Arthropods That Bite, Sting, or Infest
Four of nine classes of arthropods cause local or systemic reactions.
Arthropod-Borne Infections
ERYTHEMATOUS MACULES Occur at bite sites and are usually transient.
PAPULAR URTICARIA or urticarial papules persistent for >48 hours (Figs. 28-2 and 28-3); usually < 1 cm; vesicle may form on papule. Large urticarial plaques may occur.
BULLOUS LESIONS Tense bullae with clear fluid on a slightly inflamed base (Fig. 28-4); excoriation results in erosion.
SECONDARY LESIONS Excoriations of urticarial, papular, and vesicular lesions common. Painful erosion may be secondarily infected with Staphylococcus aureus. Excoriated or secondarily infected lesions may heal with hyper- or hypopigmentation and/or raised or depressed scars, especially in more darkly pigmented individuals.
SYSTEMIC FINDINGS May occur associated with toxin or allergy to the substance injected during the bite. Many varied systemic infections can be injected during the bite.
Clinical Variations by Arthropod
MITESSarcoptes scabiei causes infestation scabies (see Scabies). Demodex folliculorum and D. brevis live in human hair follicles and sebaceous glands, causing demodicidosis (see Demodicidosis).
Food, fowl, grain, straw, harvest, and animal mite bites cause papular urticaria.
FOOD MITES Cheese, grain, or mold mites can cause mild contact dermatitis: Baker's or grocer's itch. Straw mites. Bites occur during harvest season causing dermatitis; straw itch. Harvest mite: Chiggers. Bites can cause dermatitis. One species transmits Rickettsia tsutsugamushi, the cause of scrub typhus.
Dermatophagoides species of house dust mites are implicated in the pathogenesis of asthma and atopic dermatitis. Feed on desquamated human skin and other organic detritus, living in bedding, carpets, and furniture. Bodies and excreta may have a role in asthma and other allergies. Affected persons respond with production of IgE antibodies. Fowl mites. Chicken, pigeons, etc. Bites cause papular urticaria on exposed sites. Rat mites cause painful bites and dermatitis and transmit endemic/murine typhus. House mouse mite is the vector for rickettsialpox. Cheyletiella spp. (dog and cat mites) bite pet owners causing pruritic lesions on forearms, chest, and abdomen. Canine sarcoptic mange (S. scabiei var. canis) and feline mange (Notoedres cati) cause a pruritic dermatosis in pet owners.
TICKS Ticks attach and feed painlessly. Secretions can produce local bite reactions (erythema), febrile illness, and paralysis. Blacklegged or Ixodes tick, lone star tick, and dog tick are vectors for diseases. Erythema migrans (see Fig. 25-81), characteristic of primary Lyme disease or borreliosis, occurs at the bite site of an infected Ixodes tick that transmits Borrelia burgdorferi, B. mayonii (Midwestern U.S.).
Lymphocytoma cutis (see Fig. 25-82) also occurs at the site of bite of an infected Ixodes tick.
SPIDERSBrown recluse spider bites can result in mild local urticarial reactions to full-thickness skin necrosis. Associated with a maculopapular exanthem, fever, headache, malaise, arthralgia, and nausea/vomiting. Most lesions diagnosed as brown recluse spider bites are bite reactions to other arthropods. Widow spiders inject a neurotoxin (α-latrotoxin) that produces bite site reactions as well as varying degrees of systemic toxicity.
INSECTSPubic lice, head lice, body lice papular urticaria, excoriations, and secondary infections (see Pediculosis Capitis).
Mosquitoes. Bites usually present as papular urticaria (Fig. 28-3) on exposed sites; reactions can be urticaria, eczematous, or granulomatous.
Black Flies. Anesthetic is injected, resulting in a painless initial bite; may subsequently become painful with itching, erythema, and edema. Black fly fever is characterized by fever, nausea, and generalized lymphadenitis.
Midges. Bites produce immediate pain with erythema at bite site with 2- to 3-mm papule and vesicles, followed by indurated nodules (up to 1 cm) persisting for many months.
Tabanidae or horse flies. Bites painful with papular urticaria; rarely associated with anaphylaxis.
Dermatobia hominis (human botfly) in tropical regions causes furuncular myiasis, painful lesions that resemble pyogenic granuloma or abscess. Female botfly captures a mosquito, attaches its eggs to the mosquito body, and then releases the mosquito. Eggs hatch on the mosquito becoming larvae and are deposited on human skin. Larvae use bite site as portal of entry into skin. A pruritic papule develops at the site, slowly enlarging over several weeks into a domed nodule (resembles a furuncle) with a central pore (Fig. 28-5). Larvae drop out after 8 weeks to pupate in soil.
House Flies. Larvae deposited into any exposed skin site (ear, nose, paranasal sinuses, mouth, eye, anus, and vagina) or at any wound site (leg ulcers, ulcerated squamous and basal cell carcinomas, hematomas, or umbilical stump) and grow into maggots, which can be seen on the surface of wound causing wound myiasis (Fig. 28-6). Maggot debridement therapy is used to selectively debride necrotic wound tissue using certain species of maggot.
Cimex lectularius or bedbugs bite exposed skin (face, neck, arms, or hands) of sleeping humans. Feeding takes 5 to 10 minutes. Grouped papular urticaria (Fig. 28-2) occur at bite sites, usually, but now always, in groups of three ("breakfast, lunch, and dinner"). Bedbug hides in crevices of walls, mattresses, and furniture. Reddish brown streaks may be seen on mattress; bedbugs defecate old blood meal while ingesting a new meal.
Reduviid or kissing bugs bite usually present as papular urticaria; severe reactions can produce necrosis and ulceration. Subfamily of reduviid bugs transmits Trypanosoma cruzi, the agent of Chagas disease.
Fleas. Papular urticaria at bite site. Dog fleas often live in carpeting and bite exposed lower legs. Secondary changes of excoriation, prurigo nodularis, and S. aureus infection occur.
Tunga Penetrans or Chigoe Flea. Papule, nodule, or vesicle (6 to 8 mm in diameter) with a central black dot (tungiasis) produced by posterior part of the flea's abdominal segments. As eggs mature, papule becomes a black, pea-sized nodule (Fig. 28-7). With severe infestation, nodules and plaques appear with a honeycombed appearance. Ulceration, inflammation, and secondary infection can occur. Most common on feet, especially under the toenails, webspaces, plantar aspect of the feet, and sparing weight-bearing areas; in sunbathers, any area of exposed skin.
Female bee, hornet, or wasp sting producing immediate burning/pain, followed by intense, local, erythematous reaction with swelling and urticaria. Severe systemic reactions occur in individuals who are sensitized, with angioedema/generalized urticaria and/or respiratory insufficiency from laryngeal edema or bronchospasm and/or shock.
Fire and harvester ants produce local skin necrosis and systemic reactions to sting; bite reaction begins as an intense local inflammatory reaction that evolves to a sterile pustule.
Caterpillar/moth contact can produce burning/itching sensation, papular urticaria, irritation caused by histamine release, allergic contact dermatitis (Fig. 28-8), and/or systemic reactions. Wind-borne hairs can cause keratoconjunctivitis.
PREVENTION Apply insect repellent such as diethyltoluamide (DEET) to skin and permethrin spray to clothing. Use screens, nets, and clothing. Treat flea-infested cats and dogs; spray household with insecticides (e.g., malathion, 1% to 4% dust).
LARVAE IN SKINTungiasis. Remove flea with needle, scalpel, or curette; topical petrolatum to suffocate fleas; topical ivermectin; oral thiabendazole or metrifonate for heavy infestations.
FURUNCULAR MYIASIS Suffocate larvae by covering with petrolatum and removing the following day.
GLUCOCORTICOIDS Give potent topical glucocorticoids for a short duration for intense pruritus. Oral glucocorticoids can be given for persistent pruritus.
ANTIMICROBIAL AGENTS Secondary infection antibiotic treatment with topical agents.
SYSTEMIC INFECTION/INFESTATION Treat with appropriate antimicrobial agent.