The parasites that cause malaria belong to a group of micro-organisms called protozoans. There are four main species that cause malaria in humans:
- Plasmodium falciparum
- Plasmodium vivax
- Plasmodium ovale
- Plasmodium malariae
Malaria parasites have a complex life cycle that involves replication in both mosquitoes and in humans.
While in humans, malaria parasites infect red blood cells and multiply within them. After multiplication has occurred, the red blood cells burst releasing the newly produced parasites. The released parasites then begin this cycle again by infecting other red blood cells.
An unusual aspect of this process is that the bursting of all infected red blood cells occurs synchronously. For example, in Plasmodium vivax, infected red blood cells burst synchronously every 48 hours. This gives rise to the characteristic “tertian fever” pattern that is seen in disease caused by this species (that is, a fever that relapses every two days). Similarly, in Plasmodium malariae, infected red blood cells burst synchronously ever 72 hours. This gives rise to the characteristic “quartan fever” pattern that is seen in disease caused by this species (that is, a fever that relapses every three days).
Interestingly, all malaria species seem to have synchronous bursting cycles with periods that are multiples of 24 hours. It is not known why this occurs, but the regularity of this process lends itself naturally to mathematical modelling (Greischar at al. 2014).
References
Greischar, M.A., A.F. Read, and O. Bjornstad. 2014. Synchrony in malaria infections: how intensifying within-host competition can be adaptive. American Naturalisty 183:E36-E49