Mosquito-borne disease is a serious concern, with millions of people worldwide impacted by pathogens spread by these blood-sucking insects. In Australia, there are more than 5,000 cases of human illness caused by the mosquito-borne Ross River virus and Barmah Forest virus every year.
When mosquitoes bite, they inject saliva into the skin. Our bodies react to this mozzie spit, causing a skin irritation. The severity of the “itchy bite” varies from person to person and, in some cases, severe allergic reactions can occur.
Mosquito control programs can reduce the rates of human disease but the first line of defence remains personal protection measures: avoiding known mosquito habitats (such as wetlands) and peak biting times (dusk), wearing protective clothing (often treated with insecticides) and using bed nets and insect repellents.
But not all mosquito repellents are equal. There are over 60 individual repellent formulations currently registered including aerosols, creams, lotions, pump sprays, wipes, wrist bands and sticks. Despite this diversity of products, there are only a handful of active ingredients, the most common of which are DEET (diethyltoluamide) and Picaridin.
All topical insect repellents sold in Australia must be registered with the Australian Pesticides and Veterinary Medicines Authority (APVMA). The APVMA assess products for their efficacy and safety. This also means the label must display the active ingredients and their concentration.
Active ingredients
DEET has been shown to be effective at preventing bites from a wide range of pest mosquitoes. It’s considered the gold standard in mosquito repellents. But there is still not unanimous agreement on how DEET works. While it’s generally thought to prevent bites by inhibiting the host seeking stimuli, there is also some evidence that the mosquitoes smell, and are repelled, by DEET.
Picaridin works just as well as DEET and is generally considered to be a more pleasant product to use as it is odourless.
There is some resistance to the use of these chemical repellents because they’re unpleasant to use, are thought to damage clothing or belongings or are perceived to pose adverse health effects. But despite the widespread use of DEET, and increasingly picaridin, there are very few serious adverse reactions reported. As such, the two products have been endorsed by health authorities internationally as a safe way to avoid mosquito bites.
Formulations
There is often confusion about how the concentration of a repellent determines the level of protection: the concentration of a repellent relates to the duration of protection, not the amount of mosquitoes that are kept at bay.
Repellents containing DEET at concentrations of 80% or 10%, for example, will both protect against mosquito bites for about two hours. While the protection provided by the 10% formulation may stop after a few hours, the 80% formulation will continue to provide protection for over ten hours.
When choosing a repellent, it’s worth keeping in mind just how long you’ll need to protect yourself. For a quick trip to the park, a low-concentration repellent will do the trick. But if you’re off on a major bush walk or fishing trip, a higher concentration repellent will be required. Alternatively, you’ll need to reapply a lower concentration repellent more frequently.
So does formulation matter?
Probably not. The choice of active ingredient is really the most important decision. The choice of formulation is probably best guided by the ease of application. Sprays are generally most effective for arms and legs while creams and wipes are good for the face.
For complete protection, the entire surface of exposed skin must be covered. For this reason, it is probably best to apply repellent to your hands first, and then rub into exposed skin. A spray “here and there” won’t offer protection. Spraying repellent on clothes and/or belongings won’t help either.
What about ‘natural’ repellents?
Products derived from plants are often considered a safer alternative to the chemical products such as DEET and picaridin. Homemade concoctions of essential oils, particularly Eucalyptus and Melaleuca oils, are often promoted as suitable repellents.
First, it’s important to note that these products have the potential to cause skin reaction.
Second, studies have shown that essential oils provide only limited protection from biting mosquitoes. Registered commercial products that contain botanical extracts offer some protection but will need to be reapplied far more frequently than even the low concentration DEET- or picaridin-based repellents.
Despite its popularity, citronella has repeatedly been shown to be less effective than DEET.
The Australian native plant Corymbia citriodora (lemon-scented Gum) plant sits in an interesting position among botanical repellents. While the essential oil from this plant doesn’t demonstrate substantial repellent activity, the by-product of the hydrodistillation process has been shown to be a very effective repellent. The active ingredient is p-menthane-3, 8-diol (PMD). In Australia, PMD is listed as, “Oil of Lemon Eucalyptus being acid modified extract of lemon eucalyptus (Corymbia citriodora)”.
Although patches or plastic wrist bands are registered as repellents by APVMA, these only offer very limited and localised protection (generally only immediately around the product, if at all).
Ultrasonic repellents have been sold in many forms for many years. The most recent incarnation is smartphone apps. The failure of sound to repel mosquitoes has been shown time and time again. They simply don’t work.
What’s best?
A group of experts recently reviewed the benefits of a range of strategies, from insecticides to taking vitamin B supplements.
Their conclusion? As well as insecticide-treated bed nets and clothing, topical insect repellents provide the best protection. DEET- and picaridin-based repellents are a cheap, safe and effective way to prevent mosquito-borne disease.