While killing insects would seem to be the best way of controlling the diseases they spread, some scientists are releasing more of the biting bug, re-engineered to stall epidemics.
On a late October morning in a hilly, middle-class neighbourhood in central Singapore, a crowd was forming for a surprising event. Top government officials, community leaders and a throng of media with cameras gathered around a group of scientists who had brought a most unusual gift: boxes of mosquitoes.
With a shout of “one, two, three”, they opened the boxes and released 3,000 of the insects into the air above Singapore’s Braddell Heights. In a country where the warm, tropical climate is ideal for mosquitoes, most people are used to swatting the insects rather than releasing more of them into the environment.
Fortunately, the residents of Braddell Heights had been prepared months in advance for this event, and welcomed the release. They knew that the mosquitoes would not bite – and that they were participating in an important study on the impact of mosquitoes infected with Wolbachia, a bacterium that hinders the insects’ fertility and blocks the ability of viruses like Zika to spread.
Singapore isn’t the only country throwing mosquitoes into the air. Across Asia and Latin America, scientists are trying out radical new methods to defeat Aedes aegypti, and the less widespread Aedes albopictus – the mosquito species that spreads the dengue, chikungunya, and Zika viruses.
Many of these experiments involve altering mosquitoes in the lab to render them harmless or infertile – with Wolbachia, irradiation, or even genetic modification. But the project in Singapore, along with an even larger trial in Yogyakarta, Indonesia, are showing that the release of these altered insects near human habitation is as much about persuading people about the effectiveness of this approach as it is about scientific innovation.
This new approach to controlling insects that spread disease – known as biological vectors – began several years ago, when alarm bells first rang over the rise of dengue, today considered the world’s fastest spreading tropical disease. Incidents of dengue have increased 30-fold in the past five decades, according to the World Health Organization. It has expanded from causing severe epidemics in just nine countries before 1970 to more than 100 countries today, many of them in Asia and Latin America.
The recent emergence of Zika, with its frightening links to brain damage in babies, has given experiments in vector control added urgency. There is no treatment yet for either dengue or Zika.
Despite this, there is some reticence among members of the public about projects to tackle the Aedes mosquito. Some of this has its roots in previous attempts to control this deadly insect. In the mid-20th Century, the prevalence of mosquito-borne diseases like malaria and yellow fever led to large-scale sanitation programs and the widespread use of a powerful new insecticide: Dichlorodiphenyltrichloroethane (DDT).
After widespread use, DDT was eventually banned in 2004 due to concerns over its impact on human health and the environment. It has been linked to cancer in humans and a decline in predatory birds.
Despite this, the use of DDT did reduce numbers of mosquitos. This success led to complacency after the 1970s, says Duane Gubler, emeritus professor of infectious diseases at the Duke-NUS Medical School in Singapore. Brazil, for instance, was declared Aedes-free in 1958 but as measures were relaxed, the insect began to reappear in the 1970s. One genetic study, published in July this year, suggests that mosquitoes from non-eradicated areas in Venezuela recolonised northern Brazil, later expanding their distribution southwards.
At the same time, Gubler notes, two global trends were nurturing a mosquito comeback. Explosive, chaotic urbanisation created the perfect environment for the city-loving Aedes, as well as providing the perfect conditions for spreading viruses quickly. The growth in global transport and travel also aided their spread to new areas. Treated bed nets, often used to help prevent malaria, are also relatively useless against Aedes, which tends to be active during the day. Growing insecticide resistance added to the problem.
A new weapon
Described by one scientist as “the biggest thing since DDT”, the use of Wolbachia-infected mosquitoes has become the most widespread of the new vector control experiments, with lab studies or field trials now underway in a dozen countries. The enthusiasm is understandable: Wolbachia seems like the perfect weapon against mosquito-borne disease.
It is found in more than half of all insects, but not usually in viral-pathogen-carrying mosquitoes. The microbe not only protects its host from diseases like dengue and Zika, but is also naturally built to spread rapidly through its host population. Plus, it does not transmit to humans or animals.
One approach to rolling out this Zika and dengue resistant bacterium, pioneered in 2011 by scientists at Monash University in Australia and propagated through the World Mosquito Program (formerly known as Eliminate Dengue), is to release both male and female Wolbachia-infected mosquitoes into the environment. Wolbachia is only passed from females to their offspring so releasing females with this bacterium means that it will be spread throughout the mosquito population, making them resistant to Zika and dengue.
Ten countries—Australia, Brazil, Colombia, Indonesia, Sri Lanka, India, Vietnam, Kiribati, Fiji and Vanuatu—have signed up for this approach.
Singapore, like China, is trying a second approach, aimed at suppressing the population of mosquitoes by releasing only male mosquitoes. Males infected with Wolbachia are unable to fertilise the eggs of non-infected females.
The method chosen is partly to do with cost. The second approach is more expensive because it involves sorting males from females in the lab and, unlike the first approach, requires sustained release of mosquitoes – at least until the population crashes, and perhaps even afterwards. Last year, following successful trials, China set up facilities in Guangzhou to produce five million Wolbachia-infected mosquitoes a week.
Public acceptance also plays a critical role in the choice of technology used, as scientists have learnt from past controversies, not only with DDT, but also unsuccessful trials of sterilised mosquitoes in India in the 1970s, which led to public panic over the chemicals being used.
By Vaishnavi Chandrashekhar