Lifecycle of Plasmodium

Plasmodium requires two hosts for completion of the lifecycle. They are: Human and Female

• Human Female
• Female anaphase mosquito.

Mosquito is a primary or definitive host where the sexual cycle of plasmodium occurs. Human is a secondary or intermediate host where the asexual cycle of plasmodium occurs.

Asexual cycle(schizogony) in man:

Schizogony is the asexual multiplication of schizont ( fully growing and multiplying trophozoite stage of plasmodium) to produce merozoites. It occurs in human liver( liver schizogony) cell and RBC ( Erythrocytic schizogony).

Liver schizogony:

It involves following three phases:

Pre-Erythrocytic cycle:

When infected mosquito bites, it injects a large number of sporozoites along with saliva into the human blood. All sporozoites then migrate towards liver for shelter and further growth. They enter into the liver cell and grow into rounded schizont which divides by multiple fission to produce a large number of crypto merozoites. They either attack fresh liver cell to start exo-erythrocytic cycle or attack fresh RBC to start the erythrocytic cycle. This phase is about 8 days. This period is called pre- patent period ( a period between initial infections of sporozoites until initiation of erythrocytic cycle.)

Exo- erythrocytic cycle:

It is the second phase of asexual multiplication of plasmodium in the liver cell. It starts when crypto merozoites enter into the fresh liver cell and grow into rounded schizont. Schizont then undergoes multiple fission, to produce meta crypto merozoites. They are of two types smaller are called micro metacrypyomerozoites which attack fresh RBC. Larger are called macro meta crypto merozoites which attack liver cell to continue the exo-erythrocytic cycle.

Exo-erythrocytic cycle is absent in Plasmodium falciparum.cle:

Post -erythrocytic cycle:

It is the third phase of asexual multiplication of plasmodium in the liver cell. It occurs when erythrocytic merozoites infect the fresh liver cell.

Erythrocytic schizogony:

It is the growth, development and asexual multiplication of Plasmodium in RBC. It starts when crypto merozoites enters into the RBC and passes through different stages which are discussed below:

Trophozoite stage

It is the feeding stage of Plasmodium. It mainly feeds on cytoplasmic nutrients and haemoglobin. It breaks haemoglobin into haematin and globin with the help of lysolecithin. Globin is a protein so used as a food by trophozoite. Haematin in iron pigment which is converted by trophozoite into haemozoin.( Toxic Malarial Pigment)

Signet- ring stage:

As trophozoite grows, a large vacuole is also developed at the centre that pushes nucleus and cytoplasm towards the periphery. Because of this trophozoite assume the shape of the signet ring.

Amoeboid stage:

With further growth, vacuole disappears because of which the shape of trophozoite become irregular like that of amoeba so-called amoeboid stage.

Schizont stage:

Now the trophozoite became double in size. This fully grown trophozoite is called schizont. At this stage smaller granules appear inside the RBC called Schaffner dots.

Rosette stage:

The nucleus of schizont divides into 12 to 24 nuclei which are arranged in a circular manner. After cytoplasmic division, each nucleus derives cytoplasm and develop into merozoites. They are arranged inside RBC like petals in the rose so-called rosette stage. Haemozoin pigment is accumulated at the centre.

When infected RBC burst haemozoin and merozoites are released into the plasma. Haemozoin cause high fever whereas merozoites again infect fresh RBC to continue the erythrocytic cycle. This cycle is completed in about 48 hours and this cycle is also called the cycle of Golgi. Some merozoites may also infect the fresh liver cell to start post erythrocytic cycle.

Formation of gametocytes:

After several erythrocytic cycles, some merozoites enters into the RBC but do not develop into trophozoite. They become enlarged, rounded and develop into two types of gametocytes i.e. microgametocyte and macrogametocyte. For further development they need mosquito host i.e. they must be sucked by mosquito along with blood otherwise, they will die within few days.