Life Cycle of Plasmodium vivax (Malarial Parasite)

Systematie Position

Kingdom : Protista
Phylum : Protozoa
Class : Sporozoa
Genus : Plasmodium
Species : vivax

Habits 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 many 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 many 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.

ii. Signet Ring Stage:-

As the merozoites grow a vacuole appears in the center and the nucleus are 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.

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:
3. Asexual sehizogony
4. Sexual gamogony
5. 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 many 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 many 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.

ii. Signet Ring Stage:-

As the merozoites grow a vacuole appears in the center and the nucleus are 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.

iii. Amoeboid Stage: –

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.

iv. Schizont Stage:-

Parasite grows in size, becomes rounded and almost completely fills the RBC called Schizont.

v. Rosette Stage:-

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 developed 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 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.