The world's scientific and social network for malaria professionals
Subscribe to free Newsletter | 10087 malaria professionals are enjoying the free benefits of MalariaWorld today

They’re off – and running? Dengue resistant Aedes aegypti

September 22, 2011 - 16:41 -- Mark Benedict

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.

Comments

Bart G.J. Knols's picture
Submitted by Bart G.J. Knols on

I could not agree more with your views, Mark. This is a remarkable achievement, and may yield a lot more good news in future. There are three 'ifs' for me...

1) As you say..."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.", but having been in the Caribbean recently, where huge numbers of aegypti were around (and dengue), I am concerned about what would happen to mosquito control in this region and other parts of the world once dengue would no longer be transmitted by aegypti... Most likely, particularly in poor economies, mosquito control would come to a stand-still, and mosquito numbers would rise to the point that it becomes unbearable. Then mosquito control would still have to be conducted...

2) Dengue can also be transmitted by Aedes albopictus. Admittedly, it is a poorer vector, but it can still do the job...

3) Last, let's not forget that aegypti is a perfect vector of Chikungunya virus...which is rapidly spreading around the world. It would be good to know if Wolbachia also interferes with this virus, because you'd be back to mosquito control if the disease shows up...

Oh, and since we're talking dengue...we are about the get DengueWorld going soon, so keep your dengue stories for the new platform...

Bart

Ole Skovmand's picture
Submitted by Ole Skovmand on

In the French Pacific islands, dengue is widespread. islands are ideal for this sort of technology, and these islands live from state support from France (that is being reduced) and tourism, that due to economic problems in Japan, US, France and Australia are less common than before. Having dengue removed might help the tourist sector. They could start with Tahiti and Morea