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Key Points:

  • The combination of DEET applied to skin and permethrin to clothing will provide >99% protection.
  • Historically, DEET-based repellents were the gold standard of protection under circumstances in which it is crucial to be protected against disease-transmitting insect bites.
  • Low-concentration (5% to 15%) DEET, picaridin, and biological repellents are acceptable for preventing nuisance bites in areas without malaria, West Nile Virus or other serious endemic illnesses.
  • The fear of “DEET toxicity” prevents many people from using repellents properly; this fear is unfounded. DEET, however, may be less cosmetically acceptable (i.e., have more side effects, such as skin irritation) than non-DEET repellents
  • Permethrin is an insecticide that is chemically related to natural pyrethrum. It kills mosquitoes and ticks that land on permethrin-treated fabric or gear, but is nontoxic for human use.

What’s New?

  • The Health Guide continues to recommend DEET repellents, in concentrations ≥30%, for travelers going to malaria endemic areas. Picaridin (20% concentration) is now an acceptable alternative, and is recommended by the CDC and WHO.
  • Concerning DEET-repellent, the UK recommendation allows the use of DEET up to 30% in pregnant women and children older than 2 months.
  • DEET has never been shown to cause fetal malformations.


Mosquitoes are ubiquitous insects. They are found in every region of the world except Antarctica. Mosquitoes breed in standing water in diverse aquatic habitats, including fresh water (even if heavily polluted), saltwater marshes, brackish water, and even water found in discarded containers and old tires.

Both male and female mosquitoes feed on flower or fruit nectar, but only female mosquitoes bite; they require a blood meal every 3 to 4 days for the protein necessary to produce eggs. Mosquitoes can be divided generally into two types: daytime and nighttime biters. Those mosquitoes that transmit malaria and Japanese encephalitis (Anopheles and Culex mosquitoes) bite mostly at twilight or during the night, whereas Aedes mosquitoes, which transmit dengue and yellow fever, are daytime biters. Mosquitoes also bite indoors, so you need to prevent mosquitoes from gaining entry into living and sleeping quarters and to eliminate those that might already be there. The most common mosquito-transmitted diseases that you need to protect yourself from in tropical and subtropical climates are the following:

  • Malaria
  • Dengue fever
  • Chikungunya fever
  • Pika virus
  • Yellow fever (best prevented by vaccination)
  • Filariasis
  • Viral encephalitis (e.g., Japanese encephalitis, Venezuelan equine encephalitis)
  • Miscellaneous viral illnesses. In addition to Rift Valley fever, West Nile fever, and Sindbis fever, there are about 30 rarely diagnosed viral illnesses, such as epidemic polyarthritis, that are also mosquito transmitted.

Note: Mosquitoes cannot transmit HIV. The virus neither survives nor replicates in mosquitoes, and the blood from the last bitten person is not transmitted into the next bitten person.

Ticks and Biting Flies

The same personal protection measures that you use against mosquitoes will also protect you against ticks and biting flies—insects that transmit Lyme disease, tick-borne encephalitis, relapsing fever, typhus, leishmaniasis, onchocerciasis, trypanosomiasis, and several other tropical and infectious diseases. Of these diseases, leishmaniasis, transmitted by sand flies, is the most common.

You will want to avoid mosquitoes and biting flies for another reason—insect bites, even without the risk of disease, can make you miserable. Bites usually cause localized swelling and itching, and certain bites, such as from black flies, are very painful. Bites can also become infected, usually from excessive scratching (excoriation). Rarely, bites can cause systemic reactions, including anaphylaxis, from a person’s sensitivity to the insect’s salivary antigens. Protecting yourself from insect bites entails more than just applying an insect repellent to your skin. A multi-pronged approach is essential. The combined use of a skin repellent, permethrin-treated clothing, and/or shelters is the best way to avoid insect bites. By using the personal protection methods described in this chapter, you can achieve more than 90% protection against mosquitoes and other biting insects. Not every mosquito or insect carries disease, but just one bite from an infected mosquito or other insect can make you sick

Insect Repellents

Insect repellents fall into two categories, (1) chemical and (2) botanical.

Chemical Repellents

Repellents containing DEET (acronym for the chemical diethyltoluamide) are the most effective and widely used. DEET was developed in the 1930s by the U.S. Department of Agriculture and registered for use by the general public in 1957. Over 200 million persons now use DEET-containing repellents annually worldwide. In the past 45 years, people have applied DEET more than 10 billion times. Features of DEET are:

  • Repels insects for up to 12 hours
  • Effective against more species of biting insects than any other repellent
  • The best studied and analyzed of all repellents
  • Remains the standard of chemical insect repellents
  • Forty years of testing more than 20,000 compounds has not led to a better repellent being brought to market.

How DEET Works The female mosquito in search of a blood meal is guided by several clues, the most important being heat, moisture, carbon dioxide, and odor. DEET works in part by masking the insect-attracting odor of carbon dioxide and lactic acid given off by the human body. At very close range DEET also appears to work by interfering with an electrophysiological homing mechanism in the mosquito’s antennae.

DEET is effective in relatively small amounts provided it is spread evenly and completely over all exposed areas. DEET, however, has little “spatial activity,” meaning that nearby, untreated skin is still likely to be bitten. Factors playing roles in any repellent’s effectiveness include its concentration, the frequency and uniformity of application, and the number and species of insects attempting to bite. Evaporation and absorption from the skin surface, wash-off from rain or sweat, higher temperatures, or a windy environment all reduce effectiveness. Higher concentrations of DEET provide longer-lasting protection, but as the concentration of applied DEET climbs above 50%, each incremental increase provides relatively less additional protection. Extended-release formulations, however, have made it possible to reduce the concentration of DEET without sacrificing duration of action. DEET is most effective against mosquitoes and ticks, less so against gnats, black flies, biting flies, fleas, and mites. It has no effect against bees and wasps.

A repellent with a ≥30% concentration of DEET, or 20% picaridin, is preferred when there is the threat insect-transmitted disease, such as malaria, dengue, or West Nile virus. If a 15% picaridin repellent is used, it should be reapplied more frequently. Note that the protection times of each product can vary, depending on whether the data is from the manufacturer or from another testing entity.

DEET Toxicity/Safety

It is not known exactly how many adverse reactions might be caused by DEET, but millions of people have used DEET over the last 40+ years without significant problems. The Environmental Protection Agency (EPA) completed a comprehensive re- evaluation of DEET in 1998 and concluded: “As long as consumers follow label directions and take proper precautions, insect repellents containing DEET do not present a health concern.” Recent medical literature from Canada suggests that DEET does not pose a significant or substantial extra risk to infants and children, and the American Academy of Pediatrics currently recommends that children older than 2 months of age can safely use DEET up to 30% concentration. Adverse side effects fall primarily into two categories:

Dermatologic Side Effects Preparations containing less than 50% DEET are almost free of side effects when applied to the skin of adults. Skin reactions (contact dermatitis) may include itching, hives, blisters, or redness. Contact with the mouth can cause transient burning or stinging of the lips, tongue, and oral mucosa.

Neurologic Side Effects, Including Seizures Concerns over the potential neurologic toxicity of DEET are based on a very small number of case reports in the medical literature. Between 1961 and 2000, there were only 23 reported cases of seizures and other neurologic symptoms associated with the use of DEET. Six of these cases were deliberate ingestions. Regarding the remaining cases:

  • Details were often poorly documented and the symptoms could not be positively attributed to DEET.
  • Most cases involved “heavy, frequent, or whole-body” application of DEET.
  • There was no correlation seen between the severity of side effects and the concentration of DEET applied.

Given the small numbers of neurologic side effects reported since the 1960s and the 50 to 80 million people using DEET each year in the United States, the risk of serious side effects appears to be very low. If properly applied, DEET-containing repellents can be regarded as safe.

The potential of greater DEET toxicity in children has been a concern because their thinner skin and greater body surface area-to-weight ratio theoretically could enhance DEET absorption. This concern, however, has not been upheld by scientific study, and neurologic toxicity in children has not been substantiated by detailed surveillance. In a 1994 report reviewing 9,086 cases of DEET exposure from 71 poison control centers in the United States, the most severe reactions to DEET were caused by inhalation or eye contact, not skin application. The report also reached the following conclusion: There was no correlation between the severity of symptoms and age, gender, or concentration of applied DEET.

These reports, and others, indicate that side effects from proper DEET use are rare. Despite years of use and millions of applications, no clear pattern of DEET toxicity has emerged. It should be noted in this respect that the EPA does not require a cautionary statement on the label of repellents warning about the possibility of seizures or other neurologic side effects.

Although a direct link between DEET and significant health problems is extremely remote, the EPA recommends you follow these precautions to minimize any possible risks:

  • Apply just enough repellent to lightly cover the areas of exposed skin. Do not saturate the skin. DEET may also be applied to clothing.
  • Do not get DEET in eyes or mouth and avoid applying repellents to children’s hands to prevent contact with these areas.
  • Avoid inhaling DEET aerosol or spray.
  • Wear long sleeves and long pants, when possible, to reduce the skin surface area that needs to be treated with DEET.
  • Don’t apply repellents on open cuts, or inflamed or irritated skin.
  • Shower or wash repellent-treated skin after coming indoors.

Until recently, manufacturers of 5% to 10% DEET repellents have made label claims that their products are safer for use in children. However, because there is no evidence that DEET toxicity correlates with DEET concentration, the EPA has ruled that manufacturers can no longer place child safety claims on products. One other potential problem with the low-concentration “for children” formulas is that they usually last for shorter periods of time, requiring more frequent application of the repellent, with potentially greater risk of toxicity.

New Repellents—Picaridin & Eucalyptus

In April of 2005, the Centers for Disease Control and Prevention (CDC) began recommending two new active ingredients as safe, effective repellents:

Picaridin This is a synthetic repellent developed by Bayer Corporation and has been used worldwide since 1998. Derived from pepper, it is labeled for use against ticks, mosquitoes, and flies. The manufacturer claims DEET-like efficacy against mosquitoes, lasting 2-8 hours, depending on the species of mosquito and concentration of the active ingredient used. As opposed to DEET, picaridin is nearly odorless, does not cause skin irritation, and has no adverse effect on plastics. Picardin is now sold in the U.S. as Cutter Advanced Insect Repellent (7% picaridin), Cutter Sport (15% picardin), Sawyer Premium 20% Picardin, and Natraperl 8 Hour (20% picardin). Only the 20% formulation should be used when you need to protect against malaria.

What to Use The manufacturer states that picaridin is “as effective as DEET” in field trials that compared 19.2% picaridin against 30% to 35% DEET. In these trials, picaridin did show comparable effectiveness to DEET.

Eucalyptus (PMD: p-menthane, 3,8-diol) Marketed products containing eucalyptus include Repel Oil of Eucalyptus Repellent and OFF! Botanical, In Europe, the product is sold as Mosiguard Natural.

A derivative, p-menthane-3, 8-diol (PMD), isolated from the oil of the lemon eucalyptus plant was brought to the US market in 2002. A 30% PMD preparation appears to provide protection comparable to 20% DEET but requires more frequent reapplication to maintain the same level of protection. PMD-based repellents show low toxicity, but care must be taken to keep them out of the eyes because PMD can cause significant eye irritation. In April 2005, the CDC added PMD to its list of approved repellents, joining DEET and picaridin-based repellents.

Botanical Repellents

Long before the advent of synthetic chemicals, people used plant-derived substances to repel mosquitoes. Most plant-based insect repellents currently on the market contain essential oils from one of the following plants: citronella, cedar, lemon eucalyptus, peppermint, lemon grass, geranium, and soybeans. Citronella is the most common botanical oil found in natural repellents. When compared with DEET, however, citronella and most other essential oils give only short-lasting protection, lasting anywhere from minutes to less than 2 hours. Exceptions are the soybean oil-based and lemon eucalyptus repellents (see above); they give 90 to 120 minutes of protection. Note: Although it is “natural,” p-menthane-3, 8-diol (PMD, above) is now prepared synthetically.

One reason natural repellents are popular is because some consumers are concerned about “DEET toxicity” and they prefer a “nonchemical” repellent, despite DEET’s safety and effectiveness. Others reasons cited include a dislike of the odor and DEET’S adverse effects on synthetic fabrics and plastics. The true safety profile of natural repellents has yet to be determined. Plant-derived repellents are not inherently safe just because they are “natural.” Citronella, for example, caused the death of a 21-month-old child after ingestion of only one ounce of the oil. Drinking eucalyptus oil has also caused poisonings and fatalities.

How Low Should You Go?

Despite the lack of scientific evidence showing that higher DEET concentrations are more toxic, manufacturers have responded to consumer fears of “DEET toxicity” by producing repellents with DEET concentrations in the 5% to 10% range. These low-concentration products may be perfectly acceptable for preventing nuisance bites, but may not provide enough protection in areas of the world where insect-borne diseases are a real threat.

What Should the Traveler Do?

The Health Guide believes that travelers (including children) who are visiting areas where insect-transmitted infectious and tropical diseases are found should use a DEET repellent with a concentration of 30% to 35%. Under conditions of high temperatures or humidity, which increases loss of repellent from the skin surface, or when there is intense insect-biting activity, higher concentrations of DEET may be justified (100% DEET is rated at 10 hours.) Choosing a controlled-release formulation of DEET (e.g., Ultrathon) is another way to prolong the efficacy of a repellent (up to 12 hours) without requiring the use of DEET concentrations over 35%. There is no convincing evidence that infants and children are harmed by DEET if the repellent is used according to the label. If parents choose to use a lower-concentration (<30%) DEET repellent on their infants or young children, additional measures of protection, such as permethrin-treated clothing, mosquito nets, and elimination of indoor insects, should be used.

Recently, the CDC began recommending the synthetic repellent picaridin as a DEET alternative. The Health Guide believe you should use only the 20% formulation if you will be exposed to malaria.

The Bottom Line

Using DEET is an essential step in preventing insect bites. If you are traveling in an area where insect-borne disease is a real threat, the most prudent repellent choice is one that contains DEET or 20% picaridin. These products are effective and necessary to safeguard your health. Because of their relatively poor efficacy, the Health Guide does not recommend that plant-derived repellents be used when either children or adults are traveling to areas where insect-borne diseases may be found.


Clothing provides a physical barrier to biting insects, provided it is sufficiently thick or tightly woven. For increased protection, especially when there is more intense mosquito activity (e.g., in the evening), you should wear, weather permitting, long-sleeved shirts and trousers. Tucking your pant leg into your socks or boots can prevent both mosquito bites and tick attachment.

Chemically Treated Clothing

Clothing protection is dramatically increased when the fabric is sprayed or impregnated with a chemical that will either repel or directly kill any insect that alights on the fabric. Both DEET and permethrin are used as clothing treatments, but DEET has been largely replaced for this purpose by the more-effective permethrin.


Unlike DEET, which is used primarily on the skin, permethrin is applied to fabric, such as clothing or bed nets. Permethrin, however, is not a repellent—it is a powerful, rapidly acting contact insecticide that knocks down, or kills, insects that come in contact with it. Features of permethrin:

  • Permethrin kills or stuns insects touching treated fabric.
  • Permethrin adheres tightly to fabric and will last through multiple washings. It will not harm or stain fabric, even silk.
  • Unlike DEET, permethrin will not soften plastic or synthetic materials.
  • It is effective against mosquitoes, ticks, flies, and chiggers. Permethrin is more effective against ticks than DEET.
  • Permethrin is biodegradable and does not accumulate in the environment. However, it should not be disposed of in ways that will harm marine life.

Permethrin is a synthetic chemical analog of the naturally occurring insecticide pyrethrum that is found in chrysanthemums. It acts as a neurotoxin. The stunning and direct killing effects of permethrin are caused by its blockage of sodium transport in insect nerve fibers. Although highly toxic to insects, permethrin is not hazardous to mammals; skin absorption of the chemical is extremely low, and any absorbed permethrin is rapidly metabolized. To date, no cases of human toxicity, carcinogenicity, or mutagenicity have been reported. In fact, products such as 5% ELIMITE anti-scabies cream contain permethrin, and are safe enough to be applied overnight directly to the scalp and skin for medical purposes.

DEET and Permethrin—The Ideal Combination?

The best way to avoid insect bites—and the diseases that insects transmit—is to apply a DEET repellent to your exposed skin and treat your clothing with permethrin. The effectiveness of this combination is confirmed by many studies. In one study, conducted in Alaska, the use of permethrin-treated clothing and 33% DEET afforded 99.9% protection. In comparison, untreated control subjects sustained more than 1,000 bites per hour!

Preventing Tick Bites

The measures described are also dramatically effective against ticks. Protection against tick attachment is 100% possible when using the combination of DEET and permethrin. Tip: also wear a permethrin-treated bandana and treat your socks. This deters ticks from attaching about the head and neck and crawling up your legs from your ankles. If you are not wearing long sleeves and long pants, you need to rely more on a DEET skin repellent and vigilant self-inspection for ticks attached to your skin, especially about the legs, thighs, neck, and waist—the regions where ticks tend to migrate.

Treating Mosquito Bites

Topical corticosteroid creams and ointments can reduce redness, itching, and swelling. Avoid any topical skin preparation containing an anesthetic. Both lidocaine and benzocaine can cause localized and generalized allergic reactions. Oral antihistamines, however, can be effective in reducing the symptoms of mosquito bites. An ammonium solution (AfterBite) applied to the bite can help relieve itching.

Bed Nets

Herodotus mentioned bed nets in the 5th century b.c. when he described how fishermen were protected by nets that were naturally impregnated with fish oil. Mosquito nets now play an important role in preventing malaria and other insect-borne illnesses such as leishmaniasis, filariasis, and encephalitis. Nets are less important in preventing dengue because the Aedes mosquitoes that transmit it are daytime biters.

Treatment of nets with insecticidal or repellent compounds started in the 1930s in the USSR, using lysol, and in the American and German armies during World War II, using DDT. In 1973, photostable insecticidal pyrethroids, developed as molecular analogs of the natural plant insecticide pyrethrum, were synthesized and found to be highly effective against mosquitoes when applied to fabric. In 1984, field trials of permethrin- impregnated nets were first carried out. These and subsequent trials demonstrated that (1) permethrin binds tightly to nylon, polyester, and cotton; (2) insecticidal fabric levels can be maintained for 6 to 12 months; (3) permethrin-treated nets kill insects that land on it; and (4) treated nets reduce mosquito counts in dwellings. Studies from many countries show that malaria rates are reduced in communities where permethrin-treated nets are used. Many tropical countries now have public health programs that supply permethrin-impregnated nets to villages in malaria-endemic areas.

The use of bed nets and other personal protection measures against bites is increasing among travelers as more are being exposed to multidrug-resistant malaria, for which no prophylactic drug regimen is 100% effective. The prevention of mosquito bites, in fact, is the best defense against malaria and other insect-transmitted illnesses. Bed nets, however, have certain problems. They may not be well fitted and can be torn, allowing insects to enter. This problem can be overcome by using a permethrin-treated net. Insects always stop first on the net before going through an opening. Permethrin kills or knocks down these insects before they have a chance to enter through a hole or feed through the net on a body part touching it. Lack of adequate ventilation in a hot climate can also be a problem, more so with tightly-woven nets. A tightly woven net might have 300 holes per square inch versus a bed net with a mesh size of with 156 holes per square inch. A tighter mesh bed net, however, will keep out tiny sand flies.

Types of Nets

Bed nets come in various shapes and sizes. Conical nets hang from a single attachment point in the ceiling; rectangular nets are slightly more spacious, but require an attachment at each corner. The type of mosquito net you use depends on several factors. If you are traveling solo from location to location, the net should be compact, light, and easy to set up. If there are two of you in a fixed location for an extended period of time, then a larger model is preferable, even though it may be heavier and take more time to install. Some types of bed net used by travelers are

  • The Spider—This popular conical net (Fig. 8.4) has a large mesh (156 holes per square inch) for better ventilation, can cover a king-sized bed, but is compact and lightweight (17 oz.).

Products shown in this chapter are available through Travel Medicine, Inc., 800-TRAVMED.

  • SleepScreen—This free-standing net sets up with easy-to-assemble fiberglass poles and is carried in a small stuff sack that fits easily into a pack or carry-on luggage. The small mesh size also keeps out sand flies and no-see-ums. Larger models have nylon floors. This type of net can be set up indoors over a bed or used outdoors.

Insect Proofing Your Sleeping Quarters

According to a large European survey of tourists who have visited east Africa, sleeping in air-conditioned rooms significantly reduces the incidence of malaria. Those travelers not staying in well-screened or air-conditioned rooms should spray living and sleeping quarters in the evening with a pyrethroid-containing insecticide. Brands such as RAID Flying Insect Spray and Green Thumb Flying Insect Killer are commonly available in the United States and Canada but aerosols such as these can’t be transported by air so you will have to find their local equivalent.

In East Africa, Doom Insect Spray, containing permethrin and pyrethrum, is widely used; similar preparations are available in many other countries. One spraying should last several days because of the residual effect of the insecticide. Note: When spraying indoors, vacate the sprayed areas until the product “settles.” Don’t spray on food or eating surfaces.”