Repellents

JAMES LOGAN: Hello, my name is Professor James Logan, and I’m the head of the Department of Disease Control and the director of ARCTEC, the London School of Hygiene and Tropical Medicine. And in this presentation, we’re going to be giving an overview of insect repellents. And by the end of this presentation, you should have an understanding of what repellents are and their use as a personal protection method, potentially for the prevention of disease transmission. Additionally, you should have an understanding of the different methods of efficacy testing used to see how well a repellent actually works.

You should have a bit of an idea of the evidence base for how well they work and, in particular, how well they work in terms of preventing disease transmission. Now insect repellents have been used for centuries as a form of personal protection against insect bites. And there are lots of different types of repellent products that are available for personal use. They include things like lotions or formulations that can be applied to the skin. They also include treated clothing that can be worn, or special repellents such as mosquito coils, plugins, and vaporizers that can be used around the household or outdoors.

Now although some studies have demonstrated reductions in the transmission of diseases, such as malaria and leishmaniasis, actually, overall, there are too few trials that have ever been done to show whether repellents can actually reduce disease transmission at a community level. Nevertheless, repellents do provide high levels of individual protection– that for sure– and especially against outdoor and day-biting insects. So they should absolutely be recommended. What repellents aim to do is to prevent the bites which ultimately would have an effect, in theory, on the transmission of the pathogens that cause those diseases.

Whether that actually works in practise or not is very, very much dependent on very many factors, including the environment, the geography, the transmission dynamics of those diseases, and the vectors themselves as well. And much more work needs to be done to try and understand exactly how those types of repellents could work to control those types of diseases in the future. Now how repellents work is a little bit of a controversial topic. Lots of studies have looked at ways in which repellents work. And some have shown that repellents are actively detected by olfactory receptors on the insect.

And others have shown that repellents interfere with odour receptors that are normally tuned in to human odours, or to animal orders, and therefore interfere with the host location process. But one thing is for sure, repellents do affect an insect’s sense of smell in some way. And that seems to be where the mode of action for repellents lies. There are topical repellents that can be applied to the skin and come in different shapes and sizes. So you can get sprays, creams, lotions, roll-ons, wipes. There are lots of different applications for topical repellents. They can be quite a confusing area when you have to choose what repellent to use for protection.

Now there are several multiple– there are several active ingredients on the market that can be used. And there are four main active ingredients, and those include DEET, or N,N-diethyl-m-toluamide, which is the chemical name, also a icaridin or picaridin. The third one is PMD, or P-Menthane-Diol, which is also sometimes called lemon eucalyptus oil, and IR3535. Now all four of those active ingredients work really well. If you were to put them on the skin and into a cage of mosquitoes, you would find that no mosquitoes would bite you. The length of time that they last depends on the formulation. And that can differ between different types of products and different manufacturers as well.

Topical repellents target insects which bite generally during the day, but they are also useful for dawn and dusk biters. The biggest limitation to topical repellents, when you think about it from a disease control point of view, is the compliance by individuals. It’s quite hard to get people to use repellents every single day, so compliance can be a bit of an issue. But of course, there’s also potentially a lack of knowledge or attitudes and issues. And this can lead to the improper use of repellents, leaving individuals at risk of being bitten.

So if repellents are to be used as a method for disease control, there’s a lot of work to do to understand the behaviours that might drive people to not want to use them or to use them, and also to look at the most appropriate situations that repellents could be used for this practise. The mechanism of spatial repellency is pretty clear. So spatial repellents aim to prevent insects from entering a space that is normally occupied by the host, the human host, for example, and reduce basically the encounters between the human and the host. And these repellent chemicals are detected at a distance. And ultimately, the use of spatial repellents hopes to reduce disease transmission.

So they work by deterrents, essentially, preventing entry into a room or a space. Some of these spatial repellents have more of an irritant effect, and they disturb the behaviour of the insect. And some actually have an insecticidal effect. So they can kill, or they can cause sublethal toxicity effects, but ultimately reducing the contact between the insect and the host, and therefore, hopefully, reducing disease transmission. Clothing, shoes, bed nets, and camping gear can be treated with a pesticide or an insecticide called permethrin, which aims to kill and repel insects, such as mosquitoes or other arthropods, like ticks. Clothing and other products can be purchased pretreated, or products can actually be treated using products that you can apply.

So you either spray on the permethrin onto the clothing or dip it or wash it in a solution which adds the permethrin to the fabric. This sort of thing has actually been used for many years by the military. And in fact, it’s still used today. Some recent work has been done to demonstrate how well permethrin actually works in practise. And when applied to, for example, short sleeves, and shorts, pr short trousers, you can achieve around 50% protection against mosquito biting. If you apply permethrin to long sleeves and long trousers, where there’s a much greater coverage, you can actually achieve around 91% protection against bites and blood-feeding insects.

So even if you’re not completely covered by the permethrin-treated clothing, it still has a sort of spatial repellent effect preventing insects from biting. So it can be a very good option, particularly when combined with topical repellents. This can be a good way of protection. Now when it comes to the development and testing of repellents, that are different ways that this can be done. Now this tends to start with laboratory testing. And so, for example, you might have a new active ingredient, or potentially a new formulation that needs to be tested.

This can be done in arm and cage tests within a laboratory environment, where an individual would apply the products to their arm and insert their arm into a cage, and count how many mosquitoes out of a standard number of mosquitoes land and bite. Now this would then be compared with a control arm, where there is no active ingredients applied. And one of the first things to do with a new active ingredient is to determine the effective dose. And then what’s really important is to figure out how long a repellent actually lasts for. And what we use to determine this is something called Complete Protection Time, or CPT.

And that’s the time between the application of the repellent and the first mosquito landing and probing on the skin. So this can be done and replicated with several volunteers or participants to work out a median complete protection time to give you the number that would be put onto a product in terms of the number of hours that the product actually lasts for. Then the last way of testing is in a full field trial. And this would be to examine the efficacy of repellents under complete field conditions with participants that would sit outside and measure how many mosquitoes land on them. And you can see in the picture here, somebody is sitting with a tube and his legs exposed.

And he will use that tube to remove mosquitoes that land on his leg, and pop them into the cups that you can see there, to be able to count them. Now the length of protection time offered by repellents can vary between products and active ingredients. And most repellents that are on the market should have gone through rigorous testing to show that they’re effective before they can actually be commercialised. And how long they last for should be put onto the label. So people should always follow the label instructions. Ultimately, if you get wet, if you sweat, or if it’s hot, the repellent can rub off. And if that happens, then the repellent needs to be reapplied more frequently.

Higher concentration repellents tend to be effective for a longer period of time. But that doesn’t necessarily always follow through. So for example, lower concentration repellents that have a very good formulation can actually last for longer than higher concentration repellents. So the formulation itself is extremely important when it comes to repellents. So the label should always be followed. As a general rule, though, the active ingredients do tend to have slightly different lengths of efficacy. So for example, DEET tends to last above six hours, depending on the formulation. PMD usually lasts a little bit shorter, between four to six hours. Again, it depends on the formulation.

Icaridin can last up to six hours, and IR3535, can last anywhere between 7 and 10 hours, depending again on the formulation. Now these are some very rough guides, and give you a bit of an indication. It doesn’t mean that any repellent there is better than the other, because they all repel extremely well. And they just last for different lengths of time. And that will be dictated partly by the active ingredient, but quite a lot by the formulation. So just to summarise, in this presentation, we have looked at what a repellent is and how they work. We’ve talked a bit about the types of repellents that can be used, so you can now make good recommendations.

We’ve talked about how repellents are tested for consumer use and to determine their efficacy. We’ve also looked at how repellents can potentially be used or what the evidence base is for the use of repellents when it comes to disease transmission. And finally, we’ve covered a few myths as well, to find out which ones are correct and which ones aren’t.