140 Arthropod Bites and Stings
Key Points• Anaphylaxis, typically caused by Hymenoptera stings, is the most serious complication of all arthropod encounters and should be treated with steroids, epinephrine, and antihistamines when needed.
• Massive envenomations (>10 stings per kilogram or >100 stings per person) by Hymenoptera merit close monitoring for systemic effects of the venom.
• Latrodectism should be treated with adequate analgesia and benzodiazepines, but antivenom should be considered for severe cases.
• Dapsone, hyperbaric oxygen, colchicine, and electric shock therapy are no more effective than supportive care for the treatment of true dermonecrotic arachnidism.
Perspective
Arthropods are the most diverse, widespread, and numerous of all animal phyla inhabiting the planet. Not surprisingly, their contact with humans is a common occurrence. In 2009, 40,657 calls related to arthropods were made to poison centers in the United States, and although most do not require hospital attention, many patients will still come to the emergency department (ED) complaining of a bite or sting from an unknown or unidentified insect.1 Fortunately, the vast majority of these patients can be treated with supportive care and medications for pruritus and pain; the challenge for the emergency physician (EP) is identifying more serious and rare complications of these encounters. The most clinically significant arthropods are summarized in Table 140.1.
Table 140.1 Common Clinical Manifestations of Arthropod Envenomation
| Bees and wasps | Urticarial eruptions, anaphylaxis, rhabdomyolysis, ARF, ARDS (after massive envenomations) |
| Widow spiders | Pain, muscle spasm, local diaphoresis, tachycardia, hypertension |
| Recluse spiders | Dermonecrosis; hemolysis, DIC, ARDS (rarely) |
| Scabies | Migratory pruritus, secondary infections |
| Ants | Urticarial and papular dermatitis, anaphylaxis risk |
| Scorpions | Pain, tingling, cranial neuropathy, ataxia, pancreatitis, DIC, ARDS (exotic species) |
| Caterpillars | Painful dermatitis, ocular and mucosal irritation |
| Mites | Papular urticarial dermatitis |
| Ticks | Local tissue reaction, tick paralysis, infectious complications |
| Reduviid bug | Bullous lesions, infectious complications |
| Lice and fleas | Papular urticarial dermatitis |
| Mosquitoes | Urticaria, pruritus, infectious complications |
| Tarantulas | Local pain (bite), urticarial dermatitis, ocular irritation (hairs) |
| Centipedes | Local pain |
| Millipedes | Skin discoloration from oily extractions |
ARDS, Acute respiratory distress syndrome; ARF, acute renal failure; DIC, disseminated intravascular coagulation.
Hymenoptera
Pathophysiology
Apidae (bees), Vespidae (wasps, yellow jackets, hornets), and Formicidae (ants) are the most clinically significant groups of arthropods for two reasons. First, the incidence of Hymenoptera venom allergy has been estimated to be 0.8% to 5% in the general population and is increasing, particularly in young people.2 Second, because of their complex social organization, multiple stings are more likely to occur during Hymenoptera encounters than with arthropods that do not build nests or hives.
Recent research indicates that the major allergens in Hymenoptera venom are phospholipases and hyaluronidases, as well as mellitin, a peptide that causes degranulation of mast cells.3 Hymenoptera venom is delivered via an ovipositor stinger and gland, although some anatomic variation does exist. Male bees have no stingers and are incapable of stinging when threatened. Females have barbed stingers that become lodged in human skin and eviscerate the bee after venom delivery. The retained stinger and venom sac can be removed with tweezers. Africanized “killer” bees deserve special mention in that (1) they are far more aggressive and territorial than the more docile domesticated varieties, (2) are known to pursue perceived threats for up to 1 km, and (3) do so in much larger swarms. Africanized bees are difficult to distinguish morphologically from domesticated bees, but fortunately, this distinction is of little clinical significance because of venom homology between Hymenoptera Apidae. In contrast to bees, vespids (wasps, yellow jackets, and hornets) have the ability to withdraw their stinger from the victim and deliver multiple stings. Most severe allergic reactions to Hymenoptera are due to encounters with vespids, particularly wasps and yellow jackets.3,4
Presenting Signs and Symptoms
Massive envenomations are considered those in which the victim sustains more than 100 stings or more than 10 stings per kilogram (Fig. 140.1). Such cases merit special respect and victims should be considered for admission because of an increased risk for systemic symptoms, including nausea, vomiting, diarrhea, edema, dyspnea, hypotension, and rhabdomyolysis. Rarely, glomerulonephritis, acute renal failure, and acute respiratory distress syndrome can occur.5–7
Diagnostic Testing
No laboratory testing is necessary in cases limited to cutaneous symptoms from submassive envenomations. Massive envenomations or systemic reactions require investigation to evaluate for rhabdomyolysis, renal failure, or cardiac ischemia.5,6,8 Appropriate testing should include a basic chemistry panel and creatine phosphokinase (CPK) level.
Anaphylaxis and Allergic Reactions
It has been estimated that 40 deaths occur per year in this country as a result of anaphylaxis from Hymenoptera stings.4 Anaphylaxis is an IgE-mediated type I hypersensitivity reaction that leads to mast cell and basophil degranulation of vasoactive mediators, cytokines, prostaglandins, and platelet-activating factor. Some initial symptoms can be mild and include itchy eyes, urticaria, or cough. However, the symptoms can progress rapidly to shortness of breath, stridor, angioedema, and shock. Treatment should be initiated immediately and includes epinephrine, steroids, antihistamines, and bronchodilators (if bronchospasm is present). All available data suggest that failure or delay in the administration of epinephrine increases the chance for death from anaphylaxis. The risk for anaphylaxis with any event is dependent on the severity of the patient’s previous reaction, and it seems to be proportional to the rate of symptom onset. Once the symptoms have been controlled, patients should be observed for at least 2 hours to ensure resolution of the symptoms. Patients with persistent cardiopulmonary symptoms should be admitted to the hospital. An outline of anaphylaxis treatment is found in Box 140.1 and Table 140.2.2–5
Box 140.1 Treatment of Anaphylaxis Caused by Arthropod Venom or antivenom Therapy
Symptoms of allergy and anaphylaxis may be variable and include perioral or pharyngeal tingling, shortness of breath, tachypnea, bronchospasm and wheezing, stridor, chest pain, sudden tachycardia, hypotension, angioedema, and urticaria.
Bees and wasps are the most common sources of insect allergic reactions. Because animal-derived antibody products can also result in allergic reactions or anaphylaxis, each patient receiving antivenom must be monitored carefully. The antivenom infusion must be stopped immediately if allergic symptoms such as those listed develop.
Skin testing is a very imperfect (not sensitive, not specific) predictor of subsequent allergic reactions to antivenom.
Pretreatment includes antihistamines (e.g., diphenhydramine, 25 to 50 mg intravenously [IV], plus ranitidine, 50 mg IV) and antipyretics (acetaminophen, 500 mg or 15 mg/kg orally [PO]).
Treatment of any significant allergic reaction is prompt administration of epinephrine.
Steroids are recommended to prevent delayed allergic effects.
Pretreatment with steroids can be done in high-risk patients requiring antivenom.
Nebulized bronchodilators and supplemental oxygen can be used for bronchospasm.
Warn patients about the risk and signs of serum sickness, which occurs within 7 to 10 days of the envenomation or administration of antivenom. Serum sickness is characterized by a diffuse macular or urticarial rash, arthralgias, back pain, and sometimes hematuria. Therapy is a 10- to 14-day course of prednisone, 1 to 2 mg/kg/day PO, with tapering.
Table 140.2 Other Arthropod Rashes and Treatment
| ARTHROPOD | SIGNS AND SYMPTOMS | TREATMENT |
|---|---|---|
| Fleas and mites | Pruritic, erythematous, red papules | Oral or topical antihistamines, topical steroid cream Antimicrobials for secondary infections |
| Scabies | Significant nocturnal pruritus, intertriginous skin thickening, papules. The diagnosis can be made with microscopy of skin scrapings. Finger web spaces, wrists, elbows, and unscratched skin are the most productive sites for sampling | Topical and oral antihistamines |
| Topical scabicides: 5% permethrin cream applied once for 8-14 hr, then washed off. May be repeated in 1 wk; treatment failure typically results from incorrect application | ||
| Lindane cream no longer recommended | ||
| Ivermectin, 200 mcg/kg orally once, second dose recommended 14 days later. Only for topical treatment failure | ||
| Norwegian scabies |
