Arthropods (phylum Arthopoda) account for a higher percentage of morbidity and mortality to humans than do mammalian bites, snake bites, or marine envenomation. They are ubiquitous in domestic dwellings, caves, and campsites and in wilderness settings such as deserts, forests, and lakes. Although arthropods are most intrusive during warmer months, many are active throughout the winter, particularly indoors. Arthropods are also nocturnal and often bite unsuspecting victims while they are sleeping. Encounters with humans are generally defensive, accidental, or reactive. An individual stung by an insect or bitten by an arachnid may experience pain and local swelling, an anaphylactic reaction, or life-threatening toxicity. This review discusses the clinical presentation and latest treatment recommendations for bites and stings from spiders, scorpions, bees, ants, ticks and centipedes of North America.
Key points
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Black widow spider bites cause painful muscle spasms, secondary to neurotoxicity, and are responsive to antivenom therapy.
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Brown recluse spider bites result in hematotoxicity and most commonly manifest locally as skin necrosis.
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Scorpion stings in North America produce severe localized pain with occasional neurotoxic systemic effects.
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Hymenoptera stings from bees and wasps can result in local skin reaction to severe anaphylactic reactions and are responsible for more fatalities than any other venomous arthropod.
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Fire ant stings can cause multiple painful localized skin reaction and pustules.
Spiders
There are nearly 40,000 species of spiders worldwide (class Arachnida). Most species cannot inflict serious bites to humans because they do not have fangs long enough to penetrate the human skin. As a result, most exposures are often unnoticed and do not need treatment. In North America, approximately 50 species of arachnids potentially cause human morbidity. Spiders use their venom to paralyze and liquefy their prey. There are only a few medically relevant spiders that produce toxic venoms that can lead to local reactions, systemic illnesses, hematoxicity, and neurotoxicity.
Black Widow Spiders
Introduction
The Latrodectus genus of spiders includes 5 primary species found in North America: Latrodectus mactans , Latrodectus bishop , Latrodectus geometricus , Latrodectus hesperus , and Latrodectus variolus . They live in dimly lit, secluded areas such as woodpiles, stonewalls, cabins, barns, stables, and outhouses. They are present in southern Canada and every US state except Alaska. Black widows are jet black with an iconic red hourglass marking on the ventral aspect of the abdomen ( Fig. 1 ). The red hourglass is specific to L mactans ; other species have distinctive ventral markings, such as triangles and spots. There is a seasonal variation in the number of black widow bites, starting to increase in spring, peaking in September, and reaching a nadir in January to February.
Black widows are docile and nocturnal and bite when their web is disturbed. The female black widow is generally considered poisonous to humans and is more aggressive if guarding her egg sac. The male black widow spider has smaller jaws with minimal venom production and is not significantly poisonous to humans. These spiders use striated muscles to control the amount of venom they inject, and about 15% of bites do not deliver venom. The venom’s toxicity is due to the presence of α-latrotoxin. This toxin facilitates exocytosis of synaptic vesicles and the release of the neurotransmitters norepinephrine, γ-aminobutyric acid, and acetylcholine. The toxin also causes degeneration of motor end plates, resulting in denervation. The venom destabilizes nerve cell membranes by opening ion channels, causing a large influx of calcium into the cell and depletion of acetylcholine from presynaptic nerve terminals.
Patient evaluation overview
Latrodectism is the clinical syndrome that follows a black widow bite. The bite produces a pinprick sensation that often goes unnoticed. With careful examination, 2 small fang marks may be noticed. Within the first few hours, local irritation develops, including erythema, urticaria, or a characteristic halo-shaped target lesion. These local symptoms may be followed by generalized symptoms of pain and muscle spasms in the chest, abdomen, and lower back. Typically, the pain is concentrated to the chest with upper extremity bites and abdomen with lower extremity envenomation. Abdominal rigidity can be severe and may be mistaken as an acute abdomen. Signs and symptoms in small children are wound erythema, irritability, constant crying, sialorrhea, agitation, and seizures. Victims experience pain on the wound site, muscle spasms, abdominal and thoracic pain, and fine tremors. About one-third of patients will go on to have systemic symptoms. These systemic symptoms include hypertension, sweating, salivation, dyspnea with increased broncho-secretions, and seizures. Less common effects include myocarditis, compartment syndrome of the upper extremity, and priapism. Death is rare from black widow envenomation alone, with no recent cases reported in the US literature, and only a few documented worldwide.
Pharmacologic treatment options
Tetanus immunization should be updated, but antibiotics are unnecessary unless there is evidence of a wound infection. Oral and parenteral analgesics are administered if pain is severe. Muscle spasms may require large doses of benzodiazepines. With administration of these drugs, attention should be given to the patient’s airway status because of the concomitant neurotoxicity of the venom.
Historically, administration of calcium gluconate was considered because of concern for the development of hypocalcemia following black widow envenomation. This practice is currently not advocated, because studies have proven no benefit to the administration of calcium. Likewise, dantrolene administration has not been shown to be clinically efficacious for muscle spasms.
Nonpharmacologic treatment options
Pain at the bite site may be relieved with application of an ice pack. The wound can be cleansed with soap and irrigated with water.
Combination therapies
In extreme cases with severe symptoms, Latrodectus antivenom is recommended. Currently, in North America, the most widely available product is black widow antivenin (Lyovac, Merck). Black widow antivenin is a horse serum–derived product containing immunoglobulin G antibodies to L mactans venom. The dose of antivenom is one vial diluted in 50 mL of normal saline administered intravenously over 15 minutes. A more highly purified equine F(ab) 2 antibody black widow spider antivenom is also under investigation. The use of Latrodectus -specific antivenom is restricted to patients with severe envenomation (eg, seizures, hypertensive crisis, respiratory compromise, or intractable pain), with no allergic contraindications, in whom opioids and benzodiazepines are ineffective. If available, young children and elderly patients with severe toxicity should receive antivenom early in the clinical course.
Treatment resistance/complications
There has traditionally been reluctance to use antivenom because of concern for anaphylaxis. Two reviews of antivenom use in the United States have demonstrated low rates of adverse reactions. There have only been 2 deaths reported after black widow antivenom administration. Patients receiving antivenom may experience flulike symptoms or serum sickness 1 to 3 weeks following treatment. This entity is generally self-limited and responsive to antihistamines and steroids.
Evaluation of outcome and long-term recommendations
In adults, the pain will gradually subside after several hours but may remain for 2 to 3 days. A small child bitten by a black widow spider has a greater chance of morbidity and mortality. As with snake envenomation, the volume of distribution and milligrams per kilograms dose of the venom is relatively larger in children than adults. A dose that may cause painful muscle spasms in an adult may lead to respiratory arrest in a child. Any symptomatic patient who has suffered a bite from a black widow spider should be admitted for observation and pain control. Pregnant patients should undergo fetal monitoring. If there is cardiopulmonary compromise or seizures, the patient should be admitted to the intensive care unit for stabilization and antivenom administration. Symptoms of latrodectism typically last days, but some patients can have intermittent muscle weakness and spasms for weeks.
Brown Recluse Spiders
Introduction
The 6 species of recluse spiders in North America are Loxosceles arizonica , Loxosceles deserta , Loxosceles devia , Loxosceles laeta , Loxosceles rufescens , and Loxosceles recluse . Of these, L recluse is the most common. These spiders are reclusive nocturnal hunters and are active from spring to fall. Victims typically are bitten on the extremities while rummaging in confined spaces such as a closet or an attic, while putting on a boot, or when using a blanket or sleeping bag that a spider is trapped in.
The brown recluse gets its name because of its reclusive nature and brown- or fawn-colored body. It is approximately 1 to 5 cm in length, with a characteristic violin- or fiddle-shaped marking on the dorsal cephalothorax ( Fig. 2 ). They have long, slender legs and 6 eyes rather than eight, which is the norm for other spiders.
The venom of the recluse spider, per volume, is more potent than that of the rattlesnake and can cause extensive skin necrosis. The venom acts directly on cell walls, causing immediate injury and cell death. It contains the calcium-dependent enzyme sphingomyelinase D, which in combination with C-reactive protein has a direct lytic effect on RBCs. The local tissue destructive effects are due to hemolytic enzymes and a levarterenol-like substance that induces vasoconstriction. Following cell wall damage, intravascular coagulation causes a cascade of clotting abnormalities and local polymorphonuclear leukocyte infiltration, culminating in a necrotic ulcer.
Patient evaluation overview
Most brown recluse bites occur in predawn hours and are often painless. The seasonality of brown recluse bites and the geographic area should be considered when making this diagnosis. One study demonstrated that 95% of brown recluse bites occurred between the months of April and October. L reclusa is primarily found in the south central United States. The clinical response to loxoscelism ranges from cutaneous irritation (necrotic arachnidism) to a life-threatening systemic reaction. Most signs and symptoms of envenomation are localized to the bite area. Most (90%) result in nothing more than a local reaction and resolve spontaneously. Within a few hours, the patient experiences itching, swelling, erythema, and tenderness over the bite site. Classically, erythema surrounds a dull, blue-gray macule circumscribed by a ring or halo of pallor. Gradually, over 3 to 4 days, the wound forms a necrotic base with a central black eschar. In 7 to 14 days, the wound develops a full necrotic ulceration. Bites that are in fatty areas, such as the thigh or buttocks, tend to cause more extensive necrosis. Several sources call for stricter diagnostic criteria and claim that the diagnosis of necrotic arachnidism secondary to brown recluse spiders is overreported and often mistaken for skin abscesses (eg, MRSA [methicillin-resistant Staphylococcus aureus ]) and other dermatologic causes.
The systemic reaction, which is less common than the cutaneous reaction, is associated with a higher morbidity. The reaction rarely correlates with the severity of the cutaneous lesion. Within 24 to 72 hours following the envenomation, the patient experiences fever, chills, myalgias, and arthralgias. In severe systemic reactions, the patient may suffer coagulopathies, hypotension, jaundice, disseminated intravascular coagulation (DIC), seizures, renal failure, and hemolytic anemia. In rare cases, a patient may succumb to the systemic reaction.
Hobo Spider
The Hobo spider ( Eratigena agrestis , formerly Tegenaria ), also known as the aggressive house spider, is found in the Pacific Northwest region of the United States and Canada. This spider was traditionally included with Loxosceles species when discussing cases of necrotic arachnidism; however, recent studies are calling this into question. Similar symptoms lead many to incorrectly attribute hobo spider bites to that of the brown recluse, which is less indigenous to the northwest United States. This species is more aggressive and bites with minor provocation. Hobo spiders are brown with gray markings and have a herringbone pattern on the abdomen ( Fig. 3 ).
Pharmacologic treatment options
The management of envenomation by the brown recluse or hobo spider depends on whether the reaction is local or systemic. It is difficult to predict which type of wound will eventually progress to a disfiguring necrotic ulcer. Tetanus immunization should be updated, but antibiotics are only indicated if there is a secondary wound infection. Antihistamines and analgesics can be beneficial. Many treatment modalities, including dapsone, triamcinolone, diphenhydramine, colchicine, and trypsin, have been studied, but none have prevented the formation of an ulcerative lesion.
Nonpharmacologic treatment options
Proper care includes wound cleansing, immobilization, and elevation of the affected extremity to reduce pain and swelling. Early application of ice to the bite area will lessen the local wound reaction, whereas heat will exacerbate the symptoms. A suggested method of treating expanding wound necrosis due to brown recluse spider bites is hyperbaric oxygen treatment. However, results with such treatment have been mixed, and little evidence exists to support its use.
Combination therapies
Although not proven in clinical trials, glucocorticoids may provide a protective effect on the red blood cell (RBC) membrane, thus slowing hemolysis. The patient should be monitored in a hospital setting for the development of DIC. Transfusion of RBCs and platelets may be necessary. Plasma exchange for refractory hemolysis has been recently described following brown recluse spider envenomation. Urine alkalinization with bicarbonate may lessen renal damage if the patient is experiencing acute hemolysis.
There continues to be ongoing research with brown recluse antivenom. However, there is little evidence to support its efficacy, particularly against local dermatologic effects. Institutions in Mexico and Brazil currently produce antivenom for Loxosceles bites, but the product is not commercially available in the United States.
Surgical treatment options
Early excision of ulcers is not recommended because wound healing is slowed and scarring is more severe if excised early in the clinical course. Complications of early surgical intervention include recurrent wound breakdown as well as long-term distal extremity dysfunction. Delayed excision of ulcers after the necrotic process has subsided (usually within 6–8 weeks), followed by secondary closure with skin grafting, is the preferred method of managing necrotic ulcers. In a normal host with appropriate wound care, most bite wounds heal well with only 10% occurrence of major scarring. Immunocompromised patients and those with diabetes mellitus often have a more prolonged and complicated healing process.
Treatment resistance/complications
Historically, the use of the polymorphonuclear leukocyte inhibitor, dapsone, was advocated to diminish scarring and subsequent surgical complications. Its use, however, has not proven effective in any large study with human or animal models. Because of the potential for dapsone to induce methemoglobinemia and hemolytic anemia, particularly in children and those patients with glucose-6-phosphate dehydrogenase deficiency, administration is not advised.
Evaluation of outcome and long-term recommendations
Patients who are asymptotic following a period of observation in the emergency department (ED) and have normal baseline laboratory values may be discharged home with close outpatient follow-up for wound care within 24 to 48 hours. Systemic effects of brown recluse spider bites are rare but can be life threatening, and the patient should be evacuated if in a remote setting.
Tarantulas
Introduction
Tarantulas are feared because they are the largest of all spiders ( Fig. 4 ). They inhabit the deserts of the western United States and Mexico, but have been discovered as far east as the Mississippi River Valley. These large, hairy spiders are relatively harmless. They are extremely shy and bite only when vigorously provoked or roughly handled.
Patient evaluation overview
Tarantula bites usually cause minimal pain and surrounding edema with minimal necrosis and no serious systemic effects. Although tarantula bites are usually of little consequence to humans, they can be more severe in domestic animals, especially canines. The growing trade of these arachnids as exotic pets should prompt the clinician to inquire about this as a possible cause an unusual skin lesion.
Pharmacologic treatment options
Tetanus prophylaxis should be updated, and nonsteroidal anti-inflammatory agents can be given to alleviate pain. If needed, the patient is treated with antihistamines and topical glucocorticoids.
Nonpharmacologic treatment options
Treatment of bites consists of local wound care, and involved limbs should be raised and immobilized.
Treatment resistance/complications
Exposure to the hairs on the abdomen of the tarantula is more concerning than the actual bite. These hairs can be flicked off in large numbers as a defense mechanism and are capable of producing urticaria and pruritus that may persist for several weeks. The hairs may also get into the eyes and cause keratoconjuctivitis or ophthalmia nodosa, a nodular, granulomatous lesion in the cornea. Patients with these complaints after exposure to a tarantula should be referred to an ophthalmologist. Without appropriate care, these eye lesions may progress to keratitis, uveitis, retinitis, and orbital cellulitis.
Spiders
There are nearly 40,000 species of spiders worldwide (class Arachnida). Most species cannot inflict serious bites to humans because they do not have fangs long enough to penetrate the human skin. As a result, most exposures are often unnoticed and do not need treatment. In North America, approximately 50 species of arachnids potentially cause human morbidity. Spiders use their venom to paralyze and liquefy their prey. There are only a few medically relevant spiders that produce toxic venoms that can lead to local reactions, systemic illnesses, hematoxicity, and neurotoxicity.
Black Widow Spiders
Introduction
The Latrodectus genus of spiders includes 5 primary species found in North America: Latrodectus mactans , Latrodectus bishop , Latrodectus geometricus , Latrodectus hesperus , and Latrodectus variolus . They live in dimly lit, secluded areas such as woodpiles, stonewalls, cabins, barns, stables, and outhouses. They are present in southern Canada and every US state except Alaska. Black widows are jet black with an iconic red hourglass marking on the ventral aspect of the abdomen ( Fig. 1 ). The red hourglass is specific to L mactans ; other species have distinctive ventral markings, such as triangles and spots. There is a seasonal variation in the number of black widow bites, starting to increase in spring, peaking in September, and reaching a nadir in January to February.
Black widows are docile and nocturnal and bite when their web is disturbed. The female black widow is generally considered poisonous to humans and is more aggressive if guarding her egg sac. The male black widow spider has smaller jaws with minimal venom production and is not significantly poisonous to humans. These spiders use striated muscles to control the amount of venom they inject, and about 15% of bites do not deliver venom. The venom’s toxicity is due to the presence of α-latrotoxin. This toxin facilitates exocytosis of synaptic vesicles and the release of the neurotransmitters norepinephrine, γ-aminobutyric acid, and acetylcholine. The toxin also causes degeneration of motor end plates, resulting in denervation. The venom destabilizes nerve cell membranes by opening ion channels, causing a large influx of calcium into the cell and depletion of acetylcholine from presynaptic nerve terminals.
Patient evaluation overview
Latrodectism is the clinical syndrome that follows a black widow bite. The bite produces a pinprick sensation that often goes unnoticed. With careful examination, 2 small fang marks may be noticed. Within the first few hours, local irritation develops, including erythema, urticaria, or a characteristic halo-shaped target lesion. These local symptoms may be followed by generalized symptoms of pain and muscle spasms in the chest, abdomen, and lower back. Typically, the pain is concentrated to the chest with upper extremity bites and abdomen with lower extremity envenomation. Abdominal rigidity can be severe and may be mistaken as an acute abdomen. Signs and symptoms in small children are wound erythema, irritability, constant crying, sialorrhea, agitation, and seizures. Victims experience pain on the wound site, muscle spasms, abdominal and thoracic pain, and fine tremors. About one-third of patients will go on to have systemic symptoms. These systemic symptoms include hypertension, sweating, salivation, dyspnea with increased broncho-secretions, and seizures. Less common effects include myocarditis, compartment syndrome of the upper extremity, and priapism. Death is rare from black widow envenomation alone, with no recent cases reported in the US literature, and only a few documented worldwide.
Pharmacologic treatment options
Tetanus immunization should be updated, but antibiotics are unnecessary unless there is evidence of a wound infection. Oral and parenteral analgesics are administered if pain is severe. Muscle spasms may require large doses of benzodiazepines. With administration of these drugs, attention should be given to the patient’s airway status because of the concomitant neurotoxicity of the venom.
Historically, administration of calcium gluconate was considered because of concern for the development of hypocalcemia following black widow envenomation. This practice is currently not advocated, because studies have proven no benefit to the administration of calcium. Likewise, dantrolene administration has not been shown to be clinically efficacious for muscle spasms.
Nonpharmacologic treatment options
Pain at the bite site may be relieved with application of an ice pack. The wound can be cleansed with soap and irrigated with water.
Combination therapies
In extreme cases with severe symptoms, Latrodectus antivenom is recommended. Currently, in North America, the most widely available product is black widow antivenin (Lyovac, Merck). Black widow antivenin is a horse serum–derived product containing immunoglobulin G antibodies to L mactans venom. The dose of antivenom is one vial diluted in 50 mL of normal saline administered intravenously over 15 minutes. A more highly purified equine F(ab) 2 antibody black widow spider antivenom is also under investigation. The use of Latrodectus -specific antivenom is restricted to patients with severe envenomation (eg, seizures, hypertensive crisis, respiratory compromise, or intractable pain), with no allergic contraindications, in whom opioids and benzodiazepines are ineffective. If available, young children and elderly patients with severe toxicity should receive antivenom early in the clinical course.
Treatment resistance/complications
There has traditionally been reluctance to use antivenom because of concern for anaphylaxis. Two reviews of antivenom use in the United States have demonstrated low rates of adverse reactions. There have only been 2 deaths reported after black widow antivenom administration. Patients receiving antivenom may experience flulike symptoms or serum sickness 1 to 3 weeks following treatment. This entity is generally self-limited and responsive to antihistamines and steroids.
Evaluation of outcome and long-term recommendations
In adults, the pain will gradually subside after several hours but may remain for 2 to 3 days. A small child bitten by a black widow spider has a greater chance of morbidity and mortality. As with snake envenomation, the volume of distribution and milligrams per kilograms dose of the venom is relatively larger in children than adults. A dose that may cause painful muscle spasms in an adult may lead to respiratory arrest in a child. Any symptomatic patient who has suffered a bite from a black widow spider should be admitted for observation and pain control. Pregnant patients should undergo fetal monitoring. If there is cardiopulmonary compromise or seizures, the patient should be admitted to the intensive care unit for stabilization and antivenom administration. Symptoms of latrodectism typically last days, but some patients can have intermittent muscle weakness and spasms for weeks.
Brown Recluse Spiders
Introduction
The 6 species of recluse spiders in North America are Loxosceles arizonica , Loxosceles deserta , Loxosceles devia , Loxosceles laeta , Loxosceles rufescens , and Loxosceles recluse . Of these, L recluse is the most common. These spiders are reclusive nocturnal hunters and are active from spring to fall. Victims typically are bitten on the extremities while rummaging in confined spaces such as a closet or an attic, while putting on a boot, or when using a blanket or sleeping bag that a spider is trapped in.
The brown recluse gets its name because of its reclusive nature and brown- or fawn-colored body. It is approximately 1 to 5 cm in length, with a characteristic violin- or fiddle-shaped marking on the dorsal cephalothorax ( Fig. 2 ). They have long, slender legs and 6 eyes rather than eight, which is the norm for other spiders.
The venom of the recluse spider, per volume, is more potent than that of the rattlesnake and can cause extensive skin necrosis. The venom acts directly on cell walls, causing immediate injury and cell death. It contains the calcium-dependent enzyme sphingomyelinase D, which in combination with C-reactive protein has a direct lytic effect on RBCs. The local tissue destructive effects are due to hemolytic enzymes and a levarterenol-like substance that induces vasoconstriction. Following cell wall damage, intravascular coagulation causes a cascade of clotting abnormalities and local polymorphonuclear leukocyte infiltration, culminating in a necrotic ulcer.
Patient evaluation overview
Most brown recluse bites occur in predawn hours and are often painless. The seasonality of brown recluse bites and the geographic area should be considered when making this diagnosis. One study demonstrated that 95% of brown recluse bites occurred between the months of April and October. L reclusa is primarily found in the south central United States. The clinical response to loxoscelism ranges from cutaneous irritation (necrotic arachnidism) to a life-threatening systemic reaction. Most signs and symptoms of envenomation are localized to the bite area. Most (90%) result in nothing more than a local reaction and resolve spontaneously. Within a few hours, the patient experiences itching, swelling, erythema, and tenderness over the bite site. Classically, erythema surrounds a dull, blue-gray macule circumscribed by a ring or halo of pallor. Gradually, over 3 to 4 days, the wound forms a necrotic base with a central black eschar. In 7 to 14 days, the wound develops a full necrotic ulceration. Bites that are in fatty areas, such as the thigh or buttocks, tend to cause more extensive necrosis. Several sources call for stricter diagnostic criteria and claim that the diagnosis of necrotic arachnidism secondary to brown recluse spiders is overreported and often mistaken for skin abscesses (eg, MRSA [methicillin-resistant Staphylococcus aureus ]) and other dermatologic causes.
The systemic reaction, which is less common than the cutaneous reaction, is associated with a higher morbidity. The reaction rarely correlates with the severity of the cutaneous lesion. Within 24 to 72 hours following the envenomation, the patient experiences fever, chills, myalgias, and arthralgias. In severe systemic reactions, the patient may suffer coagulopathies, hypotension, jaundice, disseminated intravascular coagulation (DIC), seizures, renal failure, and hemolytic anemia. In rare cases, a patient may succumb to the systemic reaction.
Hobo Spider
The Hobo spider ( Eratigena agrestis , formerly Tegenaria ), also known as the aggressive house spider, is found in the Pacific Northwest region of the United States and Canada. This spider was traditionally included with Loxosceles species when discussing cases of necrotic arachnidism; however, recent studies are calling this into question. Similar symptoms lead many to incorrectly attribute hobo spider bites to that of the brown recluse, which is less indigenous to the northwest United States. This species is more aggressive and bites with minor provocation. Hobo spiders are brown with gray markings and have a herringbone pattern on the abdomen ( Fig. 3 ).
