Stage three is the stage which ocurrs in the red blood cells (RBCs) and is called the erythrocytic cycle.

In this cycle the merozoites which have been released from the liver attach to RBCs and develop and multiply further.

The merozoites produced as a result of this development are found in the blood about 9-12 days after the person has been infected (this depends on the strain of parasite). This delay in time means that the symptoms of malaria also don't occur until over a week after being bitten, and also, a person is not infectious immediately.

There are three stages to the development of the merozoites in the RBCs.

Secondly, the merozoites develop into trophozoites.

You can see these on the image on the right. Trophozoites are identified by the ring form which they take.

Image source: CDC public health image library

The parasite continues to develop and forms schizonts. Schizonts are cells which contain many merozoites.

You can see this in the image on the right. The schizont shown contains 18 merozoites.

The schizont cells eventually rupture and the merozoites are released into the blood stream.

These merozoites then continue to develop in the same way.

Image source: CDC public health image library

This cycle, from meroziote - trophozoite - schizont - cell rupture and release of merozoites occurs about every 48-72 hours depending on the strain of parasite. Symptoms occur at the point when the RBCs eupture and so tend to occur in a cycle.

It is what happens at this stage of the parasite cycle which can cause death in humans. The parasite causes death by a number of means.

• When the RBCs which have been infected and become schizonts rupture, this can cause shock due to loss of water and electrolytes. Becuase there's not enough fluid, blood pressure falls and the vital organs don't get enough blood.
  
• The rupture of blood cells also leads to the release of large amounts of residue pigments that are very toxic and can lead to kidney failure.
  
• The invasion of red blood cells leads to haemolytic anaemia (a severe form of anaemia caused by the breakdown of RBCs) that can lead to heart failure in infants because the heart has to work harder as the body is not getting enough oxygen.
  
• Infected blood cells can block capillaries and thus affect circulation in major organs brain, lungs, kidneys leads to organ failure and death.