Insect Repellent And The Mosquito: The Battle Continues

By Bob Prince

Multiple species of flying and crawling insects, including mosquitoes, ticks, flies, midges, chiggers, and fleas, bite people. Although these insects are mostly a nuisance in North America, worldwide they transmit more than 100 bacterial, protozoan, parasitic, and rickettsial diseases to humans. A single bite from an infectious vector is sufficient to transmit disease.

Mosquitoes transmit more diseases to humans than any other biting insect. Mosquitoes are the vectors responsible for transmitting several forms of viral encephalitis, yellow fever, dengue fever, bancroftian filariasis, and epidemic polyarthritis to humans; more than 700,000,000 people are infected yearly. Malaria, which is transmitted by the bite of a mosquito infected with the single-cell protozoan Plasmodium, is responsible for 3,000,000 deaths annually.

In 1999, West Nile virus was detected for the first time in the Western hemisphere; 8 people died from the virus in the New York City area. By January 2005, the virus, carried by mosquitoes, had spread to all 48 states. More than 3000 cases of West Nile virus infection were reported to the US Centers for Disease Control and Prevention (CDC) in both 2006 and 2007.

Scientists do not yet fully understand how biting insects find their hosts. Mosquitoes are the best studied of the biting insects, and they are known to use visual, thermal, and olfactory stimuli to locate a bloodmeal. For mosquitoes that feed during the daytime, host movement and dark-colored clothing may initiate the orientation toward an individual. Visual stimuli appear to be important for in-flight orientation, particularly over long ranges, whereas olfactory stimuli become more important as a mosquito nears its host.

Carbon dioxide and lactic acid from the skin and the breath appear to be the main insect attractants. Carbon dioxide can attract mosquitoes from more than 100 feet away. Skin warmth and moisture serve as attractants at close range. Volatile compounds derived from sebum, eccrine and apocrine sweat glands, and/or the bacterial action of the cutaneous microflora on these secretions may also act as chemoattractants. One study showed that drinking alcohol increases one’s attractiveness to mosquitoes. Malaria infection itself also increases the attractiveness of that person to mosquitoes.

Floral fragrances found in perfumes, lotions, soaps, and hair care products may attract biting insects.

Despite the obvious desirability of finding an effective oral insect repellent, no such agent has been identified. Ingested garlic, brewer’s yeast, and thiamine are not effective at repelling insects. The quest to develop the perfect topical repellent has been an ongoing scientific goal for years but has yet to be achieved.

To be effective, an insect repellent must be volatile enough to maintain an effective repellent vapor concentration at the skin surface, but it must not evaporate so rapidly that it quickly loses its effectiveness. Multiple factors play a role in the effectiveness; these factors include the concentration, frequency, and uniformity of application; the user’s activity level and overall attractiveness to blood-sucking arthropods; and the number and species of potentially biting organisms.

The effectiveness of any repellent is reduced by abrasion from clothing; evaporation and absorption from the skin surface; wash-off from sweat, rain, or water; a windy environment; and high ambient temperatures. (Each 10°C increase in temperature can lead to as much as a 50% reduction in protection time.) Moderate levels of physical activity have been shown to decrease the efficacy of N, N- diethyl-3-methylbenzamide (DEET) – based insect repellents by as much as 40%.

The commercially available insect repellents do not cloak the user in a chemical veil of protection. Any untreated exposed skin can be readily bitten by hungry arthropods. Protection from both the nuisance and the health risks associated with insect bites is best achieved by avoiding infested habitats, wearing protective clothing, and applying adequate insect repellent.

Bob Prince ia an author who specializes in mosquito repellent. He has written articles on natural mesquito repellent, electronic mosquito repellent, and Off mosquito repellent just to name a couple.