Cohort study. Hemoglobin (Hb) and red blood cell (RBC) polymorphisms that give rise to HbE, alpha-thalassemia, G6PD-deficiency, and ABO blood groups occur at high frequency along the Thailand-Cambodia border, where Plasmodium vivax and P. falciparum have been and continue to be transmitted. To determine whether these Hb/RBC polymorphisms have been naturally selected because they confer protection against malaria and malaria-associated anemia, we will conduct a cohort study of ethnic Khmer in Cambodia. Approximately 1000 individuals of all ages will be genotyped for the four polymorphisms listed above and then followed for 5 years to determine the mean incidence rates for both P. vivax and P. falciparum malaria, stratified by genotype. Incidence rate ratios (IRRs) will be calculated for each polymorphism relative to the wildtype genotype. Differences between Hb levels during acute episodes of malaria and Hb levels at baseline will also be calculated to determine if Hb/RBC polymorphisms influence the degree of malaria-associated anemia.

P. vivax collection study. Unlike P. falciparum, P. vivax cannot be efficiently cultivated in vitro. Improved cultivation methods are needed to make progress on nearly all aspects of P. vivax malariology, including pathogenesis, naturally-acquired immunity, vaccination, and antimalarial drug resistance. We plan to improve both short- and long-term cultivation methods in order to test various hypotheses of P. vivax pathogenesis and protection. Using freshly obtained parasite isolates from individuals with P. vivax malaria, we will test whether Hb/RBC polymorphisms influence potentially pathogenic properties of P. vivax parasites, such as their ability to bind non-infected RBCs and other host cells. It is believed that P. vivax selectively invades reticutocytes. This tropism has frustrated attempts at long-term cultivation of this parasite, which requires a constant source of reticulocyte-rich blood not easily obtained even in developed countries. The host reticulocyte receptor that mediates the highly selective tropism of P. vivax has not been identified. Fresh P. vivax parasites will also be used in in vitro experiments to identify the putative receptor that defines reticulocyte tropism. Any P. vivax ligand that bound selectively to a reticulocyte receptor will then be discovered and worked up as a promising vaccine candidate.