Plasmodium vivax is one of the two protozoan parasites responsible for most of the human morbidity and mortality due to malaria (the other being P. falciparum). It is responsible for an estimated 80 million to 300 million clinical cases every year—including severe disease and death. The estimated number of annual infections is between 130 million and 400 million. Despite this large burden of disease, P. vivax is sometimes overlooked in the shadow of the enormous problem caused by P. falciparum in sub-Saharan Africa. (Mueller et al. 2009 and references therein) Based on where they live, an estimated 2.6 billion people were at possible risk of infection with P. vivax as of 2005 (Guerra et al. 2006).
Plasmodium vivax is geographically the most widely distributed cause of malaria in humans (Mueller et al. 2009) and is responsible for more than 50% of all malaria cases outside Africa. It is endemic in the Middle East, Asia, and the Western Pacific, with a lower prevalence in Central and South America. Probably more than 80% of the worldwide vivax malaria burden lies in south and southeast Asia, although many masked infections may go undetected in Africa. The total burden of all malaria in the Americas is low compared with that in Africa, but P. vivax seems to account for more than 70% of malaria in the Americas and locally causes a substantial clinical and socioeconomic burden. The geographic distribution of endemic vivax malaria overlaps with that of endemic falciparum malaria, except in temperate zones, such as the Korean peninsula, where only vivax malaria occurs, and in much of sub-Saharan Africa, where many human populations lack the Duffy blood-group-antigen, which P. vivax requires to invade red blood cells. (Sattabongkot et al. 2004 and references therein; Mueller et al. 2009 and references therein)
Plasmodium vivax is generally viewed as causing benign uncomplicated malaria. Vivax malaria differs in a variety of clinical features from falciparum malaria, most famously in the development in P. vivax of dormant forms in the liver known as "hypnozoites", which cause subsequent infections in the blood known as relapses. However, it has become increasingly clear that P. vivax infections can result in severe clinical symptoms similar to those caused by P. falciparum. Plasmodium vivax and P. falciparum often co-exist in many parts of the world. In Thailand and many other malaria-endemic countries, P. falciparum and P. vivax are often transmitted by the same vector species. (Sattabongkot et al. 2004 and references therein; Mueller et al. 2009 and references therein)
The length of the dormancy before relapse varies between different strains of P. vivax. Hulden and Hulden (2011) proposed that besides being genetically determined by the specific strain, a relapse of vivax malaria is induced by the bites of uninfected 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)
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)
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).
