Chapter 34 Arthropod bites and stings
1. What are arthropods? Are most arthropods harmful to humans?
2. Describe various ways arthropods injure humans. What insect or arachnid would cause the injury?
• Allergic sensitization (salivary proteins and enzymes in bee venom): Bed bugs, mosquitos, bees, and ants
3. How do you diagnose a bite or sting?
The annoyance of the mosquito or the immediate pain of the bee sting rarely poses a problem of recognition. Diagnostic problems arise when the arthropod is not seen or felt but leaves a nonspecific papular rash with redness, pain, itching, or swelling. A careful history, common knowledge of local arthropod populations, and a physical examination with a high index of suspicion is necessary. Arthropod bites may present with itchy, red papules in a particular distribution or pattern that suggests an area of potential arthropod exposure. For example, grouped bites around an exposed ankle suggest a nonflying arthropod such as a flea, mite, chigger mite, or bedbug. Mysterious bites are often secondary to a zoonotic source, which includes exposures to pets, pests such as rats and mice, and nesting birds or bats near the home or workplace. Consultation with a veterinarian or entomologist is extremely beneficial in unusual cases. A biopsy of the skin lesion often will show perivascular dermatitis with eosinophils.
4. Why are some bites and stings extremely painful or dangerous while others are simply itchy, red, irritating papules?
5. One person on a hike is “eaten alive” by mosquitos while his companions are not bothered at all. Do ectoparasites such as mosquitos, ticks, fleas, and mites bite randomly?
No. Most arthropods are attracted to their hosts by a number of physical and chemical stimuli such as warmth, composition of sweat, carbon dioxide, vibrations, and odor. On the other hand, lipids derived from the degenerative epidermal cells often have a repellent quality to them. Thus, to arthropods, some human hosts are far more attractive than others.
6. Why are bite reactions to ectoparasites so much different in different people?
The severity of bug bite reactions depends most on the immunologic status of the patient. In general, a severely symptomatic eruption may result from only a few bites in an immunologically sensitized person, while many bites may produce no symptoms at all in a person with an acquired immunologic tolerance.
• Stage 2: After a 2-week period of exposure, a delayed red papule begins within 24 hours after a bite and subsides within a week.
• Stage 3: An immediate wheal after a bite, followed by a delayed papule 24 hours later, becomes the clinical presentation.
• Stage 5: With continual exposure, immunologic tolerance begins, and there is no reaction after even prolonged exposure to insect bites; that is, with the immediate as well as the delayed response is lost.
Dahl MV: Cutaneous reactions to arthropod bites and stings, Clin Cases Dermatol 3:11–16, 1991.
7. How do you treat bee stings?
The best treatment for stings is prevention. Exposure to areas of wildflowers, dandelions, or clover fields should be avoided. Always wear shoes. Brightly colored clothing, flowery designs, and the use of colognes, perfumes, and scented soaps should be minimized. When confronted by agitated bees or wasps, avoid rapid movements and either stand still or withdraw very slowly to prevent further aggravation.
The venom-containing barbed stinger, if still present at the site of the sting, should be removed by gently scraping the skin horizontally with a dull knife or credit card (Fig. 34-1). Stinger removal with forceps compresses the venom gland, forcing more venom into the skin, and should be avoided. Symptomatic care with rest, elevation, and ice to the area are helpful. Antihistamines may also be useful. Early signs of systemic toxicity or allergic reactions should be noted.
9. How do you treat anaphylactic syndrome from a bee sting?
If a reaction begins, treatment includes subcutaneous injection of epinephrine, 1:1000 in aqueous solution. This may be repeated in 15 to 20 minutes. In addition, intravenous diphenhydramine and cortisone as well as oxygen, fluids, vasopressors, and bronchodilators may also be used as needed. Since most fatalities occur within the first hour, early intervention by the allergic patient with a self-administered epinephrine injection may prevent a reaction from developing. Emergency treatment kits may be prescribed and carried at all times (Ana-Kit by Hollister-Stier [Spokane, WA], or EpiPen/EpiPen Jr by Dey Pharma [Basking Ridge, NJ]). Education and sting avoidance and evaluation for venom immunotherapy should be considered.
10. What are the unique characteristics of fire ants and their sting?
The fire ant, accidentally introduced into the United States in the 1930s from Brazil, now occupies most of the Southeast, limited only by cold winter weather in the north and the deserts of the Southwest. When the ants are provoked, they may attack en masse, administering up to several thousand stings to a victim. The sting is actually initiated by the ant biting the flesh and then pivoting slightly and repeatedly stinging the victim to form an annular eruption of sterile pustules and sting reaction sites. The sting site develops a large wheal and flare punctuated by a small sterile pustule, which ulcerates and potentially scars. The large erythematous wheal will then gradually desquamate over the next couple of days, and the sting site resolves normally. The unusual fire ant venom, a piperidine alkaloid, has become a leading cause of anaphylactic reaction to venomous animals in the United States. Treatment of the ant stings is symptomatic. Select patients with severe local or generalized sting reactions may be considered for immunotherapy.
11. What species of spiders are medically important?
The brown recluse (Loxosceles reclusa) and the female black widow (Latrodectus mactans) (Fig. 34-2) are the most serious spiders in the United States. The hobo spider (Tegenaria agrestis) in the Pacific Northwest has been reported to produce dermonecrotic arachnidism, a serious necrotic reaction in the skin induced by the venom. However, whether or not this spider produces dermonecrotic arachnidism is controversial; some authorities believe that the reported bites have come from other causes. As many as 16 other species may bite humans, causing a small amount of pain or even necrosis at the site, but these reactions are transient, self-limiting, and require little attention. Most species are either too small or the biting mouthparts will not penetrate the human skin and, therefore, inject an insignificant amount of venom.
12. How do you diagnose and treat the black widow spider bite?
The female black widow spider is 10 to 15 mm long and has a black, globose abdomen, often with a red hourglass figure on the ventral surface. This spider is found in all 48 contiguous states. The venom is a potent neurotoxin.
Figure 34-2. Female black widow spider demonstrating the characteristic hour glass on the ventral surface. (iStock_6222259)
Elston DM: What’s eating you? Latrodectus mactans (the black widow spider), Cutis 69:257–258, 2002.
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