Variant genes and the spleen in Plasmodium vivax malaria

It is generally accepted that Plasmodium vivax, the most widely distributed human malaria, does not cytoadhere in the deep capillaries of inner organs and thus this malaria parasite must have evolved splenic evasion mechanism in addition to sequestration. The spleen is a uniquely adapted lymphoid organ whose central function is the selective clearance of cell and other particles from the blood, and microbes including malaria. Splenomegaly is a hallmark of malaria and no other disease seems to exacerbate this organ as this disease does. Besides this major selective clearance function however, the spleen is also an erythropoietic organ which, under stress conditions, can be responsible for close to 40% of the RBC populations. Data obtained in experimental infections of human patients with P. vivax showed that anaemia is associated with acute and chronic infections and it has been postulated that the continued parasitemia might have been sufficient to infect and destroy most circulating reticulocytes. We review here the basis of our current knowledge of variant genes in P. vivax and the structure and function of the spleen during malaria. Based on this data, we propose that P. vivax specifically adhere to barrier cells in the human spleen allowing the parasite to escape spleen-clearance while favouring the release of merozoites in an environment where reticulocytes, the predominant, if not exclusive, host cell of P. vivax, are stored before their release into circulation to compensate for the anaemia associated with vivax malaria.