Arthropod Bites and Stings

Arthropods, comprised of insects and arachnids, account for up to 1 million emergency department visits annually in the United States.¹ Arthropods' most significant effects on humans are as vectors for infectious diseases, and direct effects of their bites and stings are typically only a self-limited nuisance (Table 1^2–5 ). Local effects can be treated symptomatically with topical or oral antihistamines, calamine lotion, topical corticosteroids, cold compresses, or, in severe cases, systemic glucocorticoids. There are limited data to support one treatment over another.

Spider bites are blamed for many necrotic wounds, but most spiders are harmless to humans. Many suspected bites are caused by other conditions such as Staphylococcus aureus infection.⁶ Spider bites typically present as a solitary papule, pustule, or wheal, and require only local wound care, analgesics, and tetanus vaccine prophylaxis. The two spiders of most medical significance in the United States are widow (Latrodectus) and recluse spiders (Loxosceles).

WIDOW SPIDERS

Widow spiders include more than 30 species with worldwide distribution. They are medium-sized spiders (up to 4 cm), characterized by shiny, dark-colored bodies with ventral red or yellow abdominal demarcations² (Figure 1). The black widow (Latrodectus mactans) is the most predominant of the five widow spiders in the United States. Widows are rarely found inside the home and are typically encountered in shady enclosed spaces outdoors (e.g., sheds, yard debris, gardening equipment). Most bites do not result in systemic envenomation, referred to as latrodectism, characterized by excess acetylcholine release.⁷ When present, symptoms of latrodectism include muscle spasm and diaphoresis starting at the involved extremity and migrating proximally. Patients can present with nausea, tachycardia, hypertension, restlessness, and, rarely, severe abdominal rigidity or chest pain, which can be mistaken for peritonitis or myocardial infarction.² Infants and children typically present as inconsolable with generalized erythema and excessive drooling. Symptoms typically come in waves and continue for 48 to 72 hours but are rarely life-threatening.^7,8

Envenomation severity can be graded, which helps determine treatment⁸ (Table 2^2,7,8 ). Mild presentations can be treated with nonopioid oral pain medications, but opioids are sometimes indicated for poorly controlled pain. Benzodiazepines are widely recommended to treat painful muscle spasms; however, no trial data support their use.^2,8 Calcium and magnesium have demonstrated no benefit and are not recommended.^2,8 Widow spider bites are rarely life-threatening, and the use of antivenom is controversial. Limited data have demonstrated decreased pain duration with antivenom, balanced with a risk of allergic reaction in up to 5% of patients.^7–9 Antivenom is not widely available in the United States and is generally only considered in patients with severe symptoms not responsive to supportive care.

RECLUSE SPIDERS

Recluse spiders are found predominantly in the Southwest and southern Midwest and are best known for the brown recluse (Loxosceles reclusa), which are small (2 cm or less) brown spiders with a dark violin-shaped pattern on the anterior thorax (Figure 2). Many harmless spiders look similar and are frequently misidentified as brown recluses. Recluses are most commonly encountered indoors in dark, quiet areas such as furniture, clothes, and bedsheets. Recluse spiders are not aggressive by nature and only bite when antagonized, usually on the trunk if the patient inadvertently rolls onto or traps the spider in clothes. Bites typically resolve in one week or less, but an estimated 10% become necrotic after 24 to 48 hours, which may require weeks to months to heal and can result in scarring. Brown recluse bites are distinguished from other spider bites by the presence of a single, flat lesion without associated swelling.¹⁰

Recluse bites are generally managed conservatively, with most studied therapies (including dapsone, corticosteroids, antihistamines, early surgical excision, and hyperbaric oxygen) shown to be ineffective or cause harm.^2,11 Good wound care with soap and water and tetanus vaccine prophylaxis (if indicated) are recommended in all cases. Antibiotics are not indicated unless evidence of a secondary infection is present. Antivenom is not available in the United States and is not recommended. A smaller number of patients may require surgical revision of the resulting scar after necrosis.

Loxosceles venom can cause hemolysis, which is more common in children and not predicted by the extent of cutaneous involvement. Life-threatening complications such as angioedema, renal failure, rhabdomyolysis, and death are rare.¹² Patients with signs of systemic loxoscelism should be admitted and treated supportively.

Scorpions are arachnids that envenomate their prey through stings using their tails. Most scorpion stings are associated with self-limited pain, and fatalities are rare in the developed world. Stings typically occur on the extremities because the scorpions are cornered in hidden places or inside shoes and crevices.¹³ Stings are immediately painful and often associated with local paresthesias. The site is often difficult to identify, although local erythema, edema, and muscle fasciculations are common. Severe systemic envenomation occurs in less than 5% of stings and is more common in children. Centruroides (including bark and striped scorpions) are the only scorpions in the United States associated with severe envenomation.¹³ The severity of envenomation determines therapy, and multiple grading scales have been developed (Table 3^14–18 ). More severe envenomations are characterized by cranial nerve abnormalities, somatic neuromuscular dysfunction, and autonomic dysfunction. In the most severe cases, airway compromise, respiratory arrest, pulmonary edema, myocardial infarction, cardiac dysrhythmias, and pancreatitis can result. Cardiomyopathy can occur in severe systemic envenomation secondary to catecholamine surge and is clinically similar to takotsubo cardiomyopathy.

Treatment is primarily supportive, ranging from local care in mild cases to sedation, ventilator support, and inotropic agents in the most severe systemic envenomations. Centruroides antivenom is available in the United States. Limited data suggest it can shorten recovery times, but a cost-effectiveness analysis supports its use only in severe envenomation cases.^19,20

The Hymenoptera order consists of more than 100,000 species. The three clinically relevant groups capable of stinging are Apidae (bees), Vespidae (wasps, yellow jackets, hornets), and Formicidae (ants)^21–23 (Table 4^22,24,25 ). Generally, stings occur by accidental contact or proximity to a disrupted nest.²¹ Four categories of Hymenoptera sting reactions are described, with distinct clinical findings and recommended treatment¹⁴ (Table 5^21–25 ).

Hymenoptera envenomation causes an average of 62 deaths each year; however, most stings are self-limited and treated supportively.²⁶ Most deaths are caused by anaphylaxis and rarely massive envenomation. The risk of severe systemic reaction is highest in older adults, small children, those receiving multiple stings, and when there is a short time to symptom onset.^23,24

Patients with a history of systemic allergic reaction to an insect sting have an approximately 50% risk of recurrent systemic reaction and should be evaluated with skin testing and, if positive, treated with venom immunotherapy, which can decrease the risk to less than 3%.^27,28 Figure 3 outlines the recommended approach for patients with a history of allergic reactions to insect stings.²⁷

APIDAE (HONEYBEES, BUMBLEBEES)

The Apis (honeybee) and Bombus (bumblebee) genera are the most commonly encountered bees. They are typically docile and only sting when threatened or provoked. When a honeybee stings, it leaves the stinger in the skin and dies shortly after. Bumblebee stingers do not detach, permitting multiple stings.

If identified, the barbed stinger should be removed as quickly as possible to avoid additional envenomation, although most venom is injected within the first 60 seconds (Table 6²³ ). Foreign body granulomas, epidermoid cysts, or secondary infection may occur if the stinger is left behind.²³

VESPIDAE (WASPS, YELLOW JACKETS, HORNETS)

Relative to the Apidae, wasps, yellow jackets, and hornets tend to be more aggressive and are universally capable of multiple stings.^22,24 Yellow jacket stings tend to peak in the summer and autumn when populations are highest, and they characteristically swarm, making multiple stings common²⁴ (Figure 4).

The spectrum of possible reactions to vespid stings is similar to the Apidae family, except for hornets. Lower doses of venom may be required to reach lethal levels for hornet stings, and hornets tend to deliver more venom per sting. Multi-organ failure and death are more commonly associated with hornets relative to other Hymenoptera.²⁴

FORMICIDAE (STINGING ANTS)

The Solenopsis species (fire ants) is the most commonly encountered stinging ant in North America. Most fire ants are 3 mm to 8 mm long with reddish-brown or black bodies and are known to swarm and sting the lower extremity in clusters (Figure 5). Immediate- and delayed-hypersensitivity reactions occur with stings.

The biting Diptera include mosquitoes, blackflies, phlebotomine sandflies, tsetse flies, biting midges, horseflies, and stable flies. Diptera most significantly affect humans as vectors for infectious diseases. The mosquito is the most clinically relevant (Figure 6), but each biting Diptera can serve as a vector.

Some may experience an allergic reaction to the components of mosquito saliva, called Skeeter syndrome. Chronic Epstein-Barr virus infection and other lymphoproliferative syndromes have been associated with a hypersensitivity reaction to mosquito bites that include high fever, bullae, and skin necrosis.²⁹

Bedbugs (Cimex lectularius and Cimex hemipterus) are increasingly prevalent parasites that readily feed on humans and mammals. Bedbugs have ovoid-shaped 5-mm long bodies with yellow or red-brown coloration. Bedbugs typically feed at night and are commonly found on mattresses, box springs, curtains, and bedding to avoid light. Bites generally cause self-limited cutaneous reactions.^30,31 A previous American Family Physician article describes the identification and management of bedbugs (https://www.aafp.org/afp/2012/1001/p653.html).

Mites (including chiggers) are insects in their larval form and less than 1 mm long that commonly bite humans. Sarcoptes scabiei can cause a persistent infestation in humans known as scabies (https://www.aafp.org/afp/2019/0515/p635.html). Although infected chiggers can transmit scrub typhus (a vector-borne zoonosis) in South Asia, this is not endemic to the United States.³²

Fleas are small (less than 5 mm), wingless, jumping insects that feed on human and animal blood (Figure 7). In the United States, the cat flea (Ctenocephalides felis) and the squirrel flea (Oropsylla montana) are the most common and are vectors for the plague, cat-scratch fever, and flea-borne (murine) typhus.³³

Many ticks in the United States (e.g., Ixodes, Amblyomma, Dermacentor, Rhipicephalus) are vectors for significant human diseases.³ Transmission of pathogens by a tick bite is time-dependent, making prompt removal of the tick important.³⁴ The pathology and management of tickborne diseases are described in a previous American Family Physician article (https://www.aafp.org/afp/2020/0501/p530.html).

Arthropod bite and sting prevention strategies include avoiding high-risk areas (i.e., standing water, dense vegetation), minimizing outdoor activity at times of peak activity (i.e., dawn, dusk), personal protection strategies, and area control methods.^35,36

To decrease the risk of bites, individuals should wear light-colored clothing and cover all exposed skin, especially at high-risk times of the day. Pyrethroid (e.g., permethrin 4%) impregnated fabrics confer even more protection and are safe when used according to instructions.^35,36

Topical skin repellents work by stimulating avoidance behaviors or blocking receptors rather than direct toxicity.^35–37 N,N-diethyl-m-toluamide (DEET; 20% to 50%) is the most highly studied repellent with the broadest spectrum of protection (Diptera, chiggers, fleas, ticks). DEET is safe for use in people older than two months, during pregnancy (after the first trimester), and when lactating. Concentrations of 20% to 35% offer the best balance of effectiveness and safety with approximately five hours of protection.^35–38 Para-menthane-3,8-diol is a plant-based repellent derived from lemon eucalyptus with inconsistent and limited data suggesting similar effectiveness to DEET.^35,36 Icaridin is odorless and nonoily and has similar effectiveness to DEET with less durability.^35,37 Insect Repellant 3535 is a new product that offers protection against mosquito bites; however, safety data are lacking for use in children or people who are pregnant or lactating.³⁷

This article updates a previous article on this topic by Juckett.³⁹

Data Sources: A PubMed search was completed using Clinical Queries and medical subject headings with key terms spider bite; spider envenomation; scorpion sting; scorpion envenomation; hymenoptera stings; hymenoptera envenomation; hymenoptera venom allergy; hymenoptera venom immunotherapy; mosquito bites; biting diptera; flea bites; mites; chiggers; fire ants bites; bedbugs; insect prevention; arthropod prevention. The search included systematic reviews, meta-analyses, randomized controlled trials, clinical trials, significant case series, and reviews. Also searched were Essential Evidence Plus, Trip database, the Cochrane database, UpToDate, and the Centers for Disease Control and Prevention. Search dates: April 11 to 28, 2021; May 26, 2021; June 19, 2021; and Mar 9, 2022.

Figure 1, Figure 2, Figure 4, Figure 5, Figure 6, and Figure 7 were provided by the Centers for Disease Control and Prevention’s Public Health Image Library, Atlanta, Ga.

The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the U.S. Air Force, the U.S. Department of Defense, or the U.S. government.