes 48 hours in Plasmodium vivax.
d. Post-erythrocytic schizogony:-
Sometimes, some merozoites produced in erythrocytic schizogony reach the liver
cells and undergo schizogony development in liver cells. This is called post-erythrocytic
schizogony.
SEXUAL CYCLE OF Plasmodium in MAN
2. Sexual Gamogony:-
Formulation of gametocytes:
After many generations in about 4-5 is the blood some merozoites increase in
size to form two types of gametocytes; larger macro (9-10µ), less numerous
and contain large nucleus. Macro gametocytes are larger (10-12µ), more
numerous and contain smaller nucleus.
SEXUAL CYCLE OF Plasmodium IN MOSQUITO
When a female Anopheles sucks the blood of a malaria patient, the gametocytes
reach the stomach of mosquito and formation of gametes take palace as follows:
a. Gametogenesis (gemetogony) :
Process of formulation of gametes (male and female gametes).
i. Formulation of male gametes:
The nucleus of microgametocyte divides to form 6-8 daughter nuclei. The cytoplasm
gives out same number of flagella like projections and daughter nuclei enter
in each projection. These projections separate from the cytoplasm and form 6-8
haploid microgamete or male gametes. This process of formation of microgamete
is called exflagellation.
ii. Formation of female gamete:-
The mega gametocyte undergoes some reorganization to form a single haploid mega
gamete or female gamete which is ready for fertilization.
b. Fertilization:
The male gamete enters the female gamete through the fertilization cone formed
at female gamete and form diploid zygote or synkaryon. Fusion is anisogamous.
c. Ookinete stage:
The zygote remains inactive for sometimes and then elongates into a worm like
Ookinete or vermicule, which is motile. The Ookinete penetrates the stomach
wall and comes to lie below its outer epithelial layer.
d. Oocyst stage:
The Ookinete gets enclosed in a cyst. The encysted zygote is called Oocyst.
The Oocyet absorbs nourishment and grows in size.
3. Asexual Sporogony
The nucleus of Oocyet divides repeatedly to form a large number of haploid daughter
nuclei. At the same time, the cytoplasm develops vacuoles and gives numerous
cytoplasmic masses. The daughter nuclei pass into each cytoplasmic mass and
develop into slender sickle-shaped sporozoites are formed in each Oocyet. This
phase of asexual multiplication is known as sporogony.
Lastly, the Oocyet brusts and sporozoites are liberated into the haemolymph
of the mosquito. They spread throughout the haemolymph and eventually reach
the salivary glands and enter the duct of the hypopharyx. The mosquito is now
becomes infective and sporozoites get inoculated or injected the human blood
when the mosquito bites. The cycle is repeated.
In mosquito whole sexual cycle is completed in 10-12 days.
Incubation period:
The period between infection and the appearance of first symptoms is called
incubation period. It is about 10-14 days in Plasmodium vivax.
Pre-patent period:
The duration between the initial sporozoites infection and the first appearance
of parasites in the blood is called as pre-patient period. It is about 8 days
in Plasmodium vivax.
Symptoms of malaria
• Mouth becomes dry, nausea and loss of appetite
• Headache, muscular pain and joint pain
• Chill, fever (106° F) and sweating all every 48 hours.
• Chill to sweating lasts for 8-10 hours.
• Liver and spleen become enlarged.
• Due to loss of RBC’s anaemia is caused.
Control of malaria
Malaria can be controlled by three ways
1. Destruction of vector
2. Prevention of infection(prophylaxis)
3. Treatment of patient
4. Public awareness
1. Destruction of vector (Anopheles mosquito)
• Mosquito can be killed by spraying DDT, BHC, Dieldrin, Malathion etc.
• Filling up ditches, gutters and pits where the mosquito breeds.
• Water surface can be poisoned by spreading kerosene oil, petroleum etc.
• A speedly flow of water prevents the mosquito larva and pupa flourishing.
• Biological control: Certain fishes (trouts, minnows, stickle back),
ducks, dragon flies etc feed on larva and pupa of mosquito.
2. Prevention of infection (Prophylaxis)
• Use of mosquito nets.
• Screening doors, windows and ventilators.
• Using mosquito repellent creams (e.g. odomus), anti mosquito mat (e.g.
Supermat) etc.
3. Treatment of patient:
There are several drugs that kill different stages of parasite in patient. The
oldest drug is Quinine; Paludrine kills almost all stages of parasite. Daraprism
(single dose of 25 mg) is the most effective drug.
Write short note on Plasmodium falciparum
Plasmodium falciparum causes malignant tertian type of malaria. Fever recurs
every second or third day, that is, after 36 to 48 hours. Death rate is very
high because the infected red blood corpuscles tend to clump into masses, thus
blocking up small blood vessels of internal organs, such as brain, spleen, lungs,
etc. It is also known as the tropical epidemic malaria of man.
Exo-erythrocytic schizogony is absent in P. falciparum.
