Dengue is nicknamed “breakbone fever” because of the excruciating pain it causes. It infects 390,000,000 people a year, mainly in tropical and subtropical parts of the Americas, Africa, the Middle East, and southeast Asia. It is spreading rapidly; half of the world’s population is at risk. It is carried by the mosquito Aedes aegypti (“unpleasant of Egypt”). There has been no cure for dengue, no treatment, and no prevention: the mosquitoes evolve resistance to insecticides, and fly by day so that bed nets are useless.
The disease is not actually transmitted by the mosquitoes but by a virus, which the female mosquitoes imbibe in one person’s blood and transmit to another. Insects look tiny to the human eye, but entomologists dissect them to study things inside that are vastly smaller. Beginning in 1924, researchers discovered that something larger than the virus (still of course microscopic) inhabits the bloodstream of 60 percent of insects, including some mosquitoes though not A. aegypti: a bacterium called Wolbachia. Decades later they learned more about Wolbachia: that it sometimes prevents mosquito eggs from hatching, or shortens the life-spans of some insects. They speculated that, if it could shorten the reproductive lives of A. aegypti by even a little, several million people would be spared the agony of dengue. The problem was to inject Wolbachia, taken from fruit flies, into the eggs of A. aegypti. This was like sticking a needle into a balloon and then withdrawing the needle without popping the balloon – all on a microscopic scale. “We tried with many thousands of eggs… It was a tedious process that took us more than a decade,” says Scott O’Neill, whose article in the June issue of Scientific American I am condensing.
Then, Wolbachia would die out in the mosquitoes after one or two generations; ways had to be found to accustom the bacteria from fruit flies to their new home in the mosquitoes. This took till 2005.
Then, it was discovered that Wolbachia does better than shortening the life span of its host: it prevents the dengue virus from carrying on its own life cycle inside the mosquito.
Eureka! If the Wolbachia-carrying mosquitoes could be released to interbreed with wild mosquitoes, they might spread and fill a wide area with mosquitoes unable to transmit dengue.
There was much work still to do. A Wolbachia strain had to be developed that did not shorten its host’s life – the longer these mosquitoes live, the more descendants they will have. Teams spent months going door to door in the community where they wanted to experiment – Cairns in northern Australia – explaining and getting people’s permission to release cupfuls of mosquitoes near their houses. Tests had to prove that Wolbachia cannot be passed on to humans (the bacterium is too large to pass through the mosquito’s salivary duct) and that the Wolbachia-infected mosquitoes are not harmful in any other way. (Lab staff and volunteers spent three years of fifteen-minute sessions with their arms bared for mosquitoes to bite.) Australian authorities had to be satisfied that there were no bad side-effects for the environment, finally gave the scheme a “negligible risk” grading – the lowest possible – and in 2011 the releases began. Now field trials are starting in Vietnam, Indonesia, and Brazil.
And Egypt? And Iraq?
The thought that came to me as I read about this was the contrast between the two ends of the human procession. Advancing in one direction are patient and often (though not always) humble scientists. Striving to drag all of us in the opposite direction are loud anti-scientists on more than two continents.