Insect Bites

Insect Bites: Protect Yourself with the Right Repellent

There are many types of insects that you need to protect yourself from. They are:

  • Mosquitoes. They 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, malaria, and West Nile Virus. Nearly 2500 cases of West Nile virus infection were reported to the US Centers for Disease Control and Prevention (CDC) in 2004.
  • Ticks. They can transmit Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, babesiosis, tularemia, and tick paralysis.
  • Flies. They are the vectors responsible for transmitting African trypanosomiasis, leishmaniasis, onchocerciasis, and loiasis to humans.
  • Fleas. Fleabites may transmit plague.

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. 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. 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.

Types of Insect Repellent

Marketed products include OFF!, Cutter, Repel, Sawyer, Ben’s (all in multiple formulations), and Ultrathon. Registered for use by the general public since 1957, N, N-diethyl-3-methylbenzamide (previously called N, N-diethyl-m-toluamide), or DEET, remains the criterion standard of currently available insect repellents. DEET, a broad-spectrum repellent, is effective against many species of crawling and flying insects, including mosquitoes, biting flies, midges, chiggers, fleas, and ticks.

The 3M Company manufactures a polymer-based 34% DEET cream, called Ultrathon, which is the standard issue repellent given to the US military. When tested under multiple different environmental and climatic field conditions, Ultrathon was as effective as 75% DEET, providing up to 12 hours of greater than 95% protection against mosquito bites. A 25% Ultrathon aerosol spray is also now available.

Sawyer Products makes a controlled-release 20% DEET lotion, which traps the chemical in a protein particle that slowly releases it to the skin surface. This formulation provides a repellency equivalent to a standard 50% DEET preparation, lasting about 5 hours. Studies show that this formulation can reduce the penetration of DEET through the skin.

How to choose a DEET-based repellent
As a general rule, higher concentrations of DEET provide longer-lasting protection. Mathematical models of the effectiveness and persistence of repellents show that the protection is proportional to concentration of the product. This curve tends to form a plateau at higher repellent concentrations, providing relatively less additional protection for each incremental dose of DEET greater than 50%. Therefore, for casual use, the highest-available concentrations of DEET are not needed. Products with 5-35% DEET provide adequate protection under nearly all conditions. The American Academy of Pediatrics currently recommends that children older than 2 months can safely use DEET up to 30% concentration.

Higher DEET concentrations are best reserved for circumstances in which the wearer will be in an environment with a high density of insects (eg, a rain forest); when reapplication of repellent may be difficult; when traveling to an area where the risk of disease transmission from insect bites is high; and in circumstances where a rapid loss of repellent from the skin surface may occur, such as under conditions of high temperature, humidity, or rain.

Guidelines for safe and effective use of DEET insect repellents
For most uses, choose a repellent with 5-35% DEET. Over the last 50 years, fewer than 50 significant cases of DEET toxicity have been reported in the medical literature. In most of these cases, the associated signs and symptoms resolved without sequelae. Most of these cases had long-term, excessive, or inappropriate use of DEET repellents. In addition, the details of exposure were poorly documented, making causal relationships difficult to establish.

In the cases where toxicity was reported, no correlation was found between adverse effects and age, gender, or concentration of the DEET product used. Of the 6 reported deaths, 3 resulted from the deliberate ingestion of DEET repellents.

According to the EPA guidelines, thoughtful product selection and careful application of the repellent greatly reduce the risk of adverse effects. Judicious use of low-concentration DEET products is most appropriate when applying the repellents to children’s skin.

IR3535 (3-[N-butyl-N-acetyl]-aminopropionic acid) is a chemical repellent that has been available in Europe for 20 years and has been sold in the United States since 1999. This repellent is currently available exclusively through the Avon Corporation as Skin-So-Soft Bug Guard Plus IR3535, at concentrations of 7.5-15%.

IR3535 is structurally similar to the amino acid alanine, and the EPA classified it as a biopesticide. It is labeled for use against mosquitoes, ticks, and biting flies.

Data submitted by the manufacturer to the EPA revealed a protection time against mosquitoes of 2.7-4 hours and protection against ticks for as long as 4 hours. However, when tested by the United States Department of Agriculture (USDA) Laboratories in the late 1970s, IR3535 provided only 6-75 minutes of complete protection against mosquito bites, and 25% IR3535 was 1/8-1/100 as effective as 25% DEET when tested against mosquitoes.

Piperidine (Picaridin)
Piperidine-based repellents have been sold in Europe for several years, and they were introduced to the US market in 2005. Derived from pepper, this repellent is labeled for use against ticks, mosquitoes, and flies. The manufacturer claims DEET-like effectiveness against mosquitoes, lasting 2-8 hours, depending on the species, and concentration of active ingredient used. The limited data available on this product suggest that it is cosmetically pleasant and has low potential for toxicity. As of the spring of 2005, this repellent is sold exclusively as Cutter Advanced Insect Repellent, containing 7% Picaridin. In April 2005, the CDC added picaridin to its approved list of insect repellents, joining DEET, as well as a botanical repellent, p-menthane, 3,8-diol (see below).

Skin-So-Soft bath oil
Avon Corporation’s Skin-So-Soft bath oil received considerable media attention several years ago when some consumers reported it to be effective as a mosquito repellent.

Studies have shown that Skin-So-Soft bath oil has a minimal repellent effect, and it is at least 10 times less effective than 12.5% DEET. The limited mosquito repellent effect of Skin-So-Soft oil may be due to its fragrance or to other components of its formulation, which may possess some repellent activity. The product’s manufacturer has never marketed the bath oil as an insect repellent.

Marketed products include Natrapel, Buzz Away, Herbal Armor, and Green Ban. Oil of citronella is the plant-derived active ingredient found in many natural or herbal insect repellents marketed in the United States. Oil of citronella has a lemony scent and is extracted from the grass plants Cymbopogon nardus and Cymbopogon winterianus. It has been registered for use in the United States since 1948.

Conflicting data exist on the effectiveness of citronella-based products. This data variation is attributed to differences in study methodology, location, and species of the biting insects tested. One comparative laboratory study demonstrated that marketed citronella-based insect repellents protected against mosquito bites for an average of less than 20 minutes. In general, citronella-based repellents provide considerably shorter protection times than DEET repellents; therefore, they require more frequent reapplication to maintain their effectiveness.

In 1997, the EPA concluded that citronella-based insect repellents must carry the following statement on their labels: “For maximum repellent effectiveness of this product, repeat applications at one hour interval.”

Citronella products are not intended to be used as tick repellents.

Soybean oil
The marketed product is called Blocker. Released in the United States in 1997, this natural repellent combines soybean oil, geranium oil, and coconut oil in its formulation. This product has been available in Europe for several years.

In tests, this repellent usually provides longer-lasting protection than citronella-based repellents. In some studies, Blocker provided complete protection against mosquito bites for as long as 3.5 hours, and against blackflies for as long as 10 hours. The product is not labeled for use against ticks.

Marketed products include Repel Oil of Eucalyptus Repellent, OFF! Botanical, and Travel Medicine’s FiteBite. 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 has shown promise as an effective natural repellent. This repellent has been popular in China since 1978, is currently available in Europe, and 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.

UPDATE: According to Genevieve Faherty, commercial consultant for Citrefine (email communication 3/25/09):

The “PMD” registered for use in Off! Botanicals is synthetic, while Citriodiol (a mixture of p-menthane-3,8-diol and other constitutents) is made up entirely of constituents found in the oil of the eucalyptus citriodora tree leaves. Citriodiol is proven to be significantly more efficacious than SC Johnson’s synthetic PMD, plus it is the only “natural” repellent approved by the EPA and recommended by the CDC.