Mosquito-borne diseases: Fighting fire with fire

Follow

I have a decidedly “live and let live” approach to life. There are no animals in this world which I harbour any malicious feelings towards, regardless of how many of those animals would think nothing of poisoning, eating, maiming, or otherwise killing me (it’s a tough world out there). No animals, with one exception. I absolutely detest mosquitos – and not irrationally so.

You see, you might not realise it, but mosquitos are actually the most dangerous animal in the world. Yes, seriously. They may not look like much, but every year, mosquitos will spread diseases to 700 million people. That’s 10% of the human population on this planet. Many of the diseases spread by mosquitos are potentially fatal, and mosquitos are responsible for over 2 million deaths every year. Needless to say, some way of curbing the spread of mosquito-borne disease would be a huge success in combating illness worldwide.

Interestingly, an unusual but potentially effective method has been devised by Ary Hoffmann and Michale Turelli at the University of Melbourne. I say unusual, because their method of preventing mosquitos from spreading disease is to actually infect the mosquitos with a disease of their own.

When mosquitos are infected with a type of bacteria called wolbachia, it renders them unable to spread viruses such as dengue fever. Dengue is a particularly nasty disease spread by mosquitos, for which no real treatments or vaccines are available. Around 40,000 people die every year from dengue, with around 2,400 cases reported over the past few years in Northern Australia.

Wolbachia bacteria are actually surprisingly common, existing naturally in around 70% of all insects. The particular strain used in this study was discovered by Hoffmann in 1988, in Australian fruit flies. Nature, it seems, is full of serendipities. In 2011, studies showed a great success. Mosquitos infected with wolbachia cannot spread the dengue virus!

However, there was still a problem to address. Wolbachia also affected the mosquitos eggs, preventing them from hatching. While this may seem, at first, like a good thing in that it may cull the mosquito population, the problem lies in the fact that if the infected mosquitos all die off, the remaining insects will still be quite able to spread disease.

The solution, perhaps even more counterintuitively, involves giving the mosquitos resistance to insecticide. At first glance, this idea may seem unappealing, but it isn’t without merit. In fact it’s quite ingenious. Areas which are particularly prone to mosquito-borne infections tend to use insecticides as a way to control the mosquito populations and curb disease. In these regions, the non-infected mosquitos would be killed off by insecticide, so that the population of mosquitos would adapt. The end result would be a population of mosquitos which cannot spread the dengue virus.

Similarly, the insecticide resistance gene would not be able to be passed to any non-infected mosquitos. A female mosquito would pass on both the gene and the bacteria to her eggs, while any non-infected female mating with an infected male would lead to eggs which will never hatch (due to cytoplasmic incompatibility). The end result would be that the only offspring from this population of infected mosquitos would also be infected, and therefore would be unable to spread viral infections to humans.

As well as dengue, the method is promising as a way of preventing other mosquito-borne illnesses, such as yellow fever, and perhaps eventually even malaria. The latest strain of bacteria which Hoffmann and Turelli are working with, named wMelPop, is a strong blocker of dengue and other viruses. Perhaps this method could eventually help to eradicate mosquito-borne diseases altogether.

the meantime, I’m noticing that typing up this article is having the psychosomatic effect of making me feel rather itchy. I’m going to take this as a sign that I should stop writing about mosquitos now. And perhaps take a shower…

Image: A Tasmanian mosquito feeding on human blood. Credit: J. J. Harrison/Wikimedia Commons