Plasmodium malariae is one of the five described Plasmodium species that cause malaria in humans (with Plasmodium falciparum generally causing the most severe disease and most of the mortality from the disease). Although distribution of Plasmodium malariae infection is reported as being patchy, it has been observed in all major malaria-endemic regions of the world. Plasmodium malariae infections are most common in sub-Saharan Africa and the southwest Pacific, where age-specific prevalence estimates in mass blood surveys have exceeded 15 to 30%. By contrast, when P. malariae has been detected in malaria endemic regions of Asia, the Middle East, South America, and Central America, it is observed as an infrequent infection, with blood-smear light microscopy (LM) prevalence rarely exceeding 1 to 2%. Much higher levels of infection were, however, found in montagnard refugees from the Cambodian–Vietnamese border. In South America, P. malariae is thought to be a zoonotic infection because the apparently genetically identical P. brasilianum infects new-world monkeys and both monkeys and humans in endemic areas show high levels of seropositivity to P. malariae and P. brasilianum antigens. As with P. falciparum infections, infection with P. malariae in West Africa is most frequent in children under ten years of age, although in Papua New Guinea (PNG) it is common in adolescents as well. Some evidence suggests that P. malariae infections may have a mitigating effect on both P. falciparum and non-P. falciparum illness and, therefore, that treatment of P. malariae and/or P. ovale could increase the pathogenesis that is associated with other malaria species. With vaccines currently being developed against P. falciparum and P. vivax, it is important to determine the burden of infection and disease due to P. malariae and P. ovale and assess the potential for interaction between these malaria parasite species. Should negative interactions be important, then reducing the burden of P. falciparum or P. vivax morbidity could increase the burden of morbidity that is attributable to P. malariae and P. ovale. The possibility for such an effect is highlighted by studies in Brazil and PNG that found high levels of P. malariae infections in isolated populations where either P. falciparum was absent or P. vivax rare. (Mueller et al. 2007 and references therein) Plasmodium malariae can persist at low levels in a human host for decades, remaining infectious to mosquito vectors.
The human malaria parasite life cycle involves two hosts, a mosquito and a human. The life cycle is very complex, including both sexual and asexual phases (see life cycle diagram) and involves a stage in the liver as well as the blood stage, the latter being responsible for the clinical manifestations of the disease. (Centers for Disease Control Parasites and Health website)
Until recently, Plasmodium knowlesi malaria in humans was misdiagnosed as the less dangerous Plasmodium malariae malaria (molecular diagnostic techniques are important, since confusion with P. malariae is easy using conventional microscopy).
There are more than 150 named species of Plasmodium that infect various species of vertebrates. Four species are well known as true parasites of humans, utilizing humans almost exclusively as a natural intermediate host: P. falciparum, P. vivax, P. ovale and P. malariae. In recent years it has become apparent that the simian malaria parasite P. knowlesi also regularly infects humans, as well as its natural monkey intermediate hosts. (Centers for Disease Control Parasites and Health website) Furthermore, it is now apparent that there are likely two distinct Plasmodium species that have both been referred to as P. ovale (Sutherland et al. 2010; Oguike et al. 2011).
