Four species of Plasmodium 25/4/06. By Tim Beanland There are over 120 species of the parasite genus Plasmodium, but only four of these infect humans to cause malaria.

Humans can be infected by four species of Plasmodium parasites: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malariae. The main differences are between Plasmodium falciparum, which can cause severe (life-threatening) malaria, and the other three species, which cannot.

In some cases the patient feels fine or is only mildly unwell, and the infection is detected by finding parasites in the blood. This is common among people who have lived through several bouts of malaria and so have developed a degree of immunity to it, such as older children and adults in much of Africa south of the Sahara. Infection without symptoms is very rare among people with no immunity, such as travellers from a malaria-free area who become infected.

Symptomatic infection with Plasmodium falciparum in most cases causes uncomplicated malaria, which can be very unpleasant but is not an immediate threat to life. The patient typically suffers from fever with headache, often diarrhoea and vomiting, low blood haemoglobin (anaemia) and an enlarged spleen. Children may additionally suffer rapid breathing, a cough and fitting.

Background: Uncomplicated malaria

In a small number of cases, uncomplicated falciparum malaria develops into severe malaria, which is a life-threatening condition. It requires urgent hospital attention.

Background: Severe malaria

Background: Plasmodium falciparum

When P. vivax, ovale or malariae cause symptoms, the clinical picture generally resembles uncomplicated falciparum malaria, but there are some important differences. The fever in untreated vivax malaria tends to spike every 48 hours, hence it used to be called a 'tertian' malaria. In vivax malaria the parasite can also become dormant in the liver, causing recurrence after many months. Ovale malaria is like vivax only develops more slowly. Malariae malaria is the least common form and causes the least severe symptoms. If untreated it causes fevers that spike every 72 hours (hence 'quartan malaria').

Image credit: D Greenwood