The role of research in vector control

JANET HEMINGWAY: So my name’s Janet Hemingway. And I’m a specialist in vector control, having worked in this area for more than 30 years. I’m going to talk today about the role of research in vector control. So if we start with current interventions, there are several of those but perhaps fewer than you might think. The vast majority of these are insecticide based. So for malaria, these would predominantly be insecticide-treated bed nets or indoor residual spraying and to a lesser extent larval control. We then have for diseases such as dengue space spraying again using insecticides or environmental control– reducing the number of breeding sites that the Aedes mosquitoes go into.

Alongside that, we’ve also been looking at how you adapt buildings so they’re less mosquito friendly.

So what’s the role of research in the interventions that we already know work? Vector control is not a static thing. So if you’re using predominantly an insecticide-based control system, then the insects that you’re trying to control are actually going to respond to that control. And eventually, you will produce resistance to the insecticides that are being used. So one clear road of research is to document whether interventions are working well. Under what situations do they work? And what changes over time if and when resistance develops? We can then look, if we see resistance starting to come through, at altering the vector control activities that we undertake.

Another road of research with current interventions is to look at how best we layer these together. So WHO, for example, strongly advocates that we use not just one form of vector control, but multiple forms. And which forms we pick from the toolkit that we’ve got available is clearly dependent on the situation. So different environments, different insect vectors require different solutions. In practise, however, often there is only enough funding to put one intervention in place. And so clearly, we need to work out what is the best intervention that will work. Is it use of a bed net? Is it indoor residual spraying? Or is it, for example, larvaciding?

We then need to make sure that we’ve got the right evidence base to demonstrate that we’re really having an impact on disease control that we believe should come from the intervention that we’re putting in place. Other control methodologies that are not insecticide based are also required if we’re going to reduce the selection pressure on the methods that we’ve got right now. Some of those worked very well. And building an environmental control is something at the start of the vector control journey, maybe in the 1930s-1940s, was used very strongly and largely fell out of fashion over the last few decades. But that’s now coming back into use. There’s clearly a lot that can be done with buildings.

Generally, most vector control is in rural areas with poor communities. The housing stock in those areas is such that insects find it easy to get into the buildings and easy to get at the human populations within them to get the blood meals that they require for their own survival, but also to transmit the various parasites and viruses that cause the human disease. If we can actually make the house less insect friendly, then it will reduce a lot of the transmission. As we look over time at how well these interventions work, it’s important not only to think about do these interventions kill insects, but the economics of deploying these control methods.

And here, a multidisciplinary team is needed Firstly, you need to understand whether the intervention is acceptable to the local population, not just killing the insect. Because if it’s not acceptable, then it won’t be used at scale operationally when you try and utilise that. Clearly, that’s been the case with bed nets where populations that are used to using bed nets have accepted them very readily. And they have had enormous impact. Other populations, we’ve had to work much harder to get the local populations to accept using the bed nets.

And others where the housing stock and the sleeping habits of the individuals really don’t lend themselves to bed nets have meant that the intervention really doesn’t work quite as well as it might.

That really comes onto a major point that we need to make clear that there isn’t a one size fits all for vector-controlled interventions. You need to be very clear about what insect that you’re trying to control, why, where, and when you’re trying to control that insect. And then you tailor your vector control to the particular situation that you find.

If you do that, then clearly you can get an enormous impact in reduction in transmission. Much research in vector control has centred around our ability now to look at genomics, genetics, and population dynamics of the insects that we’re dealing with. That change has happened very dramatically and has opened up a lot of potential new areas. So for example, we now know much more about what insects can sense. What can they smell? What do they react to? That should open up brand new avenues for control. We should be able to disrupt the mating behaviour, for example, of these insects.

We should be able to disrupt their ability to find their food sources be that nectar to keep the males going or the blood meal. We know a lot more about the insect’s behaviour. We’ve got technology now that allows us to look at exactly how, when, and where insects enter the house and how, when, and where they seek the blood meals. This should allow us to put new interventions in place that are very finely targeted to the insects that we want to control.

Attracting insects to different sources should also be possible. So if we know what attracts these insects well, we can combine them with a control method that will allow us to reduce, for example, the environmental impact if using insecticides.

Another route of using vector control should be to actually manipulate the insects themselves genetically. There’s a lot of work going on in this area, but it’s going to be quite a while before we’re able to fully deploy that. What are the areas that we could look at here? Well, clearly, we could stop the insect from being able to vector the parasite or the virus that we’re interested in. We know naturally that many insect populations are not able to transmit some of the parasites. And we know many of the genes that are involved in stopping that.

If those can be inserted into vector populations and expressed at the right time and in the right context, then we should be able to change those insects into ones that are not able to vector the disease, at which point we really then don’t need to control the insect at all, unless we’re interested in just reducing the biting nuisance that they will still remain. We could also use genetic manipulation to reduce the vector populations themselves by introducing sterility into the system and driving down the population size.

All of these methodologies will become available over the next decade or so in terms of the potential to put them out there and use them.