Busting five myths of mosquito repellents

repellent_spraying_webbAfter a record year of West Nile virus activity in 2012, North American health authorities are on alert as the peak season of mosquitoes approaches. As of 16 July 2013, a total of 23 cases of human disease, including three deaths, had already been reported by CDC.

In many regions across the US, local mosquito control districts engage a range of strategies to reduce mosquito-borne disease risk. These may include broad scale insecticide use or the release of “mosquito fish” into derelict backyard pools. However, the first line of defense against biting mosquitoes remains the use of topical insect repellents.

In North America, the CDC provide detailed information on mosquito repellents but there still seems to be confusion amongst many in the community looking for alternatives to “chemical repellents”. These repellents are often perceived to be unsafe. For an Australian perspective, I’ve put together some guidelines on mosquito repellent use that provide a few more details than typically found on the websites of local health authorities.

In my experience of trying to promote the use of “known to be effective” repellents, I still find that there are many misconceptions and misunderstanding of how these products should be used. Below are the five myths I most commonly experience when it comes to the use of mosquito repellents.

Myth 1: Natural must be better

It isn’t surprising that most people associate “natural” products with better health. Many people 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 these perceived “healthier” choices may not provide the best outcomes.

Studies have repeatedly shown that the most effective repellents are DEET and picaridin. DEET, in particular, has been shown to be very effective. Picaridin is pretty good too. Both products have shown to be effective in local field-based tests.

Unfortunately, many, many studies throughout the world have shown that botanical based repellents provide substantially less protection against biting mosquitoes than DEET or Picaridin. Products containing citronella, lavender, peppermint and Melaleuca oils are widespread and are often promoted as “DEET-Free” alternatives to the recommended repellents. There are many botanical-based insect repellents listed in the patent literature.

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.

I often wonder if Koalas are bitten any less by mosquitoes than other Australian mammals given the amount of Eucalyptus leaves they consume! (Photo by DAVID ILIFF. License: CC-BY-SA 3.0)

What about “Oil of Lemon Eucalyptus”? That’s a botanical repellent and authorities recommend it against West Nile virus right? There is often some confusion regarding this product. It is not the essential oil, but rather a byproduct 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 recommendations by CDC 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 ingredients) 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 cage” style experiments) investigating 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 (for how long are all mosquito bites prevented). While the marketing companies may be interested in claims like “over 80% of mosquito bites prevented”, 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 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 most likely related to high concentration formulations. In the US, there are many brands available that contain over 95% DEET. In the vast majority of situations, however, most people would find that an approximate 10% DEET formulation would work perfectly well and not be associated with any of these unpleasant characteristics.

Myth 4: Apply repellent like perfume

A neighbor took great pleasure in telling me how ineffective mosquito repellents were. Repeatedly. One afternoon I saw him applying repellent to his children. The aerosol was sprayed around in the air above the kids as they jumped up and down. There was no way that the repellent was going to work.

While there is still some debate as to how DEET prevents mosquitoes bites, or how the response of mosquitoes to DEET is influenced by previous exposure or infection with a pathogen, we do know that to get the best results, the repellent should be applied as a thin covering on all exposed skin. It is for this reason I personally think creams and liquids are the best repellents to use.

Don’t apply repellent like a perfume. A spray “here and there” won’t work. Spraying it on your clothes won’t work either. Apply it in the same way as you would a sunscreen but keep in mind that you won’t need to apply it quite so often.

As summer approaches, the shelves of hardware stores and supermarkets are filled with various repellents, insecticides and traps. Some work better than others.

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 that putting on repellent is a bit of a hassle. Unfortunately, there is little scientific evidence that any of these will protect you from mosquito bites.

There is even a pill, recently approved by Canadian Health Authorities, that purports to only take 30 minutes to protect the swallower from mosquito bites. An excellent blog post, “Insect repellent you can eat – but does it work?”, refuting the claims of this product, is here. Also check this nice piece from 2012, “Homeopathic Insect Repellent: Is there anything the Natural Health Products Directorate won’t approve?

In short, there is nothing you can eat or drink that has been scientifically proven to prevent mosquito bites. That’s right, not even vitamin B.

In summary, 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. Many of these (and more) urban myths will persist for some time and perhaps it is time health authorities worked harder to communicate the benefits and effective use of the products available.

UPDATE: Thanks to BugGirl and Laurie Sullivan for the suggested tweaks!