Plasmodium

Life Cycle of Plasmodium
Plasmodium is a digenetic, intracellular parasite of liver cell and RBC of vertebrates. In Mosquito it lives in the tomach and in the wall of Alimentary canal. In vertebrates the asexual or schizogony occurs and in female Anopheles mosquito, the Sexual cycle or sporogony occurs.

Schizogony
Schizogony is divided into following sub stages :-

1. Pre-Erythrocytic Schizogony

2. Exo-Erythrocytic Schizogony

3. Erythrocytic Schizogony

4. Post-Erythrocytic Schizogony

1. Pre-Erythrocytic Schizogony
When an infected female anopheles mosquito bites a healthy man, the mosquito innoculates or injects thousands of sporozoites into the blood of men. Sporozoites are microscopic spindle or sickle shaped structure with large centrally located Nucleus. Its body is covered by thin cuticle that protects it from the invasion of WBC. It is also known as infective stage of plasmodium. Within half an hour, the sporozoites are completely disappeared from the blood and reaches the liver cell. In liver cell, it grows into round or oval shaped structure that is schizont.
Schizont reproduces by multiple fission to produce numerous first generation parasites that is cryptomerozoites. The fully infected liver cell ruptures and liberates cryptomerozoites into the Sinusoids of Liver(Sinusoids is the fluid filled space between hepatic acinus.)

2. Exo-Erythrocytic Schizogony
The cryptomerozoites invade new lover cell where it changes to schizont. Schizont multiplies by multiple fission to form Numerous second generation parasites that is meta cryptozoites. It is of two types which are Micro meta-cyprozoites and Macro meta-cryptozoites. The Macro meta-cryptozoites are large and few in number. It constantly invades new liver cell. Therefore, Liver is known as Resorviour or storage of parasite. The Micro meta-cryptozoites attacks RBC. After that Erythrocytic Schizogony starts.

3. Erythrocytic Schizogony
In Plasmodium vivax, the Micro meta-cryptozoites invade young or immature R.B.C whereas in Plasmodium falciparum, the parasites invade old and matured R.B.C. The parasite begins to feed on R.B.C by which it grows in size with the appearance of small vacoule. This stage is called as feeding stage or Trophozoite. As the trophozoite grows in size. Simultaneously, the vacoule also grows which pushes the Nucleus towards the periphery and it resembles like ring. Therefore, it is called as signet ring stage. In a gully grown parasite, the vacoule disappears and it becomes irregular in shape with numerous pseudopodia like projection. Therefore it is called as amoeboid stage. The pseudopodia like projection disappears and it becomes round or oval like structure which is known as Schizont.
In Schizont, the Nucleus divides six to twelve times to form twelve to twenty-four Nuclei. The Nuclei are pushed towards the periphery and covered by incomplete division of cytoplasm by which it looks like the petals of rose flower. Therefore, it is Roset stage. By this time a yellow or orange granules appears at the centre of infected R.B.C which is called as Hemozoin. It is a toxic chemical that is responsible for fever in malaria. The fully infected R.B.C becomes week and feeble and ruptures to liberate third generation parasite that is Erythrocytic Merozoites along with Hemozoin in Sinusoids of liver (fluid filled space between Hepatic acinus). The time period from the entry of sporozoites into the human body and up to the appearance of first malarial fever is incubation period that varies from 10-14 days. In malaria of Plasmodium vivax, the fever repeats after every 48 hrs because it requires 48 hrs for the completion of Erythrocytic cycles to release Hemozoin. Some of the merozoites attacks new liver cell to begin post Erythrocytic Schizogony.

4. Post-Erythrocytic Schizogony
After repeated Erythrocytic cycle, some of the merozoites grows in size and this larger merozoites are called as Gametocytes. Gametocytes start to circulate in the peripheral circulatory system until it is sucked by mosquito. [If the Gametocytes are not sucked by mosquito then after 3-4 days the Gametocytes are destroyed in Spleen and Liver]. In the stomach of mosquito, the Gametocytes are changed into male or female gametes.

The male gametes are small and motile in which the Nucleus divides 3-4 times and migrates towards periphery. By this time, small projection appears in the cell surface of male gametocytes which later develops into long filamentous finger like projection that is flagella and this process is called as Ex-flagellation. Individual flagella acts as male gamete which separates from main body and circulates freely in the stomach until it is fused with female gametes. The female gametocytes are larger and stationery and it changes to female gametes by the appearance of cone of reception through which male gametes enters and fertilizes the egg to form diploid zygote. The zygote remains motionless for about 24 hrs and later it becomes motile worm like and pointed. This form of zygote is known as ookinete or vermicule.

The ookinete penetrates the wall of stomach and migrates towards outside where it is covered by a cyst to form oocyst. According to Bano(1969), the first division of Nucleus in oocyst is reductional followed by repeated meiotic division to form numerous haploid Nuclei. This form is called as sporoblast. The Nuclei of sporoblast divides by multiple fission to form thousands of spores. This structure is called as sporocysts. Indivisual s[pres grows into Sporozoites. Thus, the sporocysts ruptures and liberates the sporozoites Haemocoel of mozquito. From here, the sporozoites migrates to salivary gland and remains there until it bites the new host.

Plasmodium falciparum

Kingdom: – Protista

Phylum: – Protozoa

Class: – Ciliata

Genus: – Plasmodium

Species: – falciparum

It is widely distributed in sub-saharan region of Africa and in Nepal this species is recorded from western Terai esp. in Dang district of Western Nepal. It causes cerebral malaria and the mortality rate is very high. The incubation period varies from 10 to 16 days and this species attacks old and mature R.B.C’s. In the life cycle, Post Erythrocytic Schizogony is absent. In Erythrocytic Schizogony, Schuffner’s Dots are present of which function is still unknown. In this species also fever repeats after every 48 hrs. Death is due to the blockage of arteries of vital organs like Brain, Heart and Liver.