Plasmodium

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  • Topic: Malaria, Apicomplexa, Blood
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  • Published : March 31, 2013
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Life Cycle of Plasmodium vivax (Malarial Parasite)
Systematie Position
Kingdom    :    Protista
Phylum       :    Protozoa
Class          :    Sporozoa
Genus         :    Plasmodium
Species       :    vivax
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. ii. Signet Ring Stage:-
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.

iii. Amoeboid Stage: -
As the signet ring parasite grows, vacuole disappears and...
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