Protective measures adopted by individuals and families not only help in protecting the individuals against mosquito bites and hence malaria, but also help in reducing the mosquito population by denying the blood meal essential for nourishment of the mosquito eggs in the female anopheles mosquito. Use of mosquito repellents, protective clothing and mosquito nets are important measures of personal protection against malaria. These are easy to use, safe and not very expensive. However, these should be used regularly without fail and therefore, demand such commitment form the users.
Mosquito repellents: Female mosquitoes bite human beings every 3 to 4 days for the blood meal and use visual, thermal, and most importantly, olfactory stimuli to locate a host. Carbon dioxide, released mainly from breath but also from skin, serves as a long-range airborne attractant and can be detected by mosquitoes at distances of up to 36 meters. Lactic acid, skin temperature, moisture, other volatile compounds, derived from sebum, eccrine and apocrine sweat, or the cutaneous microflora bacterial action on these secretions, may all act as attractants. Factors such as diet, general health condition, or reproductive status can also influence the odour profile of an individual and may explain the observed variation in human attractiveness to mosquitoes. In addition, floral fragrances from perfumes, soaps, lotions, and hair-care products may also attract mosquitoes and consumption of alcoholic drinks such as beer can also increase the attractiveness to mosquitoes. These attractants stimulate the chemoreceptors on the antennae of the mosquitoes and inhibition of these receptors by certain chemicals can produce mosquito repellent effect.Several synthetic and natural substances are being used as mosquito repellents. DEET (N,N-diethyl-m-toluamide [or N,N-diethyl-3-methylbenzamide]), picaridin (1-methyl-propyl 2-[2-hydroxyethyl]-1-piperidinecarboxylate [also known as KBR 3023]), PMD (p-menthane 3,8-diole [or oil of lemon eucalyptus]), MGK-326(dipropyl isocinchomeronate), MGK-264 (N-octyl bicycloheptane dicarboximide), IR3535 (ethyl butylacetylaminopropionate), and oil of citronella have been registered as insect repellents by the US Environmental Protection Agency.
Of these, DEET is the most effective, and best studied, synthetic insect repellent currently on the market. It has a remarkable safety profile after 40 years of worldwide use, although toxic reactions are known to occur. Developed by scientists at the U.S. Department of Agriculture and patented by the U.S. Army in 1946, it was subsequently registered for use by the general public in 1957. According to the estimates of the U.S. Environmental Protection Agency, more than 38% of the U.S. population uses a DEET-based broad-spectrum insect repellent every year and its worldwide use exceeds 200000000 people annually.DEET is available in 5% to 100% concentrations in multiple formulations, including solutions, lotions, creams, gels, aerosol and pump sprays, and impregnated towelettes. Products with 10% to 35% DEET will provide adequate protection under most conditions. Products with concentrations around 10% are effective for about two hours, concentration of 24% protects for about 5 hours and the efficacy plateaus at a concentration of about 30%. Generally DEET should be applied once a day and the lowest concentration effective for the amount of time spent outdoors should be chosen, especially for children. Use of 20% DEET is considered to be safe in pregnancy. In children, 30% DEET or less are considered to be safe, but these are not recommended for use in infants younger than 2 months.
Application of repellents: The repellents must be applied to lightly cover all exposed areas of skin; unprotected skin even a few centimeters away from a treated area can attract mosquitoes. It should be applied carefully over the face, avoiding contact with eyes and mouth. It should not be applied to children’s hands and after applying, repellent from the surfaces of the palms should be wiped off to prevent inadvertent contact with eyes, mouth, and genitals. Repellents should never be used over cuts, wounds, inflamed, irritated, or eczematous skin. Repellents may also be applied to clothing, window screens, mesh insect nets, tents, or sleeping bags. If DEET-treated garments are stored in a plastic bag between wearings, the repellent effect can last for many weeks.
Repellents containing DEET must be carefully applied because they can damage plastics (such as watch crystals and eyeglasses frames), rayon, other synthetic fabrics, leather, and painted or varnished surfaces. DEET does not damage natural fibers, such as cotton or wool, and has no effect on nylon. Using a DEET based insect repellent and a sunscreen together may reduce the sunscreen’s effectiveness.
How to apply mosquito repellents?
- Apply during the biting time of the mosquitoes; for anopheles, it is dusk to dawn.
- Take care to avoid contact with mucous membranes (eyes, nostrils, mouth, lips); do not spray on the face.
- Do not allow young children to apply this product; do not apply to hands or near the eyes and mouth of young children.
- Do not apply to sensitive, sunburned or damaged skin or deep skin folds.
- Use just enough repellent to cover exposed skin and/or clothing; do not use under the clothing.
- Avoid over-application.
- Wash the hands after applying the repellent.
- After returning indoors, wash treated skin with soap and water; wash treated clothing before wearing it again.
- In case of repellants formulated as sprays:
- Do not spray in enclosed areas.
- Do not spray directly onto face; spray on hands first and then rub on face.
- Repeated applications (every 3–4 hours) may be needed, especially in hot and humid climates.
- Strictly adhere to the manufacturers’ instructions and do not exceed the dosage, especially for young children.
Adverse effects of DEET: DEET is generally well tolerated, although there are some reports of unpleasant odor or greasy feel. After topical application, average dermal absorption of 100% DEET was about 5.6% and for 15% DEET in ethanol, an average of 8.4% of the dose was absorbed. Studies on DEET’s toxicity, mutagenicity and oncogenicity did not indicate any significant toxicities with normal use. Some cases of contact urticaria and irritant contact dermatitis (mostly in soldiers) have been reported; the ante-cubital fossa is particularly sensitive to developing bullous irritant contact dermatitis if DEET products are allowed to remain on this area overnight. There are stray reports of fatalities, altered mental state, rashes, skin or mucous membrane irritation, transient numb or burning lips, dizziness, disorientation, and difficulty concentrating, headache and nausea etc., following the use of DEET based repellents. The use of 20% DEET during the second and third trimesters of pregnancy did not increase any adverse neurologic, gastrointestinal tract, or dermatologic effects in infants through 1 year of age. A study by Malaria Research Centre, India revealed that of those using DEET-based cream, 11.4 per cent reported skin reaction and itching. If any adverse reaction to this product is suspected, its use should be discontinued and the treated skin should be washed.
Many plant derived insect repellents have also been tested: oils of citronella, cedar, verbena, pennyroyal, geranium, lavender, pine, cajeput, cinnamon, rosemary, basil, thyme, allspice, garlic, peppermint and neem have been reported to have repellent activity. But most of these studies are poor and most of these oils provided short-lasting protection, usually 20 minutes to less than 2 hours.
Other repellents: Many other types of repellents, such as mats, coils, lotions and vaporizers are also available, in the market. The current Indian market for these repellents is estimated to be more than Rs 500–600 crores (US$ 12–15 million) with annual growth of 7 to 10%. Allethrin, bioallethrin, d-allethrin, d-transallethrin, s-bioallethrin, prallethrin etc., are used in these mats, coils and vaporizers. These compounds vaporize without decomposition on heating at temperatures up to 400°C and produce varying repellent action on the mosquitoes, depending on the type of product and species of mosquito.
Concerns have been raised about the safety of these products. In animal studies, these products have been reported to cause adverse effects on neurological, pulmonary, endocrinal and reproductive systems and to cause developmental impairment and cancers. A questionnaire-based survey among users of these products revealed that 11.8% users comprising all age groups and both sexes complained of a variety of acute toxicity, either soon after or within a few hours of using these repellents. Breathing problems were the most common (4.2%) and were frequently accompanied with headache or eye irritation or both. Pain in the ear and throat, cough, cold, running nose, wheezing, skin irritation etc., were also reported. Some coils containing pyrethroid insecticides, particularly d-allethrin, may contain octachlorodipropyl ether (S-2, S-421) as a synergist or active ingredient and the slow smoldering of the mosquito coils (about 8 hours/coil) may release bis(chloromethyl)ether (BCME), an extremely potent lung carcinogen.
Electronic mosquito repellents, ultrasonic devices, outdoor bug “zappers,” and bat houses are not effective against mosquitoes. Repellent-impregnated wristbands also do not offer any protection.
Air-conditioning: Although it has been said that air-conditioned rooms confer protection against mosquitoes, no systematic reviews or RCTs are available on the effects of air conditioning or electric fans to prevent malaria in travelers and some studies have even suggested that air conditioning does not offer any protection against anophelines.
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