Posted on by Patrick Smith

Hitting Malaria where it hurts: how to eradicate the Malaria carrying mosquito.

Picture of a mosquito
Anopheles gambiae, the mosquito responsible for carrying the Malaria causing Plasmodium falciparum. Image from Scientists Against Malaria.

Why should you care?

When talking about the leading causes of mortality (discounting war and starvation) a few big causes pop into most people’s minds: heart disease, cancer, and HIV-AIDS. In the westernized world, one that doesn’t come to mind is malaria, largely due to our advanced medical care and simply not having a large population of the malaria carrying mosquitoes in the genus Anopheles. However, malaria still infects roughly 225 million people per year and causes roughly 1 million deaths, the majority of which are in Africa (Murray et. al, 2012). A major field of medical research is devising methods of intercepting insect-borne diseases such as malaria, yellow fever, and the insidious Zika virus before human infection. That is, somehow preventing the mosquitoes from spreading the disease or even being infected in the first place, rather than treating the symptoms of the diseases in humans. One such experimental method is to cause significant mosquito mortality by spreading the bacterium Wolbachia, a parasite that is harmless to humans but lethal to mosquitoes after a blood meal. What would this mean for the rest of the environment and organisms? In an ideal scenario the Wolbachia parasite would target and swiftly execute only Anopheles mosquitoes, which are the ones carrying the malaria causing Plasmodium, while leaving all other types of mosquitoes that help sustain an ecosystem unharmed. There is still a lot of research to be done, but we are finding out some things about this potential biocontrol already.

What are you learning about?

A group of researchers studied the bacterium Wolbachia and how it can be transmitted throughout the mosquito population (Hughes et al, 2014). More specifically, they studied Wolbachia’s mechanism of vertical transmission, i.e. from mother to offspring rather than between adults. In order for it to remain in the mosquito population over time it is not enough for Wolbachia to be transmitted through contact with another mosquito, it must also be transmitted upon egg laying. The specific questions that this study focused on was this: What factors influence the vertical transmission and effect of Wolbachia in Anopheles mosquitoes; including species of mosquito, species of Wolbachia, and the presence of other bacteria living in the mosquitoes already?

What did they find?

The first thing the researchers found was that a specific species, or strain, of the bacteria Wolbachia was better at infecting Anopheles mosquitoes. After being injected into the mosquitoes, strain wAlbB was found in significantly higher levels in the ovaries (necessary for vertical transmission) than other Wolbachia strains, and in one species of mosquito was shown to increase over time as well. This strain was used for the remainder of the experiments.

They also found that mosquitoes who had been raised on a cocktail of several oral antibiotics before infection with wAlbB transmitted it to many more of their offspring than those that were not treated with antibiotics. Over 90% of offspring from mothers treated with antibiotics had the wAlbB parasite, whereas <10% of offspring whose mothers were raised without antibiotics showed a wAlbB infection (Hughes et al, Figure 2). This implies that the bacteria that naturally live inside mosquitoes work to prevent the transmission of Wolbachia and thus reduce its effectiveness as a treatment for insect-borne diseases!

Following this discovery the researchers wanted to know more about which bacteria specifically were inhibiting the transmission of wAlbB, so they compared the species found in normal mosquitoes to those found in mosquitoes treated with antibiotics. They found one group in particular, the bacterial genus Asaia, that showed a significantly lower number in those treated with antibiotics compared to the number found in normal mosquitoes. In order to investigate further whether or not these bacteria, Asaia, were the culprits responsible for the lower wAlbB infection strength they had to create a group of mosquitoes who only had Asaia bacteria and no others (besides wAlbB, of course). They then compared the survival of these three groups of mosquitoes; those raised normally, those raised on antibiotics, and those raised on antibiotics but also infected with Asaia. They found two things: firstly, two days after having a blood meal roughly 90% of the mosquitoes raised on antibiotics, the Asaia-free group, died. This means that without Asaia living in mosquitoes Wolbachia treatment could be very effective in reducing mosquito populations. Additionally, they found almost no difference between the survival of mosquitoes with just Asaia and those with the “normal’ bacteria. This implies that Asaia is in fact a very likely cause of mosquitoes’ inherent resistance to Wolbachia infection.

Where does this leave us?

There are several useful findings in this study but in reality we are still a long way from using treatments such as Wolbachia infection as a valid weapon against insect-borne diseases. We know that certain bacteria, such as Asaia in this study, can reduce the infectivity of Wolbachia  but is that the only bacteria that has that effect? Are Asaia even present in most natural populations of mosquitoes? Further research needs to be done on the specific mechanisms that are at play in the interaction between Asaia and Wolbachia. Are there specific chemicals that Asaia uses to inhibit Wolbachia? Are some strains of Wolbachia less affected by Asaia than others? Additionally, what makes Wolbachia kill the mosquito after a blood meal and can we isolate that mechanism for use in other ways? Exciting developments are being made in the Hollywood mad scientist-like field of synthetic biology, which looks at engineering specific functions and inserting them into other organisms, or theoretically even creating an entirely new organism! Perhaps many years down the road they will be able to insert the mosquito killing potential of Wolbachia into a bacterium that is not affected by the presence of other bacteria like Asaia, resulting in an effective population level weapon against deadly insect-borne diseases like malaria and the Zika virus.

Where can you feed this new found lust for information?

  • Tired of reading? Watch this talk on the topic of genetically engineered mosquitoes.
  • For a comprehensive overview of what we know about malaria and how it interacts with its mosquito and human hosts check out this review.
  • For more on insect transmitted viruses and how they are affected by the bacteria in those insects read this article.
  • For a near unmanageable amount of current and exciting research in the field of synthetic biology that I alluded to check out this site.
  • Want to learn more about bacteria and how they interact with the other organisms in the world? Look here.

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