The Mosquito Microbiome and Its Impact of Malaria Transmission

The aim of this research paper is to outline the diverse nature of the mosquito gut biome; and by investigating it in further detail, understanding how it is involved in the transmission of malaria. Using this information, we can then look into methods to reduce the spread of malaria. It is also important to note that it is only certain types of mosquitoes that act as a vector for malaria. The genus Anopheles are the only know type of mosquito that can infect humans with malaria, and in addition to this it is only the female that bite humans and inject the plasmodium bacterium into the host.

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).

The Disease

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).

The schizonts divide into several merozoites and when the red blood cell ruptures, this is when the clinical symptoms of malaria such as colds and fever begin to show in the human host. Not all of the merozoites divide into schizonts; some differentiate into male and female gametocytes. A female anophylean mosquito can then take these up during a blood meal. Within the mosquito midgut, the male gametocyte undergoes a rapid division, creating 8 flagellated microgametes, which fertilise the female macrogamete. The resulting ookinete crosses the mosquito gut wall and encysts on the exterior of the gut wall as an oocyst. Once the oocyst bursts, hundreds of sporozoites are released into the mosquito body cavity where they eventually move into the mosquito salivary gland ready for transmission.

Anopheles Mosquitoes (vectors)

As aforementioned the female Anopheles mosquitoes are the only known vectors of the plasmodium parasite. Amongst the 380 species of mosquitoes about 60 can transmit malaria to humans. Females are the only form that spread the disease because the require blood meals for the growth of their eggs. Conversely male mosquitoes can survive on sugar sources alone.

The females lay their eggs onto water, where they develop into larvae within approximately 48 hours. Anopheles larvae then go through four more stages before they develop into pupae. During this time the larvae feed on organic matter and most importantly microorganisms (which will be discussed later) within their aquatic habitat (Merritt et al, 1992). Once the larvae develop into pupae it no longer has a mouth and therefore cannot feed. After the pupal stage the mosquito transforms through complete cycle of metamorphosis into an adult mosquito.

The main species of mosquito that carry malaria are; Anopheles gambiae, Anopheles arabiensis and Anopheles funestus (Moreno, 2010). Anopheles gambiae is probably the most well known of these because of its role in transmitting the most deadly parasite species plasmodium faliciparum. These mosquitoes prefer to live indoors and feed on humans and more vulnerable to parasite infection. They are commonly found in the hot and humid environments of equatorial Africa.

Anopheline gut microbiome

The mosquito gut has an abundance of bacterial flora within it. One of the reasons for this is because bacteria are an important larval food for mosquitoes, some of these are digested and excreted but others remain in the larvae and are still present in the adult gut. Studies have shown that mosquito larvae reared in the presence of Gentamycin (antibiotic), show less growth than without it (Wotton, 1997).

In addition to this further studies have shown that transstadial transmission of bacteria from larvae