From the EM Model
6.0 Environmental Disorders
6.1 Bites and Envenomations

Among the nearly 40,000 spider species and more than 100,000 species of insects, very few species are responsible for any morbidity and mortality in humans. Anaphylaxis is the most common reason for morbidity and mortality from Hymenoptera (wasps, bees, and ants) envenomation; it is estimated to occur in between 0.4% and 5% of the general population, with approximately 40 to 100 Americans dying because of it yearly, although these figures probably grossly underestimate the true numbers.¹ Emergency physicians should be able to recognize, evaluate, and manage spider and insect envenomations.

Case Presentation

A 45-year-old man presents complaining of chest pain and difficulty breathing; he had been mowing his lawn. His vital signs are blood pressure 85/42, pulse rate 52, respiratory rate 24, temperature 37.7° C (99.9° F), and oxygen saturation 88% on room air. Physical examination reveals an obese man who appears pale and is diaphoretic. He has dry mucous membranes. An S1 and S2 are noted, and pulmonary auscultation reveals expiratory wheezing bilaterally. Bowel sounds are diminished, and his abdomen is nontender. A diffuse urticarial rash is noted on his chest, back, arms, and legs. On further questioning, he says that a bee stung him while he was mowing.

Arthropod Envenomations

Widow

Five species of widow spiders are found in North America, Latrodectus bishopi, Latrodectus geometricus, Latrodectus hesperus, Latrodectus variolus, and Latrodectus mactans, with L. mactans being the most common. They are distributed across the continental United States and range into the southern Canadian provinces.² Many species are a shiny black with red markings, although both the color and markings can vary. The female spider is primarily responsible for clinically significant envenomations because the male’s fangs are not large enough to penetrate human skin. Widow spiders are generally not aggressive and only bite defensively.³

Systemic toxicity from a widow bite is the result of alpha-latrotoxin, a potent neurotoxin, which causes the presynaptic release of the major neurotransmitters – acetylcholine, norepinephrine, dopamine, and glutamate – throughout the entire nervous system. Repeated discharge results in depletion of these neurotransmitters from the synaptic vesicles at the nerve endings. Clinical signs and symptoms are shown in Table 1. Symptoms usually resolve in 24 to 48 hours.

Pain is the primary reason patients present for treatment following a widow spider bite. Treatment includes proper wound care, ensuring updated tetanus status, and focusing on symptomatic relief using parenteral opioids and benzodiazepines. Barbiturates, dantrolene, antihistamines, and intravenous calcium have all been used with minimal success.⁴ Latrodectus antivenin is the only treatment that can shorten the duration of symptoms, but it is not universally available. The antivenin is derived from horse serum, similar to the older Crotalidae polyvalent antivenin.

CRITICAL DECISION

Which patients with findings consistent with a black widow spider bite should be given antivenin?

Patients with severe refractory pain not relieved by parenteral opioids or benzodiazepines are candidates for antivenin.^4,5 One vial of antivenin diluted in 100 to 250 mL of saline should be infused over 2 hours.⁶ The major adverse effects from antivenin administration are anaphylaxis and delayed allergic reactions.

Recluse

There are six species of Loxosceles indigenous to the United States —Loxosceles arizonica, Loxosceles deserta, Loxosceles devia, Loxosceles blanda, Loxosceles apachia, and Loxosceles reclusa. The brown recluse (L. reclusa) is the most common recluse spider in North America. As the name implies, recluse spiders are not aggressive, preferring dark, dry locations. They are mostly nocturnal. Both sexes are venomous.⁷ Unlike widow spiders, recluse spiders are found only in distinct geographic locations, generally from Kansas to Kentucky and from southern Iowa to Louisiana. Considerable controversy arises over reported recluse bites in patients with dermonecrotic lesions that occur outside of locations where recluse spiders are known to live.^7,8 The differential diagnosis (Table 2) is broad for loxoscelism.

The most important toxin isolated from Loxosceles venom appears to be sphingomyelinase. This toxin causes dermonecrosis, platelet aggregation, and complement-related hemolysis in vitro and is thought to be responsible for dermonecrosis and systemic effects in humans.¹⁰

The initial bite is generally painless, although a stinging sensation is sometimes noted. After 2 to 8 hours, mild to severe pain develops, which is thought to result from vasospasm causing ischemia.⁷ There may be two small puncta and transient erythema, with itching, swelling, and mild to moderate tenderness. The bite site then pales, with the surrounding tissue becoming red and swollen. In lesions that progress, a characteristic bulls-eye wound develops, with a red erythematous center, a white ring of induration, and an outer blue cyanotic ring.⁵ The central area can then form an eschar and slough off over the next 5 to 7 days, leaving an ulcer that heals in varying amounts of time (6 to 8 weeks). Skin grafting has been required in some cases.¹¹

Systemic effects include fevers, chills, malaise, weakness, and nausea and vomiting, along with a generalized pruritic, morbilliform, or petechial rash.⁷ Rare effects (<1%)⁵ include hemolysis, disseminated intravascular coagulation, and rhabdomyolysis with resulting renal failure.

CRITICAL DECISION

What treatment should a patient be given who presents with findings concerning for a brown recluse spider bite?

Therapy for patients suspected of having a recluse bite is generally supportive, although various treatments have been tried without significant clinical improvement. Elevation, immobilization, application of ice, local wound care, and tetanus prophylaxis are reasonable for initial therapy. Excision may be required in severe cases, although there is controversy over the timing, with more recent recommendations advising delaying surgery until 6 to 8 weeks after the bite.⁵

Tarantulas

Tarantulas are large, hairy, mygalomorph spiders that are found primarily in the Desert Southwest but can be found as far east as the Mississippi River and as far north as Arkansas.6 In addition, tarantulas are sold as pets throughout the United States. Tarantula bites usually will cause mild stinging with minimal surrounding inflammatory reaction, no dermonecrosis, and no serious systemic toxicity.¹² Treatment for tarantula bites is generally supportive, with local wound care, tetanus prophylaxis, and oral analgesics as needed.

CRITICAL DECISION

Dermatitis and blurry vision in a patient who has recently handled a tarantula should prompt consideration of what condition?

In addition to their bites, New World tarantulas have defensive urticating hairs located on their dorsal abdomen and, when threatened, they will rise on their back legs and use leg vibrations to flick a volley of hairs at their attackers.¹³ These hairs can produce a persistent papular dermatitis. Even more concerning, the hairs can penetrate the cornea, causing a foreign body keratoconjunctivitis or ophthalmia nodosa. Ophthalmia nodosa is a chronic, granulomatous, nodular reaction that occurs as a reaction to vegetable or arthropod hairs.¹⁴ Treatment of ophthalmia nodosa requires referral to an ophthalmologist for slit lamp examination and possible surgical removal of the hairs. Topical corticosteroids have also been helpful in managing this complication.¹³

Insect Envenomations

Ants

There are more than 6,000 different species of ants, and most of them are capable of both biting and stinging. Fire ants (Solenopsis sp.) are responsible for most injuries, including anaphylaxis. Since their introduction to the United States, the red imported fire ant is thought to have been responsible for more than 80 deaths.¹⁵ Fire ant venom contains piperidines, alkaloid compounds that are neurotoxic, cytotoxic, and hemotoxic and create an intense burning sensation.¹⁵ The fire ant grabs with its jaws and then stings in a circle around the bite that develops into a necrotic lesion known as a sterile pustule. The pustule may last for several days and is pathognomic for fire ant stings.¹⁶ A large local reaction may occur similar to those with other stinging insects with pruritic edema, induration, and erythema that can affect the entire limb.¹⁶ Anaphylaxis can occur hours after a sting.

Local wound treatment includes cold compresses along with elevation of the affected extremity. Topical steroids and antihistamines can improve pruritus.

Bees, Wasps, Hornets, and Yellow Jackets

These insects belong to the Vespidae and Apidae families and are responsible for most insect-related anaphylaxis. These insects are found throughout the United States. All are capable of stinging; however, the Africanized honeybee, an aggressive hybrid resulting from an experiment intended to enhance honey production, is of the most concern. The danger arises from the multiple stings inflicted because of the species’ “swarm-and-attack” behavior; their venom is no different from that of other honeybees.¹⁷ Hymenoptera venom includes histamine, dopamine, noradrenalin, 5-hydroxytryptamine, hyaluronidase, phosphomonoesterase, alpha-D-glucosidase, phospholipase A and B, kinins, and peptides, including melitin (thought to be responsible for hemolysis and rhabdomyolysis) and apamin (thought to be responsible for neurotoxicity).¹⁸

Most stings cause transient pain, itching, and swelling. There can be delayed onset of a large local sting reaction that causes induration and edema extending beyond the sting site to the entire affected extremity. These reactions can continue to increase for 24 to 48 hours after the envenomation and take 3 to 10 days to resolve.¹⁷

Systemic reactions, including anaphylaxis, present with the above findings, in addition to other cutaneous, vascular, respiratory, musculoskeletal, and renal findings, either in isolation or in combination. Cardiac findings include bradycardia, dysrhythmias, angina, and myocardial infarction.
¹⁷
Nausea, vomiting, diarrhea, fatigue, and dizziness can occur. Hemolysis, rhabdomyolysis, and transient liver transaminase elevation have also been reported with massive envenomation.¹⁹ Acute kidney injury, with or without rhabdomyolysis or intravascular hemolysis, can also occur.²⁰

Anaphylaxis is a severe, potentially fatal, systemic allergic reaction that occurs suddenly after contact with an allergy-causing substance.²¹ Although there are no universally accepted diagnostic criteria, most authorities agree that skin or mucosal involvement and involvement of either respiratory compromise (e.g., dyspnea, wheeze-bronchospasm, stridor, hypoxemia) or reduced blood pressure or other associated symptoms of end-organ dysfunction (e.g., hypotonia [collapse], syncope, incontinence) would suggest anaphylaxis.²¹

Management of anaphylaxis includes intramuscular epinephrine, oxygen, and inhaled beta-adrenergic agents (albuterol), aggressive fluid resuscitation with multiple 10- to 20-mL/kg fluid boluses under pressure, H1 and H2 antihistamines, and corticosteroids.

CRITICAL DECISION

What is the most important initial treatment for a patient who presents with anaphylaxis from insect envenomations?

Epinephrine is the treatment of choice for anaphylaxis. Intramuscular injection in doses of 0.01 mg/kg (maximum dose, 0.5 mg) given every 5 to 15 minutes as necessary is recommended for controlling symptoms and maintaining blood pressure.²¹ Although studies have not been performed in patients experiencing anaphylaxis, both children and adults have faster absorption with higher peak plasma epinephrine levels when epinephrine is administered intramuscularly in the anterolateral thigh versus subcutaneously or intramuscularly in the deltoid.²¹

Intravenous epinephrine is an option for patients with severe hypotension or cardiac arrest unresponsive to intramuscular doses of epinephrine and fluid resuscitation. Although dosages are not firmly established, 5 to 10 mcg (0.2 mcg/kg) for hypotension and 0.1 to 0.5 mg intravenously for cardiovascular collapse have been suggested.²¹ Epinephrine infusions have also been successful in treatment of anaphylaxis with hypotension.²¹

Other treatment considerations include high-flow oxygen for respiratory symptoms or hypoxemia and albuterol for bronchospasm refractory to epinephrine. Fluid resuscitation should be guided by initial response to epinephrine. Large volumes of crystalloid can be needed in the first 5 to 10 minutes, because up to 35% of blood volume can extravasate in the first 10 minutes, along with vasodilation that results in pooling, which can decrease circulating blood volume even further.²¹ A combination of both H1- and H2-antagonists has been reported to be more effective in improving the cutaneous reactions in anaphylaxis than H1-antagonists alone.²¹

CRITICAL DECISION

For how long should patients with an anaphylactic reaction be observed?

From 1% to 20% of anaphylactic reactions from all causes will be biphasic, with onset of the second phase anywhere from 1 to 72 hours after the initial onset of anaphylaxis.²¹ Unfortunately, no reliable clinical predictors enable clinicians to identify those who are more at risk for a biphasic reaction, although some studies have suggested that patients requiring higher doses of epinephrine and those who receive delayed administration of epinephrine may be at higher risk.²¹ Given these risks, a reasonable length of time to observe patients is 4 to 6 hours with consideration of a longer period of observation or hospital admission for those with particularly severe reactions or those with refractory symptoms.²¹

Case Resolution

Anaphylactic shock was suspected in this patient who reported a bee sting; epinephrine was given intramuscularly while intravenous access was obtained. He was given a 20-mL/kg fluid bolus. Diphenhydramine, ranitidine, and methylprednisolone were given intravenously. Despite these treatments, his blood pressure remained 84/46, and another fluid bolus and intramuscular dose of epinephrine were administered, after which his blood pressure and clinical condition improved. He was admitted to the emergency department’s observation unit and monitored overnight without return of his symptoms. He was discharged home with a prescription for an auto-injection epinephrine kit and a referral to an allergist.

Summary

Although most spider bites are harmless, several spiders can cause serious morbidity with rare reports of mortality. Incorrectly attributing a skin lesion to a spider bite could delay proper diagnosis and treatment; clinicians should consider the differential carefully. Insect bites are common, and most cause little more than minor irritation. Anaphylaxis, although rare, is a potentially life-threatening condition that clinicians should be able to recognize and treat rapidly.

Pearls

• Most spider bites in the United States are harmless and require only local wound care.
• Widow spider bites are characterized by both localized and generalized pain and autonomic, neurologic, and other nonspecific complaints.
• Recluse spiders are found only in certain locations in the United States and can cause severe skin lesions and systemic effects.
• Anaphylaxis is a rare but potentially lethal condition that can be caused by an insect sting; epinephrine is the treatment of choice.
• When epinephrine is required for treatment of anaphylaxis, intramuscular injection of the anterolateral thigh has faster absorption than subcutaneous or intramuscular injection in the deltoid muscle.

Pitfalls

• Attributing a skin lesion to a spider bite without consideration of other possibilities and failing to obtain an adequate history and physical examination.
• Failing to adequately observe a patient with an anaphylactic reaction for an appropriate amount of time.
• Failing to educate an anaphylactic patient on allergen avoidance and to ensure these patients have an auto-injection epinephrine kit on discharge.

References

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