Hoffmann et al. recently reported a highly noteworthy establishment of Wolbachia-infected Aedes aegypti in two Australian towns. With the potential to greatly reduce the dengue risk in these communities, this bio-control is a remarkable demonstration of the potential for heritable factors to interfere with disease. The project is off to a great start. The big question is, can the technology finish the race? And how much push will be required to make it happen? I’ll make my prediction about where this is headed.
Ok. Yes, I know. You’ve read all about it on the BBC or CNN etc. Given my experience with such reports in popular media, they are wildly enthusiastic but scientifically a mile wide and an inch deep. (If you prefer metric, make that 1.61 km wide and 2.54 cm deep. That depth estimate remains generous.) So, even though I’m a bit behind the curve jumping on the story, here is my take.
Hoffmann et al. have reported that by releasing Wolbachia-infected Aedes aegypti in two Australian towns they were able to establish infected mosquitoes at frequencies higher than 90%. (The team was led by Scott O’neill and funded by the Gates Foundation.)
What’s special about these mosquitoes? This particular Wolbachia infection has strong suppression effects on dengue virus (I blogged about a related strain here on MW.) While establishing such a Wolbachia will have little effect on nuisance biting, the residents in the trial area can take some comfort that dengue risk is likely much, much lower.
A critical part of establishing the infection is that females that carry the Wolbachia transmit it to their progeny. Male progeny that carry it are sterile when mated to females that do not. This tends to favor or “drive” the infection in a population, but only if a critical fitness-loss threshold is not exceeded. The research team estimated that no more than 30% loss of fitness could be tolerated and spread would still occur. Based on the successful establishment of the Wolbachia, this was not exceeded.
So, what do we have here? Potentially, a method to greatly reduce risk of dengue infection by relatively small – and cheap - releases (< 300,000 in these two towns) of mosquitoes. The technology is highly suitable for dense urban areas. Will the infection persist at high frequencies into the next season? The jury is out, but this is a critical question, the answer to which will determine how the technology can be implemented and what the cost will be.
The intrinsic rate of spread and persistence of Wolbachia infected mosquitoes are critical parameters for the future use of the technology. The research team expects that unassisted spread is not likely since a critical frequency of infection must occur before this happens. On the plus side, unassisted spread reduces the costs: On the negative side, there is no control to its spread. This is not a concern for public health, but it might result in some uncomfortable transnational implications. In my view, if huge extents of Aedes aegypti populations were unable to support dengue transmission, public health would benefit.
I'll set myself up for being shown wrong and predict that the infection will spread. Aedes aegypti do not disperse far, and high frequencies of Wolbachia are certain to occur in some pockets. So while there may be boundaries where an equilibrium exists, spatial and temporal heterogeneity will mess with the parameters of this calculation. If populations were homogeneous spatially, I wouldn't be so ready to make this prediction. Check back in a year or two.
Meanwhile, the Bill and Melinda Gates Foundation and the research team has a remarkable success to their credit. This achievement creates a good model for what those developing malaria might accomplish. Let's hope that a technology that has started hot out of the gate has the stamina to finish as a large-scale real-world public health intervention.