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How tsetse biology could help their control

RICHARD SELBY: Hello, I’m Richard Selby from the Liverpool School of Tropical Medicine. And in this step we’re going to look at how tsetse biology can help with controlling tsetse flies themselves. We need to remember that tsetse themselves are not a sole problem. Just having tsetse flies in an area isn’t going to warrant a control intervention. The problem comes when the dynamics for disease spread correct. So you’ve got a good interaction between flies, mammals, sometimes reptiles, and parasites. So the problem is circulating. Principally, the reason, the main reason that tsetse control has been done historically is for livestock productivity. Tsetse fly and Trypanosomiasis cause massive losses in livestock economy across Africa.
However, tsetse control for human health reasons, sleeping sickness, are much less common, but recently have been gaining momentum. The historical control measures for tsetse– this is very historical– have been removal of host populations for the purpose of disease control. This has been done through removal of humans, in the case of sleeping sickness. There was the clearance of parts of Lake Victoria’s shoreline of humans when a sleeping sickness outbreak happened at the time of the last century. But there’s also been mass culling of wildlife species. This was done to remove the possible blood meals for tsetse flies. Both of which are completely unethical now. There’s also been purposeful vector control in the method of clearing the tsetse habitat.
In the previous step, we discussed how tsetse fly prefer very specific habitat types. So if you remove that habitat, you will essentially remove tsetse fly. This has been done on a quite large scale previously. And has relatively recently continued. The image shows an area in Ethiopia which was cleared as recently as the start of this century. However, contemporary vector control follow three major principles for controlling tsetse. Each exploit particular aspects of their behaviour and can be deployed in different ecologies. The three are, and they’re shown in the images from left to right, spraying or fogging, insecticide treated cattle, and finally, traps and targets. Spraying or fogging can be achieved either at ground level or airily, using an aeroplane.
You aim to cover known tsetse resting sites, so where they like to get out of the sun. The insecticides you use are long lasting, so deltamethrin or endosulfan. And these are emitted from an atomizer, which creates an insecticide droplet of 30 to 40 micron in size. It’s a very small amount. For aerial spraying, the aerosol is applied from just above tree canopy level. And they do it right at the start of the night time when there’s a temperature inversion. So the air is being drawn down, so you know that any released insecticide is being drawn down into the target area. Ground level spraying, however, is carried out during the day. This is because it’s easier to work during daylight hours.
Application of spray or fogging has to be repeated in cycles. And these cycles are timed to be at the point that the next generation of adult flies emerge. So the timing of this depends very heavily on the tsetse biology, because you need to know how long it will take for the pupae to erupt and the adult fly to emerge. Moving onto animal targeting vector control, you have a few options of how to get an insecticide onto an animal. But what is unanimous is you have to use a residual insecticide. You can either administer it through a dip where the animal falls into a complete pool of insecticide infused water. You can pour it on.
You go along the spine, and it diffuses across the whole animal. Or you can spray it specifically onto areas where you know the tsetse will feed. Spraying is actually the cheapest way to administer the insecticide for tsetse control, and is preferable if you’re designing an intervention using animals. It’s important to add that animal targeted vector control works very well. However, it only works in areas where you have a lot of animals. Obviously, if you’ve got an area with tsetse fly, but very few livestock, you’re not going to get the impact you need because these animals are not going to meet a lot of tsetse fly.
But if you’re in an area with thousands of livestock and you spray all of them, then tsetse fly will certainly meet those animals. There’s knock-on effects which fall under the banner of one health. The insecticides will not only kill tsetse, but also reduce other biting insects– ticks, mites, which transmit other diseases. So this is actually a very popular technique to use if you’re in an area with a lot of farming. Tsetse targets and traps are blue and black in colour. This is because the tsetse fly are drawn towards the blue. And depending on which genus of tsetse you’re dealing with, they can be odour baited to increase their efficacy. The targets specifically are always insecticide treated.
They’re treated with a long-lasting insecticide. And there are many different trap designs. They are developed over many years, depending on which tsetse fly is being targeted. And you have to use the right one in the right location for the right species. But I will say the traps are very– they’re intricately designed and they’re very expensive. They’re difficult to put out. So personally, I would suggest traps are best for monitoring populations of tsetse fly. Targets are best for control. The efficacy of targets in a control programme is reliant entirely upon getting the right number of targets into the right environment. And you have to position them throughout the habitat that you’re dealing with.
The large target, which is shown here in the picture on the right, is quite expensive. It’s a very large piece of material. They cost around $10 each. And you also need a full team of usually three people to actually put them up in place. There have been recent developments in target design, which I’ll talk about in the next slide, which will reduce the price, but we’ll cover that. Targets and traps are very good for large-scale interventions, top down, where, for example, you’re working with the government or through a large NGO. It has been trialled through community programmes, so individual villages themselves doing these interventions. And there has been some success, and there’s also been some less successful events.
This very much depends on the community that you may be working with. Tiny targets have been developed in response to the old technology. The large targets being very expensive and very difficult to justify as a control programme. Liverpool School of Tropical Medicine collaborated with IRD in Montpellier and investigated tsetse reactions to different odours, different colours, different sizes of targets, a whole host of things, and realised that you could really minimise the size of the targets and have a very strong impact on tsetse numbers. The target, which is shown in the top image, is much, much smaller than the old ones. These are 20 centimetres by 40 centimetres roughly, blue and black.
A single person can put them out, operating on their own. Which in turn means that the intervention cost is a lot cheaper. The old targets were coming in at $400 US per kilometre square for control. These new ones, the tiny targets, are $64 US per kilometre square control. Which is a fantastic thing if you’re looking at a large intervention area. The tiny targets are positioned specifically in areas where we know there are tsetse fly. In a previous step, we spoke about the different genus of flies having different environments. And what we are doing is we are focusing only on Gambian HAT foci areas. All of these foci areas are dominated by riverine tsetse flies.
So what we do is we place these targets at an interval of one every 50 metres down a river. And we do a redeployment every six months. Currently, there are tiny targets operations in Chad, Democratic Republic of Congo, Guinea, Ivory Coast, and Uganda. In all of these areas, we’ve had a massive reduction of tsetse fly numbers. And it’s gone very well. And we are in the process of handing over all operations responsibilities to the national partners in each country.
Looking now more at the tracking side of things, which I said is more for monitoring, the top three images that are shown here show the different designs that we have in light tsetse fly traps, but what is important to remember is just how important it is to know exactly what the fly populations are doing. You need to find out what the tsetse fly numbers are before you do an intervention, but we also monitor them during the intervention and afterwards to find out how well the reduction is going, and whether there are points with specific problems that we need to address. The lower images are targets in operation.
The one on the furthest left is from Yasa Bonga in the Democratic Republic of Congo. We have put these targets right along the river. This image shows a target at a ferry crossing point. This is a dug-out canoe which crosses all day, every day. And the impact has been not just shown by our tracking that we are doing, but also the local population are telling us that they are not getting bitten as much. The middle image is the team who actually put these targets down the rivers. They’re doing it from canoes.
They’re travelling the entire length of the rivers, stopping every 50 metres to place another target, and clear the vegetation around that target so the tsetse fly can really see it well. Although vector control plays quite a increasing role in sleeping sickness control, we still need the correct identification of infected individuals and their treatment. HAT is a fatal disease. So we need to identify the people who are sick and treat them so that they do not die. In a really high prevalence area, this is really well done by mobile screening teams visiting each individual village and trying to get everybody to come to the screening. It is much more effective on Trypanosomiasis gambiense than it is for rhodesiense.
Rhodesiense is very difficult to find by screening, particularly mobile screening.
This case finding and treatment reduces the human abundance of the Trypanosomiasis pathogen, as well as preventing the death of anybody that may have it. This is a graph, and output from a model, which looks at the three different Trypanosomiasis options. So we have Trypanosomiasis rhodesiense and Trypanosomiasis gambiense. And the output of this shows that vector control is equally effective on both diseases, as it reduces transmissions regardless of the dynamics of these. Rhodesiense lives in cattle, so it is very difficult to control by approaching it as a human health problem. And gambiense only lives in people, or is thought to only live in people. Either which way, if you control the tsetse fly, you’re controlling the transmission completely of this parasite.
Tsetse fly control does have issues of cost and sustainability. It’s difficult to justify keeping it going forever. So what we at Liverpool School do is we are doing tsetse fly control during a period of increased case finding, case identification, and treatment. And after we’ve had no cases, no UK cases for over four years, then we know the risk of transmission is greatly reduced, so we can start looking at scaling back tsetse control. Finally, we have some further reading suggestions for people to enjoy. Thank you very much.

Now that we have looked in depth at the biology, habitats, distribution and diseases of tsetse flies, we again join Dr Richard Selby as he explains how we can use this knowledge to facilitate their control.

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