Malaria
Human malaria is caused by five Plasmodium species. Each has distinct clinical behaviour, geographic distribution, and life cycle features.
Plasmodium falciparum is responsible for the great majority of malaria deaths worldwide and is the dominant species across sub-Saharan Africa.
- It infects red blood cells of all ages, permitting very high levels of parasitaemia.
- It is uniquely capable of sequestration of infected erythrocytes in the microvasculature of deep organs (brain, placenta, other viscera). This underlies its capacity to cause severe and cerebral malaria.
- It has no hypnozoite (dormant liver) stage, so relapse does not occur. Recrudescence from incompletely cleared blood-stage infection can occur if treatment is inadequate.
Plasmodium vivax is the most geographically widespread malaria species. It is dominant across much of Asia and Latin America, and a significant cause of malaria in parts of East Africa.
- It preferentially infects reticulocytes (young red cells), which limits peak parasitaemia and disease severity relative to falciparum.
- It produces hypnozoites in the liver. These dormant forms can reactivate months to years after the initial infection and cause clinical relapse without a new mosquito bite.
Plasmodium ovale is found predominantly in West Africa and parts of the Pacific. It is generally less common than vivax or falciparum. It also preferentially infects reticulocytes and, like vivax, forms hypnozoites that can cause relapse.
Plasmodium malariae has a worldwide but patchy distribution. It produces a chronic, low-grade infection, often with lower parasitaemia, and has no hypnozoite stage.
- It can persist in the bloodstream at very low, sometimes clinically silent levels for years to decades. It occasionally produces recrudescent disease long after presumed clearance.
- It is also specifically associated with a nephrotic syndrome from immune complex deposition in chronic infection (quartan malarial nephropathy).
Plasmodium knowlesi is a zoonotic species that naturally infects macaque monkeys in Southeast Asia. It is transmitted to humans by Anopheles mosquitoes that feed on both monkeys and people.
- It has the shortest replication cycle of any human-infecting Plasmodium species (24 hours). This can produce rapid escalation of parasitaemia and severe disease.
- It is frequently misidentified as Plasmodium malariae on blood film because of morphological similarity in early trophozoite stages. This is an important diagnostic pitfall in endemic regions of Southeast Asia.
