The Burden of Malaria

Despite intense efforts, malaria remains a leading cause of morbidity and mortality worldwide. The World Health Organization estimates that nearly half a billion clinical cases of malaria occur each year, with over one million deaths (1,2). Almost 90% of these deaths occur in sub-Saharan Africa, where the risk of severe and potentially fatal Plasmodium falciparum infection is highest in young children. Recent estimates suggest that without immediate action, the number of malaria cases is likely to double over the next two decades (3). This resurgence is the result of a combination of factors, such as the breakdown of existing control programs and health infrastructure and the rapid spread of resistance to the most effective and affordable antimalarial drugs and insecticides used to control the parasite and mosquito vector populations, respectively. Therefore novel means of controlling malaria, based on integrated control strategies and innovative approaches, have been proposed. These will require an exquisite understanding of the biology of the malaria parasite and the interactions that occur between it, the mosquito vector, and the vertebrate host.

Exploiting Weaknesses in the Plasmodium Life Cycle for Malaria Control

The life cycle of the malaria parasite (Figure 1) involves a complex relationship with a vertebrate host, in which it undergoes an asexual stage of development, and an Anopheles mosquito vector, where its sexual development occurs. This relationship has been honed over millennia of co-evolution to enable the parasite to partially evade both the human and insect immune systems and ensure its own survival (4). Infection starts with a bite of an infected mosquito, when the motile sporozoite is introduced into the skin together with mosquito saliva. The sporozoite enters the bloodstream and is rapidly carried to the liver where it invades a hepatocyte. Within the hepatocyte, the parasite undergoes a period of differentiation and multiplication to produce the pre-erythrocytic schizont, containing thousands of merozoites. The merozoite, when released from the hepatocyte, enters the bloodstream and invades an erythrocyte, differentiating through trophozoites to an erythrocytic schizont. This contains a small number of merozoites which, when released, go on to invade new erythrocytes. After a number of intra-erythrocytic cycles, a proportion of merozoites undergo sexual differentiation to produce male and female gametocytes, which are then picked up in the blood meal of an Anopheles mosquito. Once inside the midgut lumen, the gametocytes start to differentiate almost immediately. Fertilization between the microgamete and macrogamete produces a zygote that slowly elongates into a motile, invasive ookinete. The ookinete passes through the peritrophic matrix that encases the blood bolus and traverses the midgut epithelium. Upon reaching the basal lamina, the parasite transforms itself into an oocyst that bursts after approximately 7 to 16 days, depending on the parasite species and temperature, releasing thousands of sporozoites into the hemocoel. These sporozoites then specifically recognize and invade the mosquito's salivary glands, and upon the next blood feeding are injected into the vertebrate host, initiating a new cycle of infection.

Figure 1. The life cycle of Plasmodium falciparum. (Click to enlarge)

The successful completion of this cycle is a fine balance between host factors, the agonists, which promote parasite development, and include molecules on the surface of both the mosquito and vertebrate host tissues that function as receptors for parasite surface ligands and the antagonists that negatively regulate it, such as components of the mosquito and vertebrate immune systems. While agonists and antagonists to Plasmodium development in the vertebrate host have generally been well characterized, due in part to the ability to culture P. falciparum in vitro (5,6,7) and the existence of the genetically tractable Plasmodium berghei murine model of malaria (8), relatively little was known of the molecular biology of the mosquito vector until recently. The urgent need, however, to address the resurgence of malaria has prompted the development of efficient molecular and genetic tools for Anopheles mosquitoes over the past few years, aimed at a better understanding of the genetic basis of mosquito-pathogen interactions. This effort has led to the achievement of a number of prominent technological milestones, including the development of an efficient germline transformation system for Anopheles stephensi (9), an important urban vector species in Asia, and Anopheles gambiae (10) mosquitoes, the principal African malaria vector, and the parallel completion of the A. gambiae genome sequence (11). The adaptation of RNAi techniques to Anopheles mosquitoes (12,13) has since provided the missing link to perform functional studies in the mosquito vectors of human malaria, that could ultimately provide an important contribution to vector control strategies. One of the driving forces behind these studies is the idea that mosquitoes can be genetically engineered to be refractory to pathogen development, and with an effective genetic drive mechanism, a pathogen-susceptible mosquito population may be replaced with a refractory one (14,15).