AN effective personal insect repellent is still the most reliable first line of defence against biting mosquitoes. Unfortunately, there is still confusion among people looking for alternatives to 'chemical repellents', which they often (wrongly) perceived to be 'unsafe'. And, many people don't understand how commercial topical insect repellents should be used.
Agencies like America's CDC provide comprehensive and detailed information on the efficacy and use of mosquito repellents. But, for an Australian perspective, I've put together some guidelines which offer more details than local health authorities typically provide.
I've also tackled the 5 most common myths I hear surrounding mosquito repellents.
MYTH 1 – 'Natural must be better'
It isn't surprising that most people associate 'natural' products with better health. They perceive mosquito repellents derived from 'natural' products, such as plant extracts, to be healthier choices. However, when it comes to mosquito repellents, there is clear evidence that they may not provide the best outcomes.
Studies have repeatedly shown that the most effective repellents are DEET, which is particularly effective, and Picaridin, which is also very good. Both products have shown to be effective in local field-based tests.
Unfortunately, many, many international studies have shown that botanical-based repellents provide substantially less protection against biting mosquitoes than DEET or Picaridin. There are many botanical-based insect repellents listed in the patent literature. Products containing citronella, lavender, peppermint and Melaleuca oils are widely available and are often promoted as 'DEET-free' alternatives. Essential oils of Australian native plants provide significantly less protection than DEET-based repellents. Expert review panels have suggested that products containing plant extracts should not be recommended in areas of endemic mosquito-borne disease or when biting mosquitoes are abundant.
It is important to remember that botanical-based repellents WILL provide some limited protection against biting mosquitoes. The biggest problem is that they will need to be reapplied 3-4 times as often as even a low-dose DEET-based repellent to provide comparable protection. Botanical repellents may be fine for a quick trip to the backyard to hang the washing out, but not for a long session of gardening or if you're off for a hike. It is also important to remember that using undiluted essential oils can also pose an important health risk, particularly in young children.
What about Oil of Lemon Eucalyptus, you ask? That's a botanical repellent and authorities recommend it against West Nile virus, right?
There's often confusion regarding this product. It is not the essential oil, but rather a by-product of the distillation process of the leaves of Corymbia citridora. Commonly known as PMD, it has been shown to be as effective as DEET (although generally requiring higher doses for comparable protection) and is recommended by the CDC in North America. The CDC's recommendation of this product should not be seen as an endorsement for other 'botanical-based' repellents.
MYTH 2 – Stronger repellent = fewer mosquitoes
This is probably the most common mistake made when choosing a repellent. The 'strength' of a repellent (i.e. the concentration of active ingredient/s) doesn't determine how many mosquitoes are kept at bay. It determines the duration of protection provided. It basically determines how long you are protected from biting mosquitoes.
The majority of published studies (the classic 'arm-in-a-cage' style of experiment) that investigated the efficacy of repellents analyse the results in two ways – mean repellency rates (a comparison of how many mosquitoes land on a treated arm compared to an untreated arm) and mean protection time (how long it prevents all mosquito bites). Marketers make claims like 'over 80% of mosquito bites prevented', but given that it only takes one mosquito bite for a pathogen to be transmitted, I'm hoping to prevent ALL bites! We should be far more interested in protection times rather than repellency.
MYTH 3 – Chemical repellents are dangerous
Both DEET and picaridin are considered safe. If used as directed, DEET-based repellents pose no substantial health risk. Despite being used by millions of people every year, there are few examples of reported serious adverse health impacts in the scientific literature.
There are many stories circulating about mosquito repellents having an unpleasant smell or creating an unpleasant feeling on the skin. There are also reports about damage to clothing and plastics in some instances. Some of these reports may be true, but are more likely to be related to high-concentration formulations. In the US, there are many brands available that contain over 95% DEET.
However, in most situations, formulations of approximately 10% DEET work perfectly well and are not likely to result in any unpleasant characteristics.
MYTH 4 – Apply repellent like perfume
A neighbour took great pleasure in telling me over and over how ineffective mosquito repellents were. One afternoon I saw him applying repellent to his children. He sprayed the aerosol around in the air above the kids as they jumped up and down. There was no way that repellent was going to work.
There is still some debate as to how DEET prevents mosquito bites, or how the response of mosquitoes to DEET is influenced by previous exposure or infection with a pathogen that mosquitoes might carry.
However, we do know that to get the best results, the repellent should be applied as a thin covering on all exposed skin. For this reason, I think cream and liquid repellents are the best ones to use.
Don't apply repellent like a perfume. Spraying it here and there won't work, nor will spraying it on your clothes. Apply it in the same way as you would use sunscreen – but not quite as often.
MYTH 5 – These gimmicks really work!
If it sounds too good to be true, it probably is! Gimmicks such as traps, ultrasonic devices, and smartphone apps all sound very appealing if you find putting on repellent a bit of a hassle. Unfortunately, there is little scientific evidence that any of these will protect you from mosquito bites.
There's even an insect-repelling pill, recently approved by Canadian Health Authorities, that purports to protect you from mozzie bites 20 minutes after you swallow it. (Read an excellent blog, Insect repellent you can eat – but does it work? that refutes the pill maker's claims. You can also read this nice piece from 2012, Homeopathic Insect Repellent: Is there anything the Natural Health Products Directorate won't approve?)
In short, there's nothing you can eat or drink that's been scientifically proven to prevent mosquito bites. No, not even vitamin B.
To sum up, the mosquito repellents widely available in North America, Australia and many other parts of the world are perfectly safe to use and can be effective in preventing mosquito bites.
Yet, these (and other) urban myths persist.
It's time health authorities worked harder to communicate the benefits of the available products and how to use them effectively.
* Dr Webb is based at the Department of Medical Entomology, Pathology West – ICPMR Westmead & the University of Sydney. You can follow him on Twitter @Mozziebites