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THE LAST LINE OF DEFENCE AGAINST MOSQUITO BITES

Do mosquito repellents actually work? Dr Cameron Webb explains how they work and what we can do to best protect ourselves from this nuisance summertime pest. 

They spread pathogens that make thousands of Australian’s sick each summer, but it’s their incessant biting that really spoils our time outdoors. For most families, the use of topical insect repellents is the standby strategy to reduce mosquito bites and mosquito-borne disease.

Local authorities routinely recommend the use of repellents to prevent bites and public health risks. However, whilst a list of recommended active ingredients in repellents are promoted, there generally isn’t a lot of guidance on how to effectively use these formulations.

We can’t kill all the mosquitoes!

Mosquito control, whether it is in the local wetlands or around your backyard, won’t be effective in killing of all mosquitoes. There are many effective strategies available to reduce the abundance of mosquitoes around the home, from the use of ecologically sustainable larvicides or insect growth regulators by local authorities in nearby wetlands, or the use of residual insecticides around the backyard and home. These approaches will certainly help. Unfortunately, some regions in Australia don’t have effective broad scale mosquito control programs and the excessive use of insecticides around the home may have impacts on beneficial arthropods.

There will always be a role for personal protection measures, including topical and non-topical mosquito repellents, in keeping you and your family safe from mosquito bites.

Yes, some people are bitten by mosquitoes more than others

There are plenty of people who claim to be ‘mosquito magnets’. Are they really bitten more by mosquitoes or are they just more likely to complain? The science of mosquito bites shows that mosquitoes do have a preference for some people more than others, it’s just the exact reasons why that aren’t all that clear.

We know that mosquitoes need the nutritional hit from your blood to develop their eggs, but why bite some people more than others? There have been many studies, both in the field and in the laboratory, that have demonstrated that individuals vary in their attractiveness to mosquitoes. The complex processes that drive the attraction of a bloodthirsty mosquito is slowly being unravelled. It is well known that the carbon dioxide we exhale attracts mosquitoes, but when they focus their sights in on an individual, it is the chemical cocktail of compounds and microbes on our skin that determines where they strike.

With over 300 compounds potentially isolated from our skin, some will attract mosquitoes and others may actually repel them. The mix of these chemical compounds is critical. It’s even possible that the foods and drinks we consume may influence our attraction too, even though there is absolutely no evidence that there is anything we can eat or drink to completely protect ourselves from mosquito bites. Be warned, there are studies that show drinking alcohol may actually increase your attractiveness to mosquitoes!

It has been proposed that scientific study of these repellent and attractant chemical compounds on our skin may lead to personalised insect repellent formulations. That may be a long way off, but perhaps the better strategy is to use this information to develop more effective chemical lures for traps. Current mosquito trapping technologies rely on lights, carbon dioxide, and/ or some basic chemical lures. These will catch mosquitoes but they’re not completely effective in preventing mosquito bites in the backyard. Perhaps one day we will have the perfect mosquito trap. It would be a whole lot easier if there was no longer a need for topical repellents, as one of the biggest barriers to the effectiveness of repellents is whether they’re applied correctly.

Choosing and using insect repellents

Advice provided by health authorities on personal protection measures is general in nature. The key components include the use of insect repellents/ insecticides (e.g. topical repellents, mosquito coils), behavioural practices (e.g. avoiding areas during times of the day when mosquitoes are most active) and physical barriers (e.g. bed nets, wearing long sleeved shirts). This advice is pretty consistent across Australia.

For most people, topical insect repellents are their first choice. All products purporting to repel mosquitoes must be approved by the Australian Pesticides and Veterinary Medicines Authority (APVMA) who make an assessment on the effectiveness and safety of the products. There are over 100 different formulations registered, but there are surprisingly few active ingredients actually available.

Caged mosquitoes are typically used for testing personal repellents in the laboratory

Time and time again, scientific studies show that the most effective mosquito repellents widely available in Australia are N, N-diethyl-3-methylbenzamide (commonly known as DEET) and 2-(2-hydroxyethyl)-1-piperidinecarboxylic acid 1-methylpropyl ester (commonly known as picaridin).

These two repellents are routinely recommended by health authorities for mosquito bite prevention. They’ve been proven effective and, despite being used millions of times each year, there are very few health risks associated with their use reported. Just don’t drink them, get them in your eyes or apply to open skin wounds.

There is no perfect formulation to recommend. There are sprays, creams, roll-ons, gels, lotions, aerosols, and even wipes. Their effectiveness depends on the active ingredients they contain and whether they’re easy to use. Pick a formulation you find easiest to use and that will become your ‘best’ formulation.

To get the best protection from these repellent formulations, it is important to understand what their concentration means for frequency of use. The ‘stronger’ the formulation, the longer it provides protection. This means that, over short periods, a ‘tropical strength’ or ‘low dose’ formulation will provide comparable protection. However, if you’re outdoors for extended periods, you need to either reapply about every two to four hours or choose a stronger formulation.

The high dose DEET-based formulations have been shown to last over 10 hours in laboratory trials but, in reality, the repellent will be sweated, washed or rubbed off before it actually stops working.

When applying a repellent, it is important to cover all exposed skin. A dab ‘here and there’ won’t work. Spraying on your clothes won’t provide much protection either. Think of the way you’d apply sunscreen. If you miss a patch of skin, you’ll get sunburnt, miss a patch of skin when applying repellent and the mosquitoes will target in on it!

Is it safer to use plant-based repellents?

The perception that these so called ‘chemical repellents’ pose a health risk often drives attention on plant-based repellents that are considered safer to use. There is no evidence to back up these claims, as plant-based repellents are almost always shown to provide less protection against biting mosquitoes than DEET or picaridin.

In some cases, ‘homemade’ repellent formulations containing essential oils are far more likely to cause skin irritation. There are some commercial formulations that contain a range of plant extracts, most commonly tea tree oil, that will offer some protection against mosquitoes. Just keep in mind that these products will typically need to be reapplied more frequently than DEET or picaridin-based formulations to provide the same level of protection.

There is a plant-case repellent that does show great potential, however. The Australian native plant Corymbia citriodora produces an effective insect repellent, but while the essential oil from this plant doesn’t demonstrate substantial repellent activity, the by-product of the hydrodistillation process (p-menthane-3, 8-diol (PMD)) has been shown to be a very effective repellent. This is now found in insect repellents registered for use in Australia and is generally listed as ‘oil of lemon eucalyptus’. Laboratory and field studies have shown that this product can offer comparable protection from biting mosquitoes as both DEET and picaridin.

Sound too good to be true? It probably is…

Applying insect repellent can be a hassle, especially when you’re looking forward to spending time outdoors. For this reason, plenty of gimmicks have been proposed that will stop mosquito bites without needing to reach for the spray, cream, or lotion. Colourful wrist bands (see main picture, above) and patches are sold as alternatives to topical repellents. Unfortunately, there is no evidence these provide more than just a few centimetres of protection. Similarly, there is a suite of applications available for your smartphone that purport to repel blood seeking mosquitoes by emitting high frequency sounds. Again, it is just wishful thinking as there is no evidence these devices provide any protection whatsoever!

This summer, take a fresh look at the insect repellent section of your local supermarket or pharmacy. While the packaging may have been updated, not much has changed in the actual formulations for many years. Just make sure the product is registered with the APVMA, check out the recommendations for use, and enjoy spending time outside this summer with a few less itchy red lumps!

Dr Cameron Webb, NSW Health Pathology; Marie Bashir Institute of Infectious Diseases and Biosecurity, University of Sydney