Throughout the history of mankind, mosquitoes and the diseases they carry have been a scourge upon human beings across the world. It is estimated that half the people who have ever died were killed by Malaria, a disease passed from person to person by the Aedes aegypti mosquito. The mosquito is statistically the most dangerous animal on the planet – aside from humans.
The scenario in modern times is hardly this bleak, with the emergence of modern medicine as well as pesticides and other safety measures. This doesn’t change the fact that there are still around 3.9 billion people worldwide who are at risk from mosquito-borne diseases such as Malaria and Dengue fever. An article in the New York Times recently about a species of bacteria called Wolbachia had me thinking, could these bacteria possibly be the magic bullet against mosquitoes which humanity so needs?
Wolbachia is a species of bacteria unique to insects meaning, importantly, that it’s harmless to vertebrates such as birds, mammals and humans. The word bacteria for many invokes a degree of fear or scepticism, due to the innumerable deaths attributed to bacteria (although the total number of bacteria species which cause disease in humans is less than 100, compared to the millions of species on earth). Wolbachia, however, is something very different and somewhat special: it is classed as a symbiont. A symbiont is effectively an organism living within another, larger organism in a mutually beneficial way. In this case, the Wolbachia bestows a degree of protection from viruses to the insect within which it is living. This trait is crucial to the potential these fantastic little bacteria have as a key in stopping the global spread of mosquitoes and the diseases they carry.
So how does Wolbachia work? How could it stop the growing prevalence of these mosquitoes? The answer lies
in the somewhat unfortunate (for the mosquitoes) side effects that the bacteria bring about. Wolbachia is a master manipulator of the insect reproductive system, changing the composition and structure of the eggs of female mosquitoes and the sperm of males. This means the infected mosquitoes can only successfully reproduce with other mosquitoes infected with the same strain of bacteria, a phenomenon called cytoplasmic incompatibility. This spreads the bacterium through insect populations very quickly as infected males contaminate uninfected females when they reproduce, and if an uninfected male mates with an infected female the eggs laid by the female will never hatch. This causes both a reduction in mosquito population size overall (as females will only mate once) and an increase in the proportion of the population carrying Wolbachia.
The impact on population size is only one of the benefits to us humans. The other lies in the resistance to viruses and other pathogens that the symbiont offers to the mosquitoes. If the disease, for example dengue or malaria, is unable to take hold in the mosquito, it will be far less likely that the mosquito can pass on the infection to a human host through taking a blood meal. The discovery of this effect was a revelation and, indeed, it seems to work in practice too. Between 2011 and 2015, large scale releases of these Wolbachia-infected mosquitoes in Australia took place to try and stop the spread of Dengue fever. Thousands hung boxes of treated eggs outside their houses to help spread the bacteria and cases of dengue started to plummet to almost non-existent levels.
There are, however, a number of disadvantages associated with the use of Wolbachia. For one, despite
naturally infecting thousands of species of insects, Aedes aegypti isn’t one of them. In order to establish Wolbachia in a mosquito population the bacteria has to be manually inserted into the eggs of mosquitoes in order to infect the embryos, adding a layer of complication to the distribution process. In addition, while in a small, closed area it is a one-time treatment, trying to treat a substantial area such as a whole country would be very difficult as it would be possible for uninfected mosquitoes to fly in and re-establish a Wolbachia-free colony within the population. This means it would have to be a treatment repeated yearly to maintain the distribution of the Wolbachia.
It’s also crucial to remember that this is only the effect the bacteria have on mosquito populations. While mosquitoes are indeed the most prevalent living transporters of disease, there are many others such as tsetse flies and sand-flies over which Wolbachia has little to no hold. Finally, we must look towards the implications of a bacteria species with the power to alter the capacity of an entire species to reproduce and the way in which we as humans are applying it. While this particular bacterium holds no threat to humanity, perhaps we should pause and examine the implications of playing god with such a powerful bacterium – and the possible consequences of something similar being used in more malicious circumstances.