War, plague, famine, heart disease, cigarettes, road trauma: six very effective killers of human beings. But they're all amateurs when their records are compared to the number one mass murderer of all time. The humble mosquito, and the deadly diseases it carries, is estimated to have been responsible for as many as 46 billion deaths over the history of our species. That staggering number is even more frightening in context - it means that mosquitoes are alleged to have killed more than half the humans that ever lived.
So if any creature has earned the full force of the wrath of humanity, this nasty little bugger is it. Especially certain species like Aedes aegypti mosquitoes - the world's number one disease vector for deadly dengue fever, which infects between 50 and 100 million people a year around the world.
A. aegypti has evolved into the most curious and innocuous of human predators - it's the females that bite, and they more or less only feed on humans. Each bite exposes the victim to any blood-borne pathogens that the mosquito might have picked up along its way. Dengue and yellow fevers are among the most common - the mosquito contracts the virus by biting an infected victim, and then injects it along with its saliva when it stabs the next unlucky target's skin with its proboscis.
A. aegypti flies silently, so it's hard to know when you're in danger of being bitten, and it breeds and multiplies extremely effectively, needing only a teaspoon full of standing water for its larvae to hatch.
DDT-based insecticides have been effective against these little blighters, but evolution is quickly building up their resistance to this and other control measures. Fighting them with poison might be effective in the short term, but in the long run it only makes them stronger.
There is, however, a potential solution that can hijack the mosquito's breeding cycle to dramatically bring down the population and human risk factors. And it's undergoing testing in two very different ways right now.
Genetic hacking - a brilliant solutionAmerican scientist Anthony James, from UC Irvine, has made mosquito genetics the focus of his career - and his latest invention is a genetically modified mosquito designed to bring populations of Aedes aegypti down from within.
In short, the modified genes affect only the female mosquitoes, rendering them flightless. The larvae hatch on the water, and the females are unable to leave, rendering them harmless to humans and leaving them to die. The males are unaffected, so they mature normally, then mate with other females to pass the genetic modification on.
It's an extremely effective way of triggering a mosquito population crash - James and his colleagues have proven in cage-based testing in Mexico that a sufficient number of genetically hacked males can completely decimate a mosquito population within a few months. The table below shows this genetic genocide in action - within 23 and 33 weeks, the genetically modified males managed to completely destroy the otherwise stable mosquito population in James' test cages.
A. aegypti eggs make this a fantastically portable solution too - they survive for years at a time in dry conditions, then hatch in the presence of water. So you can more or less post an envelope full of millions of dry eggs to wherever in the world it's needed, and just add water. The crippled females will die where they hatch and you've got yourself a mutant force of GM males ready to start their work.
Genetically modified mosquitoes to the windBut while James' "netted laboratory" follows the traditionally cautious scientific approach, one of his partners has been decidedly more gung-ho about it.
Luke Alphey, whose company Oxitec was originally hired by James to design the flightless female genetic modification, is so confident that these genetic warriors work, and that there will be no environmental ill effects, that he has taken advantage of the lack of regulation in many areas to conduct full scale field tests in the wild.
Oxitec's historic first release of GM mosquitoes in 2009 killed an estimated 80% of the A. aegypti population on the Grand Cayman island in the Carribbean - a geographically isolated area.
More mutant, autocidal mosquitoes have been released in Malaysia, and the technique is reportedly going into large scale production in Brazil.
James sees Oxitec's full-speed-ahead approach as a potential risk to the entire science of genetic modification. "That's the difficulty of working with corporations," he told Scientific American, "I can't control corporate partners."
An ethical and environmental quandarySo it seems it's happening. And whether it's for better or for worse depends entirely on your viewpoint.
It's difficult to know exactly what the result might be when you release something like this into the wild. Will there be knock-on effects on the food chain? What will the birds and fish that feed on mosquitoes eat instead? Will the demise of A. aegypti make way for an even nastier pest? Will their removal take away the means of pollination for certain plants? And will the genetic modification itself have unforeseen repercussions down the track?
Then there's the ethics of it - advanced use of this technology could foreseeably cause A. aegypti to become extinct. Some people brave the antarctic winter to save endangered whales, others will chain themselves to trees to defend endangered frogs... But who will stand up for the mosquito? And with a world human population ticking past 7 billion and counting, should we look at A. aegypti as an effective and necessary form of human population control?
On the other hand, humans have become dominant on this planet chiefly due to our ability to manipulate our environment - and with a scientific consensus forming that the complete eradication of mosquitoes would have limited, if any, adverse environmental effects, this could be one of the most human-friendly modifications we could make to our world. And it would certainly be no worse for the environment than our habit of clear-felling forest areas.
As for "playing God" - that argument is moot. We're well and truly adept at that. We've been artificially selecting animals and plants for hundreds and thousands of years to suit our visual, olfactory and gastronomic preferences. Hardly a species that enters our lives in a significant way has not been altered over the generations to suit us better.
Why should we spare our most dangerous natural predator? Does history's greatest killer of human beings deserve a reprieve from the death penalty? What do you think?