Plasmodium falciparum is found worldwide in tropical and subtropical areas with a predominance in Africa. P. falciparum can cause severe malaria because it multiplies rapidly in the blood which can result in severe blood loss leading to anemia. Additionally, the infected red blood cells can clog small vessels due to the ability of the parasite to synthesize adhesion molecules (PfEMPs), export them to the RBC surface, and then attach to the vessel surface. This process helps the parasite evade splenic destruction.
Plasmodium vivax is found mostly in Asia, Latin America, and in some parts of Africa. Due to the population densities, P. vivax is probably the most prevalent human malaria parasite. Both P. vivax and P. ovale have dormant liver stages (“hypnozoites”) that can activate and invade the blood (which is referred to as “relapse”) several months or years after initial infection. It has been found that P. vivax can use the Duffy red blood cell antigen to enter the cell; thus Duffy negative people tend to be more resistant to P. vivax infection.
Plasmodium ovale is found mostly in Africa (particularly West Africa) and the islands of the western Pacific. P. ovale and P. vivax are both biologically and morphologically very similar. One major difference from P. vivax is that P. ovale can infect individuals who are negative for the Duffy blood group as is the case for many residents of sub-Saharan Africa. This would seem to explain the greater prevalence of P. ovale in most of Africa rather than that of P. vivax.
Plasmodium malariae is found worldwide and is the only human malaria parasite species that has a quartan cycle (three-day cycle). P. falciparum, P. vivax, and P. ovale all have tertian (two-day) cycles. P. malariae causes a long-lasting, chronic infection that can last a lifetime if left untreated. Nephrotic syndrome can occur in some chronically infected patients.
Plasmodium knowlesi is found as a natural pathogen of long-tailed and pig-tailed macaques throughout Southeast Asia. In Malaysia, it has been shown to be a significant cause of zoonotic malaria. P. knowlesi has a 24-hour replication cycle and can rapidly progress to a severe infection from a seemingly uncomplicated infection.
