What is the most dangerous animal in the world?
Well, the calculation is not so simple to make. What do you count? Direct deaths, habitat destruction, years of illness? I suppose, in many cases, humans would top the list. However, when it comes to infectious diseases, mosquitoes are head and shoulders above everyone else. While the mosquito bite, annoying it is, does not kill directly, depending on where in the world you live in, it may end up being a kiss of. There are more than 3000 species of mosquitoes in the world, however, only 3 species- Anopheles, Culex, and Aedes, are responsible for most of the diseases they carry. And they sure carry a lot! From nematode worms that cause elephantiasis to viruses that cause encephalitis, yellow fever, dengue and many other diseases.
Some might argue that the worst mosquito-transmitted disease is malaria. Malaria is caused by a protozoan parasite, Plasmodium falciparum, which gets into the human circulatory system, damages the red blood cells and causes cycling onsets of anemia, fever, headache, and other symptoms. A female mosquito, which bites an infected human, takes up a little bit of Plasmodium containing blood and carries it to the next person during the next meal, in this way maintaining the malaria transmission chain. The World Health Organisation estimates that in 2016 there were 216 million malaria cases worldwide and around 445,000 deaths caused by malaria. The effectiveness and usefulness of the current malaria vaccine (called RTS,S) is widely debated and Plasmodium parasites that are resistant to the available antimalarial drugs are common. So, rather than focusing on the malarial parasite, a number of researchers have decided to target the vector, the mosquito, itself.
Over the last decade there have been many strategies for controlling the mosquito population in malaria endemic areas. A lot of research has focused on understanding what makes the mosquito a good plasmodium vector in the first place. If we can make mosquitos less suitable hosts for the parasites maybe we can prevent their spread. Plasmodium doesn’t just go in and out of the mosquito; it actually undergoes necessary developmental processes inside it, before it is released as an infectious agent. Understanding which elements of mosquito metabolism are required for successful plasmodium development allows to make strains of mosquitoes that are incompatible with the parasites life cycle. Just this last September two studies in Science have shown that changes in mosquito gut microbiome makes mosquitos less capable of carrying malaria. One of the studies made mosquito strains that had an up-regulated immune gene expression. These “super-immunity” mosquito strains are resistant to Plasmodium infection and they also have altered microbiomes. As it turns out, the female blood-feeding mosquitoes are more attracted to these male mosquitoes with altered microbiomes. At least in closed-lab studies, these plasmodium resistant mosquitoes were overtime able to outcompete the wild type, plasmodium susceptible, males. Similarly, another study has shown that in the lab mosquitos could be colonised with a bacterium, called Serratia AS1, that makes and secretes specific anti-plasmodium proteins. AS1-carrying mosquitoes are resistant to plasmodium and also can compete with the non-resistant wild-type males. In both these studies the ultimate goal is to test the release these engineered mosquitos into the malaria-endemic areas. If the Plasmodium-resistant mosquitoes can spread in the wild environments then perhaps overtime we will see a crash in the numbers of Plasmodium parasites present in the wild mosquito populations as well.
Perhaps one of the most successful field experiments in controlling the mosquito populations have so far been achieved by a British company Oxitec. Oxitec makes genetically modified self-limiting mosquitos, also known as OX513A. These mosquitoes carry a gene, which, if the mosquito is not provided with a tetracycline antibiotic, causes havoc in the mosquito cells and kills the developing mosquito larvae. Oxitec grows OX513A in house, supplied with tetracycline the water where the mosquitos breed. Subsequently, these mosquitoes are released into the areas where malaria is prevalent and they mate with the wild type females. As there’s no tetracycline in the environment no offspring of these mating events survive and the mosquito population crashes. Successful trials with the OX513A mosquitoes have already been conducted in Brazil, Panama, and the Cayman Islands, which showed significant reduction in the local mosquito populations. Some environmental groups, however, are concerned with Oxitec’s approach because of the unknown effects the engineered mosquitos could have on the environment, without more stringently regulated release procedures. Oxitec, on the other hand, is pushing forward and is now trying to adapt their approach to other mosquito-transmitted pathogens including Zika and Dengue viruses.
Mosquito control has recently become a popular topic among techies too. For example, Microsoft’s Project Premonition makes special mosquito-traps that can be deployed using drones in forests, where they sample the mosquito populations and collect data about mosquito species abundance and the pathogens they carry. Popularised by a TED talk, a mosquito zapping laser was another start-up idea to fight malaria, although, the buzz has somewhat died down recently, probably because of the practical difficulties in making it work in the field rather than on a pre-set stage.