Before we analyse techniques of transforming bacteria within the mosquitoes or manipulating the insects themselves, it is important that we breakdown the complexity of the malaria disease and understand what I feel are the three key aspects: the disease itself, Plasmodium parasites (malaria causing parasite) and Anopheles spp. mosquitoes (vectors).
Malaria is most definitely one of the world's most threatening diseases. Not just because it kills more than 1 million people annually (WHO), with 300-500 million clinical cases of the disease, but the disease is endemic in over 100 countries with more than 80% of the deaths in sub-Saharan Africa. Most tragically however is the fact that malaria is particularly deadly towards children below the age of five. Consequently this malaria is an extremely difficult disease to control and in the future this is only set to get worse. Drugs used on the parasites have slowly become less and less effective as the parasites have developed resistance; furthermore efforts to kill the mosquito vectors have yielded a similar response as they have become immune to the various insecticides used to control mosquito populations, with predictions suggesting that the threat from malaria will only get worse in the coming years. Mosquitoes in particular are highly sensitive to temperature. Anopheles spp. mosquitoes normally do not reproduce below 16oC. With adequate moisture levels, increased temperatures will generally cause an increase in mosquito; populations, biting rates, and general activity, which will accelerate the incubation of the parasites and viruses within them (Epstein, 1998). Global warming will hence increase the geographic range within which both the mosquito and malaria parasite could survive, with enough abundance for continued transmission. Models predict that a 3oC global temperature rise can increase the number of annual malaria cases by 50-80 million (Martens, 1995).
Plasmodium (malaria causing parasite)
Figure 1- Plasmodium life cycle (Professor Jacob Koella, Imperial College London)
The parasites that are attributed to causing malaria are from the Plasmodium genus. There are 4 main types that infect humans; P.falciparum, P.vivax, P. malariae, P.ovale. These parasites do not live outside an Anopheles mosquito or human host. P.falciparum is regarded the most deadly of the malaria transmitting parasites, and often causes death in humans. This can also be true of P.vivax in some cases. Fig.1 (above) shows a simplified overview of the plasmodium life cycle, both inside and outside it's human host, however I will discuss this is a little more detail. The life cycle of all Plasmodium species is quite complex. Humans are infected when biten by an Anopheline mosquito carrying the parasite. During feeding the Sporozoites move from the mosquito salivary glands into the bloodstream and in the hepatocytes (liver cells). In the next 2-week cycle of P. falciparum, the parasites within the liver cells differentiate and undergo asexual multiplication resulting in thousands of merozoites, which burst from the hepatoctye. These merozoites invade red blood cells (erythrocytes) and undergo an additional round of multiplication to form trophozoites and then schizonts over a 48 h cycle (varies from species to species).