Next Tuesday will be the start of the National Environment Agency’s (NEA’s) Project Wolbachia rollout. Wolbachia-infected male mosquitoes will be released initially at Braddell Heights, followed by Nee Soon East and Tampines West as small scale trials of both efficacy and mosquito behaviour.As I understand it, there are three major dengue control strategies involving Wolbachia-infected Aedes mosquitoes – Aedes aegypti are not natively infected by these endosymbiotic bacteria.
The first is what NEA is intending to implement: mosquito population suppression via the release of “sterilising” male mosquitoes. When these male mosquitoes mate with wild female Aedes mosquitoes, the females are cross-infected with Wolbachia that trigger a phenomenon called “cytoplasmic incompatibility” that result in there being no viable offspring for the infected females’ lifetime. With repeated massive releases of Wolbachia-infected male mosquitoes, the overall Aedes mosquito population will be significantly reduced, and therefore human dengue cases should fall.
The second involves introducing a life-shortening Wolbachia (wMelPop) into native Aedes populations via release of laboratory-infected male and female mosquitoes. The concept is that females with shortened lifespan would also have a reduced time to transmission of dengue, hence reducing the number of human dengue cases overall. This strategy is the least likely (of the three listed here) to work, because infected females are unable to out-compete uninfected mosquitoes in the wild, and over time, both Wolbachia and mosquitoes will naturally evolve to neutralise the effect of life-shortening.
The final strategy involves introducing Wolbachia (wMel) that partially block dengue virus transmission by blocking the virus from spreading within the mosquito (it becomes immune to dengue courtesy of the endosymbiont). These Wolbachia, which confer little survival disadvantage to the Aedes mosquitoes, has the greatest potential to spread throughout any existing mosquito population. Unlike the prior two strategies, the size of the mosquito population is not affected, but the number of dengue cases will similarly fall. This is potentially the most cost-effective anti-dengue Wolbachia strategy, but it is also irreversible in the sense that the intervention cannot be “undone”, unlike the first or even the second strategies.
The early results of such Wolbachia projects have been promising in many countries. However, the medium and long-term impact of these Wolbachia strategies on dengue transmission is dependent on evolutionary forces acting on mosquitoes, Wolbachia and dengue virus. This was excellently discussed in an “evolutionary forecast” published 3 years ago, and which is well worth the time to read.
Erratum (16 October 2016)
As pointed out by A/Prof Ng (comment to this post), I have the facts of cytoplasmic incompatibility wrong. For the strategy of mosquito population suppression, the Wolbachia-infected male mosquitoes do not cross-infect the females. Rather, the Wolbachia modifies the male sperm (but the mature sperm does not contain the bacteria), which is able to fertilise the eggs of uninfected females, but the eggs are unable to develop further. The uninfected females are still able to reproduce if they were to mate with uninfected males, hence a high ratio of Wolbachia-infected to uninfected Aedes males is constantly required in order to significantly dent the mosquito population over time.