Arthropod Bites and Stings

1

What are arthropods?

Arthropods compose the largest number and most diverse group of animals on earth. They are invertebrates that arise from eggs and share these three anatomic features: segmented bodies, hard exoskeleton, and symmetrically paired and jointed appendages.

2

Are most arthropods harmful to humans?

Less than 0.5% of the 1 million named species are injurious to humans. Most are harmless, while others are even beneficial to humans.

3

What are some examples of arthropods?

Arthropods may be classified as follows:

• Arachnids (spiders, ticks, mites, scorpions)

• Insects (e.g., bees, lice, fleas, beetles, mosquitos, butterflies, moths)

• Millipedes, centipedes, and crustaceans

4

Describe various ways arthropods injure humans. What insect or arachnid would cause the injury?

• Vesication (blisters): Blister beetle

• Envenomation: Bees, ants, spiders

• Allergic sensitization: Bedbugs, mosquitos, bees, and ants

• Invasion: Human botfly; tungiasis: penetrating fleas

• Contact urticaria: Caterpillars and setae from butterflies and moths

• Necrosis: Brown recluse spider

• Secondary infection: Any bite or sting, usually staphylococci

• Vector of disease: Mosquito, tick, body louse, flies

5

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, exam, and common knowledge of local arthropod populations are necessary to make a diagnosis in this more nuanced setting.

6

How do arthropod bites or stings typically present clinically?

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.

7

Is the histopathology of an arthropod bite specific?

No, a biopsy of the skin lesion is nonspecific and often will show perivascular dermatitis with eosinophils.

8

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.

9

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.

Dahl MV. Cutaneous reactions to arthropod bites and stings. Clin Cases Dermatol . 1991;3:11–16.

10

How do you treat bee stings?

The best treatment for stings is prevention. Minimize the use of perfumes and scented soaps as well as exposure to areas of wildflowers, and when confronted by agitated bees, avoid rapid movements.

When a person is stung, 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. 33.1 ). 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.

11

What are the signs of serious systemic reactions to bee, wasp, or ant stings?

Anaphylaxis is the most serious systemic reaction and may occur with bronchospasm, urticaria, angioedema, and, finally, vascular collapse and even death.

12

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, which can be prescribed and carried by patients at all times. 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.

13

What are the unique characteristics of fire ants and their sting?

The fire ant ( Solenopsis spp.) ( Fig. 33.2 ), when provoked, may attack en masse, administering up to several thousand stings to a victim resulting in numerous sterile pustules ( Fig. 33.3 ). The fire ant venom contains a hemolytic factor, solenopsin D, a piperidine alkaloid that can cause an anaphylactic reaction. Treatment of the ant stings is symptomatic.

Nguyen SA, Napoli DC. Natural history of large local and generalized cutaneous reactions to imported fire ant stings in children. Ann Allergy Asthma Immunol . 2005;94:387–390.

14

What species of spiders are medically important?

The female black widow ( Latrodectus mactans ) ( Fig. 33.4 ) and the brown recluse ( Loxosceles reclusa ) ( Fig. 33.5 ) are the most venomous spiders in the United States. The jumping spider ( Phidippus formosus ) is the most common biting spider in the United States causing a painful bite due to venom that contains hyaluronidase.

15

Does the hobo spider (Tegenaria agrestis) bite cause a necrotic reaction?

No, the latest information on this species thoroughly discredits the original reports that made claims to its importance as a dermonecrotic species. Evidence is overwhelming that this spider is not medically important.

Vetter R, Isbister G. Do hobo spider bites cause dermonecrotic injuries? Ann Emerg Med . 2004;44:605–607.

Gaver-Wainwright M, Zack R, Foradori M, et al. Misdiagnosis of spider bites: bacterial associates, mechanical pathogen transfer and hemolytic potential of venom from the hobo spider, Tegeneria agrestis. J Med Entomol . 2011;48:382–388.

16

How do you diagnose and treat the black widow spider bite?

The black widow ( Latrodectus mactans ) is a large black spider, often with a red hourglass figure on the ventral surface. The venom is a potent neurotoxin.

Initially, the bite is acutely painful, followed by systemic symptoms of pain and muscle cramping that can mimic an acute abdomen. Treatment of this neurotoxin with antivenin should be prompt. Calcium gluconate, muscle relaxants, and pain medications are also used.

Elston DM. What's eating you? Latrodectus mactans (the black widow spider). Cutis . 2002;69:257–258.

17

How does a brown recluse spider bite present?

Unlike the black widow, the brown recluse ( Loxosceles reclusa ) produces a dermonecrotic toxin that can cause severe necrosis of the skin ( Fig. 33.6 ), as well as a hemolytic toxin that causes severe, even life-threatening, hemolysis. The brown recluse is a brown spider with a dark brown violin-like marking on the dorsum of the cephalothorax. The bite commonly occurs when a person is cleaning old storage rooms or woodpiles outdoors, where the spider resides. The initial bite may be painless but is followed by pain and necrosis. The bite then begins to show a central blue color (impending necrosis), a surrounding white area (a vasospasm and ischemia), and a peripheral red halo (inflammation), the “red, white, and blue” sign. If a larger area of necrosis develops after a few days, a rare but serious and life-threatening systemic reaction to the hemolytic toxin may occur.

18

How should brown recluse spider bites be treated?

First aid measures are important and can be remembered by the mnemonic RICE. R est, i ce, c ompression, and e levation of the bite site decrease blood flow, temperature, and enzymatic activity of the dermonecrotic toxin. Ice should be applied immediately. General wound care, tetanus toxoid, antibiotics for secondary infection, and observation for systemic hematologic problems may be necessary until the wound heals. Dapsone and corticosteroids have been used, with variable results.

Elston DM, Miller SD, Young RJ 3rd, et al. Comparison of colchicine, dapsone, triamcinolone, and diphenhydramine therapy for the treatment of brown recluse spider envenomation: a double-blind, controlled study in a rabbit model. Arch Dermatol . 2005;141:595–597.

Mold JW, Thompson DM. Management of brown recluse spider bites in primary care. J Am Board Fam Pract . 2004;17:347–352.