LIFE CYCLE OF PLASMODIUM VIVAX

• Plasmodium is a malaria fever parasite, which is a sporozoa i.e., it comes under sub-phylum sporozoa. Generally all sporozoans are parasites. Plasmodium is an endoparasite.
• They do not have any locomotory organelles. Movement takes place through blood current.
• They do not have any contractile vacuoles. So, there is no need of phagocytic mode of nutrition.
• Plasmodium is a digenetic parasite i.e., it needs two host to complete its life cycle.
• Biologically, the primary host is man and the secondary host is female Anopheles mosquito.
• There are mainly four species of Plasmodium which causes malaria:

1. Plasmodium vivax    
2. Plasmodium ovale                                                                                     
3. Plasmodium falciparum                                                                                                              
4. Plasmodium malariae

• Plasmodium vivax causes Benign tertian malaria. Here the peroxism period is about 48 hours and the incubation period is about 14 days.

SYSTEMATIC POSITION 

Phylum- Protozoa

Sub-phylum- Sporozoa

Class- Telosporea

Sub-class- Coccidia

Order- Eucoccidia

Sub-order- Haemospornia

Genus- Plasmodium

Species- vivax

• The life cycle of Plasmodum vivax completes in two phases:

1. Asexual phase (Schizogony)                                                             
2. Sexual phase (Gametogony)

• Asexual reproduction occurs by multiple fission while sexual reproduction is anisogamous type. Thus, two opposite gametes are different from each other.

a) ASEXUAL PHASE (SHIZOGONY)

• Asexual cycle completes in man.
• It completes through the following headings:

1. Pre-erythrocytic cycle
2. Exo-erythrocytic cycle
3. Erythrocytic cycle
4. Post-erythrocytic cycle

• Before entering these above four phases, infection takes place.

      INFECTION – An healthy person aquires infection, when a female Anopheles mosquito  containing sporozoites in its salivary glands, bites him for sucking blood. The mosquito punctures the skin by its probosis and first introduces some saliva into the blood stream. Along with the saliva thousands of sporozoites contained there in are also inoculated. Saliva of mosquito contain some anticoagolant named Anophelin which check the clotting of blood.

 i) PRE-ERYTHROCYTIC SCHIZOGONY

• After the inoculation of sporozoites in the blood plasma, the sporozoites circulates for about half an hour and invades the hepatic cells.
• The sporozoites in the hepatocyte grows in size and becomes spherical then it is called cryptozoites.
• The cryptozoites feed upon the cytoplasm of hepatic cell, increase in size and form schizont.
• The nucleus of schizont divides by multiple fission to form thousands of cryptomerozoite.
• Now the liver cell burst and the cryptomerozoites liberated outside to enter the Exo-erythrocytic cycle.

 ii) EXO-ERYTHROCYTIC SCHIZOGONY

• The cryptomerozoite released from the liver cells invades the fresh liver cells and form metacryptozoite.
• The metacryptozoite undergoes schizogony to form large number of metacryptomerozoites. Here two types of  metacryptomerozoites are formed i.e., Micrometacryptomerozoites and Macrometacryptomerozoites.
• The macrometacryptomerozoites enters the fresh liver cells to continue exo-erythrocytic cycle and the micrometacryptomerozoite enters the Erythrocytic cycle.

iii) ERYTHOROCYTIC SCHIZOGONY

• Now the micometacryptomerozoites after reaching the blood stream penetrate the R.B.C.
• Generally, one micrometacryptomerozoites invades one R.B.C and grow by feeding the cytoplasm of the host cell. The elongated shape of the parasite becomes rounded disc like. This stage is called Trophozoite.
• Then, a contractile vacuole appears in the cytoplasm of trophozoite and the vacuole pushes its  nucleus to one side. Now, the Trophozoite looks like a ring and so that this stage of Plasmodium is called as the Signet-ring stage.
• Later the vacuole dissappears and gradually it developes into an active amoeboid trophozoite.
• The amoeboid trophozoite feeds upon the haemoglobin of R.B.C, increase in size and fills the entire cytoplasm to form schizont.
• The haemoglobin disintegrates and form a yellowish brown pigment granules called haemozoin granules. At the same time, a large number of Schiffner’s dots appears.
• Now the nucleus of the schizont divides by schizony to form 12-24 daughter nuclei. The nuclei migrate to the peripheri of the cell and this stage appears like a flower. So, this stage is reffered as Rosette stage.
• Now, these nuclei accumulate some amount of cytoplasm to form merozoites. This stage occurs inside the R.B.C.
• Erythrocytic schizogony completes in 48 to 72 hours.

 iv) POST ERYTHROCYTIC SCHIZOGONY

• After completion of erythrocytic cycle, the erythrocyte burst liberating the merozoites and toxic waste into the plasma. During this time the patient experiences shivering.
• The newly formed merozoites attack the fresh R.B.C and after entering the R.BC, the R.B.C increases in size to become gametocytes.
• The male or microgametocyte is smaller and the female or megagametocyte is larger in size.
• The matured gametocytes are unable to develop further in the body of man. If these gametocytes present in R.B.C are not transfered  to the body of a female Anopheles mosquito, it will degenerate after a couple of days.
• Further development of gametocytes occurs inside the stomach of female Anopheles mosquito

b) SEXUAL PHASE (GAMETOGONY)

• Sexual phase of Plasmodium completes in female Anopheles mosquito.
• It completes through the following headings:

i) Ingestion by mosquito

• When a female Anopheles mosquito sucks the blood of  an infected person containing gametocytes  in R.B.Cs, the gametocytes become lodged in the cavity of gut.

ii) Gametogony

• The development of gamete is known as Gametogony.
• The gametes are of two types i.e., Microgametocyte (male), Macrogametocyte (female).

a) Microgametocyte

• Microgametocyte undergoes a process called exflagellation. In microgametocyte nucleus divides by mitosis to produce 6-8 haploid daughter nuclei. The cytoplasm outgrows into long thin and flagella like projection.
• These projection breaks away as mature male gamete, called microgamete.

b) Macrogametocyte

• A female macrogametocyte undergoes some reorganization and become macrogamete or megagamete.

iii) Fertilization

• A macrogamete gives out small cytoplasmic projection, the fertilization cone. The actively mobile microgamete enters the macrogamete through this fertilization cone.
• The nucleus of two gametes finally fuse together to form the round zygote or synkaryon.

iv) Ookinetes

The zygote becomes elongated and motile, known as ookinetes.

v) Encystment

• Ookinete penetrates through the wall of the gut, it becomes spherical and begin to encyst.
• The cyst wall is secreted partly by ookinete and partly by the gut wall.
• The encysted zygote is called Oocyst.

vi) Sporogony

• Each oocyst enters a phase of asexual multiplication known as sporogony. Its nucleus divides by meiosis forming enormous number of haploid nuclei, which arrange them self along the cytoplasmic mass. In this way about 1000 of minute slender sporozoites are formed from each oocyst.
• When the matured oocyst rupture, sporozoites are liberated into different organs in the body of mosquito but many of them penetrates the salivary gland. The mosquito now becomes infective. When it bites a healthy person thousands of sporozoites are injected in his blood along with saliva and the cycle repeats again.

Symptoms

• Symptoms of malaria appears after 4 to 5 erythrocytic cycles.
• Severe shivering chill followed by high fever is a very important symptom.
• In vivax malaria symptoms like nausea, insomnia, headache, loss of appetite, muscle pain, etc.  are commonly take place.

Pathogenesis

• Severe anaemia is caused by malarial parasites.
• Hardening and enlargement of spleen takes place.
• Leukopenia may also occur.
• Large amount of liberated haemoglobin from the destroyed RBC is  excreted with the urine and hence it is also known as black water fever.