Arthropods comprise a vast invertebrate phylum that includes arachnids (those with eight legs, such as spiders, mites, and ticks) and insects (those with six legs [subphylum Hexapoda], such as mosquitoes, gnats, and flies). Most arachnid and insect bites are summertime nuisances, but several species' bites are toxic and others are vectors of disease. This article focuses on spider, mite, tick, and insect bites of medical importance.

Spiders

Only two spiders in the United States are considered medically important with regard to bites: the black widow (Latrodectus mactans) and the brown recluse (Loxosceles reclusa). Most spiders are harmless. Many suspected spider bites are actually caused by other conditions, such as methicillin-resistant Staphylococcus aureus infection.¹ In one series of 600 suspected bites, 80% were attributable to other causes, such as pyoderma.² Without a witnessed bite and species identification, other possibilities should be considered^2,3 (Table 1³ ). Most genuine bites are single lesions that require only cleansing, cold packs, and a tetanus vaccination booster.

WIDOW SPIDERS

Widow spiders can be found worldwide. Of the five species in the United States, the three most common are L. mactans in the East, Latrodectus hesperus in the West, and Latrodectus variolus in the North. The female L. mactans is black, usually with a classic red hourglass mark on the ventral abdomen (Figure 1). Introduced brown widow spiders (Latrodectus geometricus) are found in the South, and have an orange or yellow hourglass mark and milder venom. Widow spiders typically reside in yard debris, and their bite, which feels like a pinprick, is associated with recent outdoor activity. Widow venom contains α-latrotoxin, which provokes a massive presynaptic release of acetylcholine. After a person has been bitten, crampy muscle spasms usually occur within an hour in the chest (upper extremity bites) or the abdomen (lower extremity bites). The pain may be intense enough to mimic myocardial infarction or acute abdomen, but it is rarely life threatening and typically resolves within 48 to 72 hours.⁴ Nausea, weakness, local or generalized diaphoresis, and facial edema may occur. Clark's grading system is used to assess bites and determine management (Table 2).⁴ Grade 3 bites, especially in children or older persons, may warrant antivenom and hospital admission to reduce potential complications.

Treatment of widow bites usually involves intravenous benzodiazepines for muscle spasms and narcotics for pain. These have largely replaced intravenous calcium gluconate 10% in clinical practice.⁴ An effective antivenom (L. mactans [equine]) may be considered up to 48 hours after a bite, but should be used only if severe pain persists after standard therapy. One amp of antivenom is added to 100 mL normal saline and slowly infused for 15 minutes (1 mL per minute), with the remainder given over one to two hours.⁵ Serum sickness may be a complication.

BROWN RECLUSE SPIDERS

Brown recluse, or “fiddleback,” spiders are found in the south-central United States (Figure 2) and South America. Brown recluse spiders typically have an inverted violin-shaped marking on their dorsal thorax (Figure 3). They are unusual in that they have three pairs of eyes instead of the traditional four pairs. In contrast to widow bites, recluse bites usually occur indoors because the spiders often hide in piles of clothing or in dark places behind furniture. Although the bite may be minimally painful, a tender erythematous halo soon develops, followed by central necrosis in about 40% of cases and scarring in 13% of cases.⁶ Skin grafting is rarely required.

Venom from brown recluse spiders contains sphingomyelinase D, which is thought to be responsible for neutrophil activation and skin necrosis (necrotic arachnidism). Systemic symptoms are rare in the United States, but persons may develop hemolysis, coagulopathy, and a measles-like toxic erythema rash, which responds to prednisone. A Loxosceles antivenom is manufactured in South America but is unavailable in the United States.

Management of recluse bites is controversial, and there are no published clinical trials to guide therapy. Most experts believe that overly aggressive bite management does more harm than good, and cutting out the bite site to prevent necrosis is contraindicated.³ Good wound care and minor debridement usually suffice. Antihistamines have been suggested as an unproven adjunctive therapy that poses little risk to the patient.⁷ Although controversial, dapsone at a dosage of 50 to 100 mg twice per day for 10 days has been recommended for severe bites based on its theoretical ability to prevent neutrophil degranulation/necrosis.⁸ If this treatment is used, glucose-6-phosphate dehydrogenase (G6PD) testing should be performed because dapsone may cause hemolysis in patients with G6PD deficiency.

Ticks

Ticks are vectors of multiple diseases (Table 3).^9,10 There are two major types of ticks: hard ticks (Ixodidae) have shield-like scutum with the head visible from above, whereas the less medically important soft ticks (Argasidae) have a leathery skin with the head invisible from above. Hard ticks latch onto their hosts and feed for days, whereas soft ticks are localized in animal nests. Hard ticks inhabit brushy fields and tall grass where they await potential hosts.

Tick nymphs and adults feed on different hosts, and humans are usually incidentally involved. In the case of the deer or black-legged tick (Ixodes scapularis; Figure 4), nymphs feed on mice and adults feed on deer. Both stages of ticks can transmit Lyme disease, which manifests as a characteristic bull's-eye rash (i.e., erythema migrans) at the site of an infected bite (Figure 5). Ixodes ticks are also capable of transmitting babesiosis, which is a malaria-like protozoal illness, and human granulocytic anaplasmosis.⁹

Prophylactic treatment for Lyme disease with a single 200-mg dose of doxycycline is recommended only if the following criteria are met: the tick is identifiable as I. scapularis, it has been attached for more than 36 hours (engorged), antibiotics can be administered within 72 hours of removal, the local infection rate of the vector is greater than 20%, and the patient can tolerate doxycycline.¹¹ For an interactive Lyme disease map showing reported cases across the United States from 2001 to 2011, visit http://www.cdc.gov/lyme/stats/maps/interactivemaps.html.

The American dog tick (Dermacentor variabilis) is a common hard tick that may transmit Rocky Mountain spotted fever and tularemia (Figure 6). The Lone Star tick (Amblyomma americanum) of the southeast United States is identifiable by a white spot on its shield (Figure 7). It is a potential vector of human monocytic ehrlichiosis, tularemia, Q fever, and southern tick-associated rash illness, which is a Lyme-like disease that may produce a circular rash, headache, and fever. The infectious agent has not been conclusively identified but generally responds to doxycycline.⁹ Recently, Lone Star tick bites have provoked a persistent allergy to red meat (and cetuximab [Erbitux]) caused by immunoglobulin E antibodies to galactose-alpha-1,3-galactose, better known as alpha-gal. Persons who are bitten develop delayed hives or, in some cases, anaphylaxis after eating beef, pork, or lamb.¹² Tick paralysis, which results from gravid female bites, is a toxin-mediated ascending paralysis that generally resolves after tick removal.

Tick bites can be prevented by spraying clothes with permethrin, a synthetic insecticide. In one study, persons wearing permethrin-treated socks and sneakers received almost 75% fewer bites than persons wearing untreated items.¹³ Applying diethyltoluamide (DEET)-containing repellents to exposed skin can provide additional benefit. Tucking pants into socks, wearing hats, and performing a daily tick check also reduce exposure.

Ticks should be removed with a pair of forceps (or tick-removal device), grasping the tick as close to the skin as possible. A gentle, steady pull, perpendicular to the skin, without twisting, usually dislodges the tick without leaving mouthparts behind.¹⁴ Applying viscous lidocaine 2% (Xylocaine) is another option, but is often too slow to be satisfactory.¹⁵ Petroleum jelly, alcohol, nail polish remover, and lit matches should not be used because they may cause ticks to regurgitate infectious material into the bite.¹⁴

Mites and Chiggers

Mites are tiny eight-legged arthropods that, in their larval form, cause itchy bites and sometimes transmit diseases, such as scrub typhus (overseas) and rickettsial pox. The six-legged larval form is called the chigger, also known as harvest bug or red bug (Trombicula spp.). The larvae appear on the skin as tiny red specks that crawl until they reach constrictive clothing; thus, bites tend to be located at belt or sock lines. Occasionally, the penis is severely affected, resulting in summer penile syndrome.¹⁶ The resulting welts are significantly larger than the size of the chigger and are caused by an allergic reaction to the saliva. Chiggers do not burrow into the skin.

The same precautions used for ticks are effective for chiggers. Immediate showering is likely to dislodge most of them. Bites can be treated with antihistamines, cold compresses, calamine lotion, or rubbing alcohol. No treatment has been demonstrated to be particularly effective.

Fleas

Fleas (Siphonaptera) are wingless jumping insects that feed on blood. Flea bites are usually from dog or cat fleas (Ctenocephalides), which attack humans when their preferred hosts are unavailable. For example, a common scenario involves a vacationing family returning home to fleas after their pets have been in a kennel. Typical flea bites are pruritic ankle papules, but other sites may be involved after coming in contact with infested areas. The human flea species (Pulex irritans) is usually associated with poor hygiene. Flea bites may transmit plague, tularemia, cat-scratch disease, and cat flea typhus (Rickettsia felis).

Bedbugs

Temperate bedbugs (Cimex lectularius), tropical bedbugs (Cimex hemipterus), and bat bugs (Cimex pilosellus) are oval, flat insects measuring about 5 mm in length. Bedbugs hide inside box springs, under mattresses, and behind headboards where they may survive for up to one year without feeding.¹⁷ Examination of infested bedding typically reveals reddish stains and a characteristic sweet smell sometimes likened to coriander. A previous American Family Physician article provides additional information on identifying and eradicating bedbugs (https://www.aafp.org/afp/2012/1001/p653.html).¹⁸

Diptera

Diptera (two-winged insects) include mosquitoes, midges, gnats, and flies, all of which can act as disease vectors (Table 4).¹⁹ Mosquito bites can spread West Nile virus, certain types of encephalitis, and, more recently, dengue fever, with cases reported in Key West, Fla., Hawaii, and Texas.²⁰ The vector for dengue fever is Aedes aegypti (Figure 8). Most mosquito bites in the United States provoke nothing more than an itchy welt. Typical bites appear as pruritic pink papules, often with a central punctum. Sensitivity varies widely, however, and some persons experience a severe or prolonged reaction. The more extreme cases have been termed skeeter syndrome, which describes an intense hypersensitivity reaction with fever related to allergens in mosquito saliva.²¹ Antihistamines and prednisone are used to treat this condition.

Prevention

Bite prevention strategies include avoiding high-risk sites (e.g., marshes, pools of standing water) and avoiding going outdoors at certain times of day (after dusk); wearing protective light-colored clothing, ideally treated with permethrin; and employing insect screens or bed nets. Bug zappers are ineffective and destroy beneficial insects. The most effective approach is applying insect repellents containing DEET 35% or picaridin 20%. Combination repellent-sunscreen preparations are not recommended because they have differing durations of effectiveness.²² Repellents should be applied after applying sunscreen, because applying DEET first increases skin absorption.²³

DEET is the most effective repellent and is safe if guidelines are followed. Optimal concentrations are 10% to 35%, and a sustained-release 35% preparation (Ultrathon) is effective for up to 12 hours.²⁴ Although higher concentrations may provide longer protection, those greater than 50% should be avoided because of minimal additional benefit and increased skin absorption.^24,25 DEET 10% to 30% may be used in children older than two months if an adult supervises application and if it is not applied to the lips or hands.^25,26 DEET is considered safe during pregnancy after the first trimester.^26,27

Picaridin lacks the oiliness and odor of DEET, and does not damage synthetic fabrics, although it has a shorter duration of action.²⁸ It is not approved for children younger than two years. Most natural repellents are less effective, but the best of these is oil of lemon eucalyptus, which contains the active ingredient p-menthane-3,8-diol.²⁵ Unreliable alternatives that are not recommended include ultrasonic devices, oral thiamine, garlic, and wristbands impregnated with repellents.²⁵

Data Sources: A literature search was performed in PubMed, Cochrane Database of Systematic Reviews, the Centers for Disease Control and Prevention, the Medical Letter, and Essential Evidence Plus. Initial search terms were insect bites, spider bites, bedbugs, ticks, tick-borne diseases, mites, and fleas. Secondary searches were conducted on STARI (southern tick-associated rash illness), Powassan encephalitis, and other vector-borne diseases. References were also identified from UpToDate and other pertinent review articles on these topics. Search dates: August and September 2011, and July 31, 2013.