Mosquitoes vector more diseases to humans than any other blood-feeding arthropod. They transmit malaria to 300 to 500 million people each year, resulting in as many as 3 million deaths per year.¹⁴⁸ In addition, about 30,000 international travelers visiting malaria-endemic countries contract the disease yearly.⁷⁴ Mosquitoes vector multiple arboviruses to humans, including several forms of encephalitis, epidemic polyarthritis, yellow fever, and dengue fever (see Chapter 48). Mosquitoes also transmit the larval form of the nematode that causes lymphatic filariasis.

Only female mosquitoes bite, requiring a blood-protein meal for egg production. Male mosquitoes feed solely on plant juices and flower nectar. Mosquitoes feed every 3 to 4 days, consuming up to their own weight in blood with each feeding. Certain species of mosquitoes prefer to feed at twilight or nighttime; others (such as the now-widespread Asian tiger mosquito, Aedes albopictus) bite mostly during the day. Some mosquito species are zoophilic (preferring to feed on animals, including birds, reptiles, various mammals, and amphibians), whereas others are anthropophilic (preferring human blood). Members of the genera Anopheles, Culex, and Aedes are the most common biters of humans. In some mosquitoes, seasonal switching of hosts provides a mechanism for transmitting disease from animal to human.

Mosquitoes rely on visual, thermal, and olfactory stimuli to help them locate a blood meal.* For mosquitoes that feed during the daytime, host movement and dark-colored clothing may initiate 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 are the best-studied attractants. Carbon dioxide serves as a long-range attractant, luring mosquitoes at distances of up to 36 m (118.1 feet).^56,^57,¹⁵²At close range, skin warmth and moisture serve as attractants.^15,²⁹ Volatile compounds, derived from sebum, eccrine and apocrine sweat, and/or the cutaneous microflora bacterial action on these secretions, may also act as chemoattractants.^65,^87,^103,^143,¹⁷⁰ Floral fragrances found in perfumes, lotions, soaps, and hair-care products can also lure mosquitoes.⁴⁸ One study has shown that alcohol ingestion increases the likelihood of being bitten by mosquitoes.¹⁴⁹ Mosquitoes are more attracted to individuals infected with transmittable malaria than to uninfected people or to treated individuals who are no longer infected.⁸⁹

There can be significant variability in the attractiveness of different individuals to the same or different species of mosquitoes ^22,^32,⁸¹—a point that travelers should keep in mind when visiting new areas. In some studies, men have been bitten more than women, and adults more than children.^81,¹¹³ Other studies have shown that repellents based on N,N-diethyl-3-methylbenzamide (DEET) (previously called N,N-diethyl-m-toluamide) may protect women more poorly than they do men.⁶⁰ Heavyset people are more likely to attract mosquitoes, perhaps due to their greater relative heat or carbon dioxide output.¹²⁸ Of note is that some studies have failed to confirm these findings.²²

Mosquito bites commonly produce small local wheal reactions and associated itching. More dramatic reactions, including generalized urticaria, angioedema, and anaphylaxis have been reported in highly sensitive individuals. Rare “skeeter syndrome” has been documented in five young children who had dramatic localized redness, swelling, warmth and induration develop within hours of a mosquito bite. This entity mimics cellulitis but resolves spontaneously and is associated with significantly elevated serum levels of IgE and IgG to mosquito salivary gland antigens.¹⁵¹ Antihistamines may be useful when taken prophylactically to reduce the intensity of mosquito bite reactions: Compared with placebo, levocetirizine was shown in one study to reduce the size of the 24-hour bite papules by 71% and the accompanying pruritus by 56%.⁷⁵

During the day, many mosquito species tend to rest in cool, dark areas, such as on dense vegetation, or in hollow tree stumps, animal burrows, and caves. To complete their life cycle, mosquitoes also require standing water, which may be found in tree holes, woodland pools, marshes, drainage ditches, or puddles. To minimize the chances of being bitten by mosquitoes, campsites should ideally be situated as far away from these sites as possible.

Blackflies (Family Simuliidae)

At 2 to 5 mm (0.1 to 0.2 inch) in length, blackflies ^20,^39,^59,^79,¹⁰⁴ (Figure 47-1, B) are smaller than mosquitoes. They have short antennae, stout humpbacked bodies, and broad wings. Blackflies are found globally wherever there are fast-running, clear rivers or streams, which they require for larval development. In temperate regions, adults are most prevalent in late spring and early summer and are most likely encountered near larval habitats, where they are difficult to avoid. However, unlike most mosquitoes, blackflies tend to bite during the daytime. They primarily use visual cues to locate a host. Dark moving objects are particularly attractive, but carbon dioxide and body warmth also serve as attractants. Only the female bites, taking up to 5 minutes to feed. Blackflies may be present at high densities, inflicting numerous bites on their victims.

Blackflies are particularly attracted to the eyes, nostrils, and ears of their hosts. They often crawl under clothing or into the hair to feed. The insect’s mouthparts tear the skin surface, producing a pool of blood from which the fly feeds. Blood loss from the bite site often continues after the blackfly has departed. The resulting intensely pruritic, painful, and edematous papules are typically slow to heal. Rare systemic reactions, including fever, urticaria, anaphylaxis, and even death have been reported following blackfly bites. Although these flies are not known to transmit disease to humans in North America, in the tropics, blackflies are vectors of the parasite Onchocerca volvulus, which causes the disease river blindness.

Biting Midges (Family Ceratopogonidae)

Also known as no-see-ums, sand gnats, sand fleas, and sand flies, biting midges ^20,^39,^59,^79,¹⁰⁴ are small, slender flies (<2 mm [0.1 inch] body length) with narrow wings (Figure 47-1, C). Their small size makes them difficult to see, and they can pass readily through common window screens. Biting midges occur from low to high latitudes worldwide. They breed most commonly in salt marshes, but larvae also develop in moist organic matter associated with freshwater wetlands and irrigated pastures. Despite their inconspicuous size, female midges are aggressive biters, frequently attacking in swarms and inflicting multiple painful and pruritic bites within minutes. Midges often crawl into the hair before biting. Depending on the species, midges may bite during the day or at nighttime. Their activity is greatest during calm weather, declining as wind speed increases. They may be avoided by moving to open areas with greater airflow or into shelters with no-see-um netting. Biting midges are not known to transmit diseases to humans in temperate regions but vector filarial and microbial parasites in tropical regions.

Tabanids (Family Tabanidae)

The family Tabanidae (Figure 47-1, D) includes horseflies, deerflies, greenheads, and yellow flies.^20,^39,^59,^79,¹⁰⁴ These insects are relatively large (10 to 20 mm [0.4 to 0.8 inch]) robust fliers, with numerous species worldwide. Tabanids breed in aquatic or semiaquatic environments, with a life cycle of over 1 year. They are able to fly for miles and rely primarily on vision to locate a host by movement. Dark-colored clothing may increase the likelihood of being bitten. These flies are most active on warm, overcast days. Only the females bite, using scissor-like mouthparts to create within the skin a bleeding slash, which is slow to heal. Despite their size, these flies usually bite painlessly, but the resulting reaction can include intense itching, secondary infection, and, rarely, systemic reactions, such as urticaria or anaphylaxis. Because the adult fly usually lives only about 1 month, and only one generation emerges per year, the potential season for being bitten is relatively short in higher latitudes and altitudes. In the United States, deerflies have been shown to be capable of transmitting tularemia to humans; in Africa, the deerfly may vector the filarial parasitic worm, Loa loa.

Sand Flies (Family Psychodidae)

Sand flies * are tiny (2 to 3 mm [0.1 inch]), hairy, and long-legged flies, with multisegmented antennae and a characteristic V-shape to the wings when at rest. (Figure 47-1, E ). Only female sand flies are blood feeders, feeding mostly during calm, windless nights and resting during the day in animal burrows, tree holes, or caves, which should be avoided. Larvae develop in moist organic matter within such habitats. Most sand fly bites occur on the face and neck, but all exposed skin may be attacked.

In tropical and subtropical climates, sand flies have been shown to vector multiple cutaneous, mucocutaneous, and systemic diseases, including bartonellosis and three forms of leishmaniasis. Visceral leishmaniasis is endemic in the tropical and subtropical regions of Central and South America, Africa, Asia, the Mediterranean, and Southern Europe. The only sand fly–transmitted disease in the United States has been cutaneous leishmaniasis, reported in Texas and Oklahoma.

Tsetse Flies (Family Glossinidae)

Tsetse flies ^20,^39,^59,^79,¹⁰⁴ are found only in tropical Africa. They are 7 to 14 mm (0.3 to 0.6 inch) long, yellowish-brown, with wings that fold over their backs, giving them the appearance of honeybees at rest (Figure 47-1, F ). Both sexes bite, feeding in daytime on a wide variety of mammals, including humans. Light-colored, thickly-woven, loose-fitting clothing may deter biting. Tsetse flies seem to rely primarily on vision and movement to identify their hosts. Their bites are painful and may cause petechiae or pruritic wheals. Tsetse flies vector African trypanosomiasis (sleeping sickness).

Stable Flies (Family Muscidae)

Stable flies ^59,⁷⁹ resemble common houseflies, and are most often encountered in coastal areas. Unlike a housefly, which rests with its body parallel to the surface, a stable fly rests with its head held higher than its posterior (Figure 47-1, G). Both male and female stable flies are vicious daytime biters, requiring a blood meal every 48 hours in order to survive. If disturbed, they will attempt to feed multiple times, preferring to bite the lower extremities. Horses and cattle are the preferred hosts, but hungry stable flies readily bite humans. These flies have knife-like mouthparts that they use to puncture flesh before pumping up the blood. Stable flies breed in decaying vegetation (e.g., wet hay bales, beach seaweed) and herbivore manure and are frequently found congregated on sunny walls. Moving away from these breeding habitats will reduce the likelihood of being bitten. Stable fly bites are generally self-limited. They are not known to transmit disease to humans.

Kissing Bugs (Family Reduviidae)

Kissing or Chagas bugs * (triatomine or assassin bugs) are large (10 to 30 mm [0.4 to 1.2 inches] adult length) insects with cone-shaped heads, overlapping wings, and an alternating pattern of dull orange and dark brown stripes on the lateral abdomen (Figure 47-1, H ). Kissing bugs get their name from a tendency to bite around the human mouth, but they also bite other parts of the body. Both male and female kissing bugs bite, requiring a blood meal in order to mature through each of five nymphal growth stages. They are nocturnal feeders, attracted to their hosts by warmth, carbon dioxide, and odor. During the day, they rest in trees or indoors in crevices of house walls and ceilings. Kissing bug bites are initially painless, but frequent exposure to the bites can produce erythema, edema, and pruritus at the bite sites. Kissing bugs are the vector for Trypanosoma cruzi, the causative agent of Chagas’ disease, which has been reported in Central and South America, as well as in the southwestern United States. Chagas’ disease symptoms are more severe with long-term repeated inoculations and are also more harmful in South America than farther north. Kissing bugs often feed on rodents that may live within thatch roofing material. Avoiding sleeping under thatch roofing in Chagas-endemic areas may reduce the probability of infection.

Fleas (Family Pulicidae)

Adult fleas ^20,^39,^59,^79,¹⁰⁴ are small (2 to 6 mm [0.1 to 0.2 inch]), wingless insects with powerful legs that enable adults to jump distances of up to 30 cm (11.9 inches) (Figure 47-1, I ). Hungry adult fleas of both sexes feed on humans and other warm-blooded mammals and birds. Different flea species are associated with specific hosts, and they may be especially abundant around rodent and bird nests, which serve as both adult and larval habitats and should be avoided. Fleas usually move actively on the host, probing and biting several times, resulting in grouped lesions of pruritic papules. Fleas are capable of transmitting sylvatic plague and murine typhus.

Chigger Mites (Family Trombiculidae)

Trombiculid mites ^20,^39,^59,^79,¹⁰⁴ (Figure 47-1, J ) may be found worldwide. Commonly known as chiggers, redbugs, or harvest mites, these reddish-yellow insects are readily encountered in damp, grassy, and wooded areas, especially along the margins of forests, where they may number in the thousands. Only the tiny (less than 0.2 mm [0.1 inch]) larval stages are parasitic, feeding on mammals, birds, reptiles, and amphibians. Chiggers are most active in the summer and early autumn. They usually infest humans by crawling up the shoes and legs, preferring to attach to skin at areas where the clothing fits tightly, such as at the tops of socks or around the elastic edges of underwear. Chiggers do not burrow into the skin or actively suck blood. Rather, they pierce skin with their mouthparts and secrete a proteolytic salivary fluid that dissolves host tissue, which they in turn suck up. If undisturbed, chiggers may feed for several days, before dropping off. In humans, this rarely occurs, because the larvae usually cause enough irritation that they are dislodged by scratching. The host response to chigger bites is brisk, often leading to intensely pruritic, bright-red 1- to 2-cm (0.4- to 0.8-inch) nodules.

Chiggers are especially abundant in warm, moist temperate climates (e.g., the southeastern United States) and tropical areas of high mammal density, including livestock pastures and natural parks and reserves, where moist grassy or understory areas can be found. These areas may seem like attractive camping areas but should be avoided. In Asia, chiggers may serve as vectors of scrub typhus. Rickettsialpox is also transmitted by a mite bite.

Ticks (Families Ixodidae and Argasidae)

(See Chapter 51.)

Ticks,* like mites, are arachnids rather than insects. There are hard ticks (family Ixodidae) and soft ticks (family Argasidae) (Figure 47-1, K and L). Hard ticks are so named because of the presence of a sclerotized plate, or scutum, that covers part of the body. Both types of ticks may be found worldwide, but hard ticks are more commonly encountered in North America. Hard ticks are usually found in weedy or shrubby areas, along trails, and at forest boundaries, where mammalian hosts, such as deer, are plentiful. Soft ticks are more resistant to desiccation than are hard ticks. Soft ticks thrive in hot and dry climates and are commonly found in animal burrows or caves.

After hatching, both genders of ticks pass through three developmental stages—larval, nymph, and adult—all of which blood-feed. Ticks are unable to fly or jump. Soft ticks are nocturnal and feed rapidly, in just a few minutes. Hard ticks most commonly “quest” for hosts during the day, often climbing vegetation and waiting still for hours or days, forelegs outstretched, until they detect the vibration or carbon dioxide plume of a passing host. When they encounter fur or skin, they climb onto the host and then crawl in search of an appropriate location on which to attach and feed. The attachment bite is usually painless. Hard ticks may remain and feed on a single host for days.

People in suspected tick habitats should check clothing frequently for the presence of ticks. If multiple ticks are seen on clothing, they are most easily removed by permanently trapping them on a piece of cellophane duct tape or by rolling a sticky tape type of lint remover across them; hundreds of small ticks can be easily removed by this method. Tape can be stuck onto the inner thigh area of pants for storage between episodes of tick removal. Laundering infested garments cannot be relied on to kill nymphs, unless the clothing is subjected to the hot cycle of the dryer.²¹

Attached ticks are more difficult to remove. Tick mouthparts are barbed, and some species of tick also secrete a cement that firmly anchors the tick into the skin. Erythema, pruritus, and edema are commonly seen at the site of a tick bite. Improper partial removal of the mouthparts may initiate a long-lasting foreign body reaction, leading to secondarily infected lesions that are slow to heal, or granuloma formation that may persist for months. (For a discussion of the best method for tick removal, see Chapter 51.)

After the tick is removed, the bite site should be cleansed with soap and water, or an antiseptic, and hands should be washed. It may be prudent to save the tick, in case later identification becomes necessary. Laboratory studies of Borrelia burgdorferi (Lyme disease)–infected ticks show that duration of attachment is directly correlated with the risk for transmission of the spirochete.* Prompt removal of attached ticks (ideally within 24 hours of attachment) will greatly reduce the likelihood of disease transmission.¹²⁵

In the United States, soft ticks of the single genus Ornithodoros are capable of transmitting to humans the Borrelia spirochete that causes relapsing fever. Three genera of hard ticks transmit disease to man: Ixodes (which vectors Lyme disease, babesiosis, tick paralysis, and Russian spring–summer encephalitis), Dermacentor (vectors tularemia, Rocky Mountain spotted fever, ehrlichiosis, Colorado tick fever, and tick paralysis), and Amblyomma (vectors tularemia, ehrlichiosis, and tick paralysis).^107,¹⁵⁵ Larval, nymph, and adult ticks may all transmit disease during feeding. Transovarial transmission also enables female ticks to directly infect their offspring.

Personal Protection

Personal protection against bites may be achieved in three ways: avoiding infested habitats, using protective clothing and/or shelters, and applying insect repellents.

Habitat Avoidance

As discussed in the sections above, knowing where to find biting arthropods’ breeding and resting places can help reduce exposure and the chance of being bitten. Mosquitoes and other nocturnal bloodsuckers are particularly active at dusk, making this a good time to be indoors. To avoid the usual resting places of biting arthropods, campsites should ideally be situated in areas that are high, dry, and open, devoid of rodent burrows and nests, and as free from vegetation as possible. Areas with standing or stagnant water should be avoided, because these are ideal breeding grounds for mosquitoes. Attempts should be made to avoid unnecessary use of lights, which attract multiple insects.

Physical Protection

Physical barriers can be extremely effective in preventing insect bites, by blocking arthropods’ access to the skin. Long-sleeved shirts, socks, long pants, and a hat will protect all but the face, neck, and hands. Tucking pants into the socks or boots makes it much more difficult for ticks or chigger mites to gain access to the skin. Rubber boots are commonly worn, particularly in tropical areas, to reduce chigger and tick contact while walking and hiking. Light-colored clothing is preferable, because it makes it easier to spot ticks and is less attractive to mosquitoes and biting flies. Ticks will find it more difficult to cling to smooth, closely woven fabrics (e.g., nylon).¹⁴⁶ Loose-fitting clothing, made out of tightly woven fabric, with a tucked-in T-shirt undergarment is particularly effective at reducing bites on the upper body. A light-colored, full-brimmed hat will protect the head and neck. Deerflies tend to land on the hat instead of the head; blackflies and biting midges are less likely to crawl to the shaded skin beneath a hat brim.

Mesh overgarments, or garments made of tightly woven material, are available to protect against insect bites. Head nets, hooded jackets, pants, and mittens are available from a number of manufacturers, in a wide range of sizes and styles (Box 47-2). Mesh garments are usually made of either polyester or nylon and, depending on the manufacturer, are available in either white or dark colors. With a mesh size of less than 0.3 mm (0.01 inch), many of these garments are woven tightly enough to exclude even biting midges and immature ticks. As with any clothing, bending or crouching may still pull the garments close enough to the skin surface to enable insects to bite through. Shannon Outdoors addresses this potential problem with a double-layered mesh that reportedly prevents mosquito penetration. Similarly, Outdoor Research manufactures head nets with a lightweight spring-steel hoop positioned to prevent the netting from collapsing against the face. Although mesh garments are effective barriers against insects, some people may find them uncomfortable during vigorous activity or in hot weather.

BOX 47-2 Manufacturers of Protective Clothing, Protective Shelters, and Insect Nets

Lightweight insect nets and mesh shelters are available to protect travelers, sleeping indoors or in the wilderness (Figure 47-2). The effectiveness of insect nets or shelters may be enhanced by treating them with a permethrin-based contact insecticide (discussed below), which can provide weeks of efficacy following a single application.

FIGURE 47-2 A to C, Protective shelters. D, Bed net.

Repellents

For many people, applying a topical insect repellent may be the most effective and easiest way to prevent arthropod bites. The quest to develop the “perfect” insect repellent has been an ongoing scientific goal for years and has yet to be achieved. The ideal agent would repel multiple species of biting arthropods, remain effective for at least 8 hours, cause no irritation to skin or mucous membranes, possess no systemic toxicity, be resistant to abrasion and washing off, and be greaseless and odorless. No presently available insect repellent meets all of these criteria. Efforts to find such a compound have been hampered by the multiplicity of variables that affect the inherent repellency of any chemical. Repellents do not all share a single mode of action, and different species of insects may react differently to the same repellent.¹³³

Many chemicals that are effective repellents evaporate or absorb into the skin too quickly to be of great usefulness. To be effective as an insect repellent, a chemical must be volatile enough to maintain an effective repellent vapor concentration at the skin surface but not evaporate so rapidly that it quickly loses its effectiveness. Multiple factors play a role in effectiveness, including concentration, frequency and uniformity of application, the user’s activity level and overall attractiveness to blood-sucking arthropods, interaction between the individual user and the repellent formulation, and the number and species of the organisms trying to bite. The effectiveness of any repellent is reduced by abrasion from clothing; evaporation and absorption from the skin surface; being washed off by sweat, rain, or water; physical activity; and a windy environment.* Each 10° C (18° F) increase in ambient temperature can lead to as much as 50% reduction in protection time.⁸³ Recent formulation developments improve the duration of effectiveness and the cosmetic properties of repellent active ingredients, such that a well-formulated mediocre active ingredient may outperform a poorly formulated stronger active ingredient. Insect repellents do not “cloak” the user in a chemical veil of protection; any untreated exposed skin can be readily bitten.¹⁰² Individuals not wearing insect repellent may be bitten with increased frequency when standing next to someone who is wearing insect repellent.¹¹¹

Chemical Repellents

DEET

DEET has been the gold standard of insect repellents for many decades. Only in the past 5 years have other repellents come to market that show comparable broad-spectrum efficacy (discussed later). DEET has been registered for use by the general public since 1957. It is effective against many species of crawling and flying insects, including mosquitoes, biting flies, midges, chiggers, fleas, and ticks. The U.S. Environmental Protection Agency (EPA) estimates that about 30% of the U.S. population uses a DEET-based product every year; worldwide use exceeds 200 million people annually.^167,¹⁶⁸ Decades of empirical testing of more than 20,000 other compounds has not yet led to the release of a clearly superior repellent.* The mechanism of action of DEET is only partially understood. One recent study demonstrated that DEET blocks a specific olfactory sensory neuron in mosquitoes, effectively masking odors that would normally attract the mosquito to a host.⁴⁰ Another study, in contrast, found that a single olfactory neuron in mosquitoes responds to DEET and induces avoidance behaviors.¹⁵⁹

DEET may be applied directly to skin, clothing, mesh insect nets or shelters, window screens, tents, or sleeping bags. Care should be taken to avoid inadvertent contact with plastics (such as watch crystals, glasses frames or lenses, other optics, equipment, and fishing line), rayon, spandex, leather, or painted and varnished surfaces, because DEET may damage these. DEET does not damage natural fibers like wool and cotton.

In the United States, DEET is sold in concentrations from 5% to 100%, in various formulations, including lotions, solutions, sprays, and impregnated wipes (Table 47-1). As a general rule, higher concentrations of DEET will provide longer-lasting protection. Duration of efficacy of DEET increases only slowly above 35% concentrations, and the longest-lasting (extended duration) formulations all use concentrations no higher than 35%. Higher-concentration and extended-duration DEET formulations are probably best reserved for circumstances in which the wearer will be in an environment with a very high density of insects (e.g., tundra in early summer, or salt marsh), where there is a high risk for disease transmission from insect bites, or under circumstances where there may be rapid loss of repellent from the skin surface, such as under conditions of high temperature and humidity or rain. Under these circumstances, reapplication of the repellent will likely be necessary to maintain its effectiveness.

TABLE 47-1 DEET-Containing Insect Repellents