Plasmodium falciparum is one of the two protozoan parasites responsible for most of the world's cases of human malaria (the other being P. vivax). Hay et al. (2010) estimated that in 2010 there were around 450 million clinical cases of P. falciparum malaria in the world. Based on where they live, an estimated 2.5 billion people were at possible risk of infection with P. falciparum as of 2005. Three quarters of people exposed to P. falciparum risk live in just ten countries. (Guerra et al. 2006). According to Hay et al. (2010), around half the world’s estimated P. falciparum clinical cases (and nearly half the statistical uncertainty in estimates) derive from just four countries: India, Nigeria, Democratic Republic of the Congo (DRC), and Myanmar (Burma). 60% of the estimated world falciparum malaria burden falls in Africa and nearly a quarter on Nigeria and DRC alone. The current falciparum malaria burden in India is very unclear, but official surveillance numbers appear to be dramatic understimates. India's malaria problem is probably exacerbated by the unique problem of urban malaria, maintained by Anopheles stephensi. (Hay et al. 2010) Plasmodium falciparum is responsible for most of the nearly one million people (many of them children) killed by malaria each year.
In contrast to P. vivax, P. falciparum does not have a dormant liver stage. Instead, the gametocytes of P. falciparum can survive in the blood for months. (Mueller et al. 2007)
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)
The most obvious features that distinguish Plasmodium vivax from P. falciparum include the development of dormant forms in the liver known as "hypnozoites", which cause subsequent infections in the blood called relapses; the appearance (sometimes before onset of clinical symptoms) of round gametocytes in the peripheral blood (i.e., not banana-shaped gametocytes like those produced by P. falciparum); a predilection (or requirement) of merozoites for reticulocytes as host cells; circulation of all blood-stage developmental forms in the peripheral blood; the absence of electron-dense protrusions (known as knobs in P. falciparum); and the presence of numerous caveolae–vesicle complexes along the surface of infected red blood cells. (Mueller et al. 2009 and references therein)
Evolution and Systematics
Systematics and Taxonomy
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).