Plasmodium vivax

Recognizable descriptions of malaria were recorded in Chinese, Indian, Egyptian and Mesopotamian texts as early as 5,000 years ago. Evidence from human DNA sequences shows the effects of malaria to be far older still, influencing human evolution across tens of thousands of years. It is no exaggeration to say that malaria has played a crucial role in human history, determining the fates of armies and empires. Malaria brought down Alexander the Great and saved Rome from Attila's hordes. Dubbed the 'King of Diseases' in the Vedas, its modern name comes from the Italian peninsula, where mal'aria or 'bad air' was thought to cause the debilitating paroxysmal tertian or quartan (three- or four-day) fevers and febrile deaths that ravaged the populace every year for millennia.
We now know that the infectious agent of malaria is not fetid swamp gas but an apicomplexan protist of the genus Plasmodium whose complex life cycle shuttles between human and mosquito hosts without any free-living stages. Four species of the parasite are known to commonly cause malaria in humans: P. falciparum, P. malariae, P. ovale, and P. vivax. Of the four, P. falciparum is the most deadly; vivax malaria was known historically as 'benign tertian' fever. However, vivax malaria is 'benign' only in that the misery the disease causes rarely ends in death. In fact P. vivax has several characteristics which make it a compelling object of study:

1. Distribution & population at risk : P.vivax is the most widely distributed human malaria, and the most common species observed in temperate regions of the world, representing the major cause of malaria outside Africa. More people were at risk from vivax malaria in 2005 than from any other species.
2. Economic impact: Malaria transmission rates are low in most regions where P.vivax is prevalent, thus protective immunity is infrequent and all ages can succumb to the disease. Men of working age, i.e., individuals whose economic productivity is among the most important, are particularly at risk.
3. Morbidity: P. vivax malaria is acute and excruciating, involving repeated episodes of high fever preceded by violent headache and chills and profuse sweating, and often accompanied by vomiting, diarrhea, and enlargement of the spleen. Uncomplicated falciparum malaria is much less intense with less pronounced paroxysms.
4. Anemia: Although parasitemias are lower than those seen with P. falciparum infections due to the preference of P.vivax to infect reticulocytes, anemia caused by chronic destruction and depletion of immature red blood cells is the most common pathological consequence.
5. Selectivity: P.vivax parasites appear limited to invasion of Duffy antigen-positive red blood cells only, which may explain the absence of the species in West Africa where Duffy negativity predominates.
6. Relapse: The hypnozoite stage of P.vivax can lie dormant in infected liver cells for months or years, providing a mechanism for the parasite to hibernate during less optimal transmission periods.