Have scientists finally found a way to eradicate dengue?
by Jason Lai
Remember that triumphant exclamation over a successful revenge against the mosquito which left you scratching incessantly on top of the annoying buzz as they hovered about your ear? Over time, we became more inventive in ways to kill mosquitoes. Besides the conventional (read: boring) sprays and insecticides, there was the great mosquito zapper. I vividly remember the triumphant smell of burnt mosquitoes and feeding them to my pet ants which found an appetite for ‘fried’ mosquitoes.
Killing mosquitoes is big business, aside from relieving us from the intrusive mosquitoes, it is a public health concern. Dengue is no stranger to residents of tropical regions as it rides high on mosquitoes to infect 50-100 million people every year1. Like any other viruses, dengue virus (DENV) survive by dependency on defined hosts and continual transmission to other hosts in a cyclical manner. While some viruses leave us with benign consequences, others prove to be too virulent for our bodies to handle; DENV is one such virus – failure to keep the symptoms under control can be fatal. Unlike other ‘hot’ viruses such as enterovirus 71 (hand, foot and mouth disease), SARS, H5N1, human papillomavirus and human immunodeficiency virus (HIV), of which primary transmission is via bodily fluids or touch, the main mode of transmission is via Aedes mosquitoes. These mosquitoes are strictly innocent; they too are victims to DENV infection just like us.
Given the relatively high prevalence of DENV in Malaysia and its devastating effect on public health as well as public coffers, there are several strategies to combat DENV. While scientists work tirelessly on the current DENV vaccine in clinical trials, the other front is to eradicate those pesky mosquitoes. As fogging works found some success in reducing the number of infections, it is yet a viable long term solution, seeing that artificial selection could lead to the prospects of breeding insecticide-resistant strains. Therefore, new strategies in eliminating Aedes mosquitoes ought to be formulated.
In August 2010, Malaysians were caught by surprise by the short notice of the government’s intention to conduct an open field trial of a genetically modified (GM) mosquito promised to bring the Aedes population down, inviting the ire of non-governmental organizations (NGOs), especially the ardent antagonists towards anything GM.
The GM mosquitoes, OX513A (also known as LA513A), were named after the genetic construct introduced into the Aedes aegypti mosquitoes[*]2. This construct was chosen due to its timed killing of larvae right before they pupate, thereby retaining the competition for resources with the naturally occurring wild type (WT) larvae, while preventing full development of OX513A mosquitoes into adults. The genetic construct produces a lethal factor, called tTAV, that regulates itself and is self-sufficient. Moreover, the production of the tTAV is exponential due to an inherent positive feedback loop mechanism, effectively killing the larvae. However, just as kryptonite weakens Superman, the presence of a drug called tetracycline weakens tTAV, sparing the larvae.
With this construct, the strategy was to mass produce male OX513A mosquitoes and release them to inseminate multiple females to produce offsprings programmed to die unless grown in an environment rich in tetracycline (watch the educational video informing residents of Cayman Islands about the trial: http://tinyurl.com/scimy-ox513a. As ingenious as the idea sounds, NGOs who protested against the use of these mosquitoes were as refractory. Their main concerns were mostly about questioning the uncertainty and unintended consequences of releasing OX513A mosquitoes. While valid, field trial with GM pests is not something new, moreover, some of them emerged to successfully eradicate the targeted pest (screw worms in North America)3, while at worst, the effect of releasing these GM pests were not as appreciable as the butterfly effect4. The other concern was tetracycline, being the compound that reverses the lethal effect, was claimed by the NGOs to be ubiquitous in our environment. To be fair, neither party has solid data to substantiate this contentious point.
In late January 2011, the mosquitoes were eventually released in an uninhabited area of a non-reserved government forested land off Jalan Tentera in Bentong district, Pahang. The results were published recently5. Although OX513A mosquitoes survive well after release in a controlled setting, the question whether they are capable of reducing A. aegypti population in their natural habitats (e.g. urbanised areas) remains unanswered. This is because the release site was way beyond the reach of their natural habitats (urbanised areas) to find their mates! Nonetheless, the open field study highlighted several characteristics of these OX513A mosquitoes in the wild, mostly corroborating results from the controlled environment in the laboratory: WT mosquitoes tend to fly further and survive better than OX513A counterparts6. These are in part explained by the fact that OX513A mosquitoes are producing the toxic tTAV in their body once they are freed from their tetracycline-laced water in the laboratory7. Fortunately though, a complementing study conducted in the Cayman Islands reported informative data8. Briefly, it was determined that the release of ~465 males per hectar per week resulted in a proportion of just under 1:4 (OX513A:WT) male mosquitoes in the open environment, translating to an approximately 1:9 (OX513A:WT) larvae. While mating efficiency was not matched to larvae production, their study realised the importance to survey the population of WT male mosquitoes to determine the adequate number of OX513A male mosquitoes that translates to effective Aedes aegypti population suppression. More importantly, there were no reported incidences of malignant consequences when these OX513A mosquitoes were released in inhabited areas.
Another successful field study was conducted in Australia9. However, the strategy employed is slightly different: instead of artificial genetic constructs, a strain of the Wolbachia (wMel) bacterium was introduced to Aedes aegypti. This bacteria was reared to colonise mosquito cells, the effect is such that infected males inseminating uninfected females result in morbidity in the offspring while infected females will always produce viable offspring. Most importantly, wMel protects these mosquitoes from DENV-2 (serological type 2)[ † ] infection, hence putting a halt to DENV transmission to humans9. In their landmark study in inhabited areas, it was shown that wMel infected mosquitoes were capable of establishing stable populations, with reduced capability of transmitting DENV to humans10.
Collectively, these data present promising future in the use of genetic engineering techniques to suppress mosquito population. Indeed, further open field studies within appropriate parameters are needed to determine if this technique can replicate the success of eradicating screw worms. The prospects of eliminating Aedes aegypti, which is the principal vector for DENV transmission, would mitigate the burden on public health and the economy. One such study is possible by applying the lessons learnt in the open field studies conducted in Malaysia and Cayman Islands, tweaking parameters such as releasing adequate numbers of OX513A male mosquitoes to outcompete WT counterparts in urban areas. Finally, given that these OX513A mosquitoes are theoretically meant to produce offspring which dies prior achieving sexual maturity, it would be important to find out if any resistant variants could arise over a long period of time.
Let’s consider a blissful scenario – mosquitoes go extinct, one less annoyance to deal with for the rest of your life. Think about it, this is achievable, “…while humans inadvertently drive beneficial species, from tuna to corals, to the edge of extinction, their best efforts can’t seriously threaten an insect with few redeeming features.”11 The question now is, would the Malaysian public be supportive of future open field trial of OX513A mosquitoes? The description of the OX513A mosquitoes is available online for free (see references 2,5,6,12) as well as my introduction above for public education. Indeed, I hope extensive communication and consultation be initiated between the public, scientists and the government to ensure an informed decision will be made on the future of this study. The ‘What if OX513A mosquito is the answer to total eradication of Aedes aegypti?’ should be given equal consideration as the cliché ‘what ifs’ of dissenting NGOs given that the preceding technology to OX513A proved successful in the screw worm case. On the flip side, in a world without mosquitoes, humans would no longer have the mosquito to take the fall for an ‘unintended’ slap and my ants might just die of withdrawal symptoms from the lack of crispy mosquitoes .
ABOUT THE AUTHOR:
Freshly baked from the National University of Singapore, Jason Lai is in a quagmire of disciplines – biology, statistics and computer science – to analyse whole genomes in the Statistical Genetics lab helmed by Professor Teo YY. A molecular biologist by training but a statistician at heart, he hopes to do his part in public discourse by encouraging open discussion of public issues related to the sciences. Jason can be contacted at [email protected]. Find out more by visiting his Scientific Malaysian profile: http://www.scientificmalaysian.com/members/jazzdaman/
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Walker, T. et al. The wMel Wolbachia strain blocks dengue and invades caged Aedes aegypti populations. Nature 476, 450-453 (2011).
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Bargielowski, I., Alphey, L., and Koella, J.C. (2011). Cost of mating and insemination capacity of a genetically modified mosquito Aedes aegypti OX513A compared to its wild type counterpart. PloS One 6, e26086.
[*] Aedes aegypti is a subgroup of Aedes mosquitoes that are predominantly found in urbanised environment. For the purpose of this article, Aedes mosquito and Aedes aegypti will be used interchangeably.
[ † ] There are four serotypes of DENV capable of eliciting dengue fever.