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

ARDS, Acute respiratory distress syndrome; ARF, acute renal failure; DIC, disseminated intravascular coagulation.

Approaching this topic can be a daunting task because there are literally millions of arthropod species and hundreds of medical significance. This chapter focuses on the venomous and immunologic effects of these organisms. It is useful to divide arthropods into broad groups based on organism similarity and clinical findings: Hymenoptera (bees, wasps, hornets, yellow jackets, ants); arachnids (spiders, ticks, scorpions); centipedes, millipedes, and caterpillars; and scabies, fleas, and lice.

Epidemiology

Arthropod venoms are complex mixtures of enzymes, proteins, histamines, and other bioamines that can either damage tissue directly or elicit an allergic response. The venoms of each family are unique, and cross-reactivity is rare with the exception of wasps, hornets, and yellow jackets (vespids). Local tissue reactions from dermal exposure to arthropod antigens result in the common pruritic, urticarial, and papular lesions seen after envenomation. The most immediately life-threatening complication of arthropod exposure is anaphylaxis either to the venom itself or to the antivenom administered in the ED. Treatment of anaphylaxis merits special discussion and is covered in the next section on 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

Ant venom is also delivered via a stinging apparatus, but ants are known to initially bite with their powerful jaws before stinging their victim and can do so en masse via a pheromone-coordinated attack. Multiple ant stings are most common with fire ants.

Presenting Signs and Symptoms

Hymenoptera stings cause immediate pain with subsequent erythema, edema, and pruritus. Fire ants are known for their particularly painful sting, which can eventually develop into a sterile pustule. Delayed type IV reactions can occur with all Hymenoptera venoms and result in larger, albeit localized reactions.

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

Fig. 140.1 Massive Hymenoptera envenomation.

(Courtesy Richard Clark, MD.)

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

Table 140.2 Other Arthropod Rashes and Treatment