No reaction to as bite does not mean ‘no bite’ as a delayed reaction time to the bite increases with age. The age of the victim has an enormous influence on the speed of the reaction to the bite: e.g. newborn infants (as they cannot have previously been bitten) do not react at all, 4-6 year olds may take 1-2 hours, and adults in their 20s may a couple of days.

Insect Repellents
The search for the perfect topical insect repellent continues. This ideal agent would repel multiple species of biting arthropods, remain effective for at least 8 hours, cause no irritation to the skin or mucous membranes, cause no systemic toxicity, be resistant to abrasion and rub-off, and be greaseless and odorless. No available insect repellent meets all of these criteria. Abrasion from clothing, evaporation and absorption from the skin surface, wash-off from sweat or rain, higher temperatures, or a windy environment all decrease repellent effectiveness. Each 10 °C increase in temperature can lead to as much as a 50% reduction in protection time. The repellents currently available must be applied to all exposed areas of skin; unprotected skin a few centimeters away from a treated area can be attacked by hungry mosquitoes.

Chemical Insect Repellents
N,N-Diethyl-3-Methylbenzamide (DEET). This remains the gold standard of currently available insect repellents. This substance was discovered and patented by the U.S. Army in 1946. It was registered for use by the general public in 1957, so has been around for awhile. It is a broad-spectrum repellent that is effective against most insects, particularly: mosquitoes, flies, chiggers, fleas, and ticks. Twenty years of testing of more than 20 000 other compounds has not resulted in another marketed chemical product with the duration of protection and broad-spectrum effectiveness of DEET.

As a general rule, higher concentrations of DEET provide longer-lasting protection. Unfortunately, no guidelines are available to help consumers decide what concentration of DEET is appropriate for their specific needs. Mathematical models of the effectiveness and persistence of mosquito repellents show that the protection offered by a repellent is proportional to the logarithm of the dose (concentration of the product). Again this is not particularly well understood by clinicians, and certainly not by travellers! In its simplicity, however, for casual use a high concentration of DEET is not needed. Products with 10% to 35% DEET will provide adequate protection under most conditions. Children under the age of 2 years would be best served by a repellent containing no more than 10% DEET. It is considered that toxicity may increase with higher concentrations. Practically if they do occur, it is invariably as a result of over-exposure to the chemical. DEET has had a remarkable safety profile during more than 40 years of use by millions of people worldwide, and is an essential product for the traveller to ‘arboviral territories’! Just one caution, however, repellents containing DEET must be carefully applied because they can damage synthetic materials such as watch straps, eyewear frames and fabrics.

Plant-Derived Repellents
No plant-derived chemicals tested to date have been able to demonstrate the broad effectiveness and duration of DEET, although a few show repellent activity. Plants whose essential oils have been reported to have repellent activity include citronella, cedar, pennyroyal, geranium, lavender, pine, cinnamon, rosemary, basil, thyme, garlic, and peppermint. When tested, most of these tended to give short-lasting protection, of less than 2 hours.

Permethrin
Pyrethrum is a powerful, rapidly acting insecticide, originally derived from the crushed and dried flowers of the daisy Chrysanthemum cinerariifolium. Permethrin is a human-made synthetic pyrethroid. It does not repel insects but works as a contact insecticide, causing nervous system toxicity that leads to the death of the insect. The chemical is effective against most insects and particularly mosquitoes, flies, ticks, and chiggers. Permethrin has low toxicity in mammals, is poorly absorbed by the skin.

Disclaimer:
Whilst every effort is made to provide current and accurate information and advice, no liability is accepted by TRAVEL ESSENTIALS, WORLDWISE or any of their subsidiaries for any incorrect information.