Etiology
The effects of arthropod bites generally result from traumatic injury, local inflammation, or hypersensitivity reactions to arthropod saliva. Although some arthropods inject venom during biting, most envenomation occurs via a stinger connected to a venom gland. Notable arthropods possessing stingers include bees, wasps, hornets, fire ants, and scorpions. Both bites and stings cause tissue injury, which may serve as a portal of entry for secondary bacterial infection.
Medically significant arthropods belong to 4 major classes as mentioned below.
Chilopoda, which includes centipedes.
Diplopoda, which includes millipedes.
Insecta, which includes members of the order Hymenoptera (such as bees, wasps, hornets, yellow jackets, and fire ants), as well as organisms from several other orders, including mosquitoes, flies, bedbugs, fleas, lice, beetles, caterpillars, moths, and kissing bugs.
Arachnida, which includes spiders, scorpions, mites, chiggers, and ticks.
Class Chilopoda
Centipedes are chilopods characterized by a cephalic biting structure connected to a venom gland. Bites typically produce 2 hemorrhagic punctures with surrounding erythema and swelling. The venom contains biochemical mediators, including metalloproteases, which induce immediate localized pain.[2] Treatment is generally supportive and includes cleansing the area with soap and water, applying ice locally, applying topical corticosteroids, and managing pain with acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs). Cases presenting with severe localized pain may benefit from local infiltration with lidocaine.
Class Diplopoda
Millipedes are members of the class Diplopoda, characterized by the presence of 2 pairs of legs per body segment. Damage from millipedes occurs through the secretion of a toxic liquid from glands located along the sides of their body segments, producing a localized, caustic-like effect on tissues. Clinically, exposure may present with an intense burning sensation, erythema, and, occasionally, vesicle formation. The toxic liquid often results in a localized area of hyperpigmentation, typically brown or black, which may persist for months.
Ocular exposure can produce more severe injury, including chemical conjunctivitis or corneal ulceration. Treatment of topical exposure is similar to that of a second-degree burn and includes topical antibiotics and analgesics. Immediate washing of the affected area with soapy water may help reduce the severity of the toxin’s effects. Management of ocular exposure requires copious irrigation, fluorescein staining to evaluate for corneal ulceration, application of topical antibiotics and cycloplegics, and referral to an ophthalmologist.
Class Insecta
Members of the class Insecta comprise approximately 60% of all arthropod species. This class includes Hymenoptera members (bees, wasps, yellow jackets, hornets, and fire ants), mosquitoes, bed bugs, fleas, flies, lice, beetles, caterpillars, moths, and kissing bugs.[3]
Caterpillars and moths: While most caterpillars and moths are harmless to humans, some cause cutaneous disease when contact with their protective hairs or spines occurs. The most common clinical manifestations of exposure are erythema, pruritus, and the development of papules or urticaria at the site of contact.
In the United States, some of the most severe clinical reactions occur following contact with the puss caterpillar, which is primarily found in the Southeast. Exposure often results in localized burning pain rather than pruritus and leads to the development of hemorrhagic, purpuric papules arranged in a grid-like pattern.
Treatment of caterpillar- and moth-induced cutaneous reactions consists primarily of symptomatic care, including local ice application, control of pruritus with topical corticosteroids and systemic antihistamines, and pain management with NSAIDs or acetaminophen. Any hairs or spines present on the skin may be removed using adhesive tape.
Bedbugs:
Cimex lectularius, the human bedbug, is an obligate blood feeder with a worldwide distribution. Adults are oval-shaped, flat, and red-brown, and measure approximately 3 to 6 mm in length. Bedbugs typically inhabit crevices in walls, floors, mattresses, cushions, bed frames, and similar structures. Bites may occur in linear arrangements and most commonly present as small papules or puncta, usually without surrounding erythema.
Treatment of bites consists primarily of symptomatic care, including the administration of topical glucocorticoids and systemic antihistamines to control pruritus. Secondary infections should be treated with appropriate antibiotics. Elimination of infestations often requires a combination of professionally applied insecticides and nonchemical measures, as complete eradication can be challenging.
Kissing bugs: Kissing bugs, or triatomine bugs, are reduviid insects capable of carrying the parasite Trypanosoma cruzi, which is the causative agent of Chagas disease. Adults typically measure 1.5 to 2.5 cm in length, are brown or black, and typically display red or yellow stripes on the abdomen. Triatomines are nocturnal, blood-feeding insects that transmit T cruzi through their feces.
Kissing bugs are commonly found in Mexico, Central America, and South America, where they inhabit thatched roofs or cracks and crevices in substandard housing. At least 11 species are capable of transmitting Chagas disease in the United States, and they have been identified in at least 31 states. Field collections in the Southern US have detected T cruzi prevalence as high as 60%. Infection occurs when feces containing the parasite are inadvertently scratched or rubbed into the bite wounds or mucous membranes, particularly the eyes or mouth.
Bites are usually painless and present as papules with hemorrhagic puncta or as vesiculobullous lesions. Localized swelling, erythema, and induration at the site of parasite entry form a chagoma. The classic manifestation of acute Chagas disease is a chagoma on the eyelid, referred to as the "Romana sign." Following an acute phase lasting several months, infection progresses to a chronic phase that may remain asymptomatic for decades. Approximately 20% to 30% of infected individuals develop cardiac or gastrointestinal complications.
Lice: Lice are parasitic insects that feed on human blood. Adults are flat, wingless, tan to grayish-white, measure 2.5 to 3.5 mm in length, and possess 6 legs. Three species infest humans: the head louse (Pediculus humanus capitis), the body louse (Pediculus humanus corporis), and the pubic or crab louse (Pthirus pubis). Head and pubic lice are not associated with disease transmission, whereas body lice can transmit typhus and louse-borne relapsing fever.
Head lice are most common in young children attending daycare or elementary school and typically infest the scalp, eyebrows, and eyelashes. The life cycle includes the egg (nit), nymph, and adult stages. Eggs are yellowish and cemented to hair shafts. Nymphs hatch from eggs in approximately 8 days and mature into obligate blood-feeding adults.
Clinical manifestations of head lice infestation include a sensation of movement in the hair, pruritus, and visible nits or lice. Diagnosis is confirmed by identifying eggs, nymphs, or adults on the scalp or hair. Treatment warrants the application of pediculicides, many of which are available over the counter, including pyrethrins and permethrin 1% lotion. Oral ivermectin (administered off-label) at an initial dose of 200 mcg/kg, repeated after 10 days, is often used. Nits should be removed with a fine-toothed comb. As eggs often survive initial treatment, retreatment is typically required 7 to 10 days later.
Pubic lice are smaller than head or body lice, measuring approximately 1 to 2 mm in length. Adults possess 6 legs, with the 2 large front legs resembling crab pinchers. Infestation is typically limited to areas of short hair, such as the pubic region, but lice may occasionally be found on eyelashes, eyebrows, and axillary or beard hair. Pubic lice undergo a life cycle consisting of eggs, nymphs, and adults, similar to head lice.
Transmission usually occurs through sexual contact, making infestations most common in adults. Clinicians should evaluate for other sexually transmitted infections when pubic lice are identified. Clinical manifestations resemble those of head lice, and diagnosis is established by visualizing lice or nits in the affected hair. Treatment is the same as for head lice infestations. Ophthalmic-grade petroleum ointment is recommended to be applied to the eyelid margins for 10 days in cases involving eyelashes.
Unlike head and pubic lice, body lice live and lay their eggs in the seams of clothing or bedding and move to the skin only to feed. Transmission occurs primarily through direct contact but may also be mediated by clothing, bed linens, or towels. In the United States, infestations are most commonly observed in homeless populations.
Body lice produce more intense pruritus than head or pubic lice, secondary to an allergic response to lice saliva. Areas of pyoderma may develop on skin covered by clothing, such as the axilla, trunk, and groin. Diagnosis typically involves identification of eggs and lice in clothing seams rather than on the skin.
Treatment often does not require pediculicide application, as improvements in hygiene, including regular showering and wearing freshly laundered clothing, usually eradicate the infestation. All clothing, bed linens, and towels should be washed in hot water and machine-dried on a high-heat cycle. Systemic antihistamines may alleviate pruritus, and antibiotics may be indicated for secondary infection. When pharmacologic therapy is necessary, agents used for head and pubic lice are appropriate. Body lice pose significant public health risks because they can transmit epidemic typhus, trench fever, and epidemic relapsing fever.[4]
Mosquitoes: Mosquitoes belong to the family Culicidae and are characterized by their compound eyes, delicate wings, long, thin legs, and proboscises, which enable biting and blood-feeding. Mosquito bites typically produce minimal trauma and are often imperceptible to the host. The most common symptom is pruritus, and the most frequent cutaneous finding is the presence of urticarial wheals. Treatment is primarily symptomatic and includes ice application and administration of topical corticosteroids and systemic antihistamines. The most significant threat posed by mosquitoes is their role as vectors for serious diseases, including malaria, filariasis, yellow fever, dengue fever, West Nile virus infection, Zika virus infection, and chikungunya.
Flies: Several medically significant flies bite humans, most notably deer flies, horse flies, sand flies, and tsetse flies. During blood feeding, these flies use specialized mouthparts to lacerate the skin, inject saliva containing anticoagulants, and ingest the resulting blood.
Some species serve as vectors for serious diseases. Tsetse flies transmit trypanosomiasis, sand flies transmit bartonellosis and leishmaniasis, and deer flies transmit tularemia. Certain flies, such as black flies and horse flies, can induce severe allergic reactions, including anaphylaxis.
Fly bites are usually painful and may cause pronounced cutaneous inflammation, leading to large urticarial wheals and papules. Treatment is primarily symptomatic and includes ice application, pain management with acetaminophen or NSAIDs, thorough cleansing of the wound, and pruritus alleviation with systemic antihistamines.
Myiasis is an infestation of fly larvae. Wound myiasis occurs when flies deposit larvae on or near a wound, where the larvae feed on surrounding tissue. Copious irrigation typically removes the larvae, though debridement may be required occasionally.
Furuncular myiasis arises when fly larvae burrow into intact skin. The most notable cause is the human botfly, which is found in the Caribbean and Central and South America. Transmission usually occurs when a female botfly lays eggs on a blood-sucking insect, such as a mosquito. When an insect bites a human, its eggs are deposited on the skin, which eventually hatch, and the larvae burrow beneath the skin, emerging fully mature after 5 to 10 weeks.
Clinically, botfly larvae produce an erythematous papule or nodule with a central punctum, representing the larva’s breathing tube. Although larvae may emerge spontaneously after 5 to 10 weeks, treatment generally involves surgical incision and extraction. Occlusion of the punctum with petroleum jelly or pork fat can also induce larval emergence within 24 hours.[5] Additional management includes updating tetanus immunization, administering antibiotics for secondary infection, and controlling pain with acetaminophen or NSAIDs.
Fleas: Fleas are wingless ectoparasites that feed on mammals and birds. Adults are thin, measure 2 to 4 mm in length, and are red to brown in color.
Flea bites typically present as erythematous papules, often with a central hemorrhagic appearance. Lesions may also manifest as urticarial papules, vesicles, or bullae. Pruritus can be intense, and scratching may lead to excoriation and secondary bacterial infection.
Treatment focuses on controlling pruritus with topical calamine lotion or corticosteroids and with systemic antihistamines. The most significant medical impact of fleas is their role as vectors for serious and potentially fatal diseases, including tularemia, endemic typhus, and bubonic plague.
Order Hymenoptera
The order Hymenoptera includes wasps, bees, yellow jackets, hornets, and fire ants. These insects deliver a painful sting using a venom-injecting caudal stinger. Venom composition is complex, and some venoms have the potential for cross-sensitization. Local reactions are most common and typically present with immediate pain, erythema, and edema at the sting site. Anaphylactic reactions may initially manifest as pruritus, facial flushing, and urticaria, rapidly progressing to wheezing, dyspnea, angioedema, stridor, vomiting, abdominal cramping, and syncope.
The Apoidea family, consisting of honeybees and bumblebees, possesses a stinger with curved barbs that remains embedded in the victim following a sting. Africanized, or “killer,” bees are highly aggressive and were imported to the United States from Brazil in the late 1950s. The current distribution of these bees is primarily in the Southwestern US, particularly Texas, Arizona, and California, with continued northward and eastward migration.
Wasps and yellow jackets: They belong to the Vespidae family. Unlike Apoidea members, these insects do not lose their stingers during an attack and are capable of stinging multiple times.
Treatment of uncomplicated stings includes manual and careful removal of the stinger, application of ice compresses, and use of analgesics. Short courses of systemic antihistamines and corticosteroids are effective for more severe localized reactions.
Fire ants: They are Hymenoptera belonging to the Formicidae family and live in large ground colonies primarily in the Southeastern US. The geographic range of these insects has gradually expanded, reaching parts of California, Maryland, and as far north as Canada. Fire ants may be red or black, are highly aggressive, and often attack in swarms.
During an attack, fire ants latch onto the victim with powerful jaws and deliver up to 10 stings using their ovipositor. The venom contains alkaloids that induce intense, burning pain. Fire ant venom may trigger anaphylaxis and may cause cross-sensitization to other Hymenoptera venoms. Most reactions are localized dermatologic findings similar to stings from the Vespidae and Apoidea families. A characteristic lesion consists of 2 central hemorrhagic puncta from the bite, surrounded by a ring of erythematous papules caused by the stings.[6] These papules progress to vesicles, then to sterile, pruritic pustules over 6 to 24 hours. Treatment is consistent with that of other Hymenoptera stings.
Arachnida
Arachnids are a class of arthropods that includes ticks, mites, scorpions, chiggers, and spiders. These organisms are primarily terrestrial and possess 4 pairs of legs, chelicerae, and a 2-part body plan consisting of the cephalothorax and abdomen.
Ticks: Nymphal and adult ticks possess 8 legs, each tipped with a pair of claws and an oval-shaped body that becomes engorged during feeding. Most ticks are classified as hard ticks (belonging to the Ixodidae family) or soft ticks (belonging to the Argasidae family).
Ticks feed by creating a small opening in the epidermis and injecting saliva containing anticoagulants or platelet aggregation inhibitors. Bites are usually painless and can produce a wide variety of rashes and dermatologic findings, thereby complicating diagnosis. Lesions often appear as an erythematous papule with surrounding erythema, although some bites present as pruritic urticarial lesions.
Ticks are also vectors for infectious diseases. Tick-borne illnesses such as Lyme disease and Rocky Mountain spotted fever may produce characteristic rashes. However, these signs are not always present or easily recognized.
The most significant impact of ticks on humans is their role as vectors for numerous serious diseases, including Rocky Mountain spotted fever, endemic typhus, ehrlichiosis, Q-fever, encephalitis, hemorrhagic fever, Lyme disease, relapsing fever, tularemia, and babesiosis. Bites from certain ticks, particularly the lone star tick, may induce α-gal allergy, a potentially life-threatening immunoglobulin E–mediated hypersensitivity to the oligosaccharide galactose-α-1,3-galactose, which is present in most mammalian meat.
Uncomplicated tick bites are managed with routine wound care, topical corticosteroid application, and systemic antihistamine treatment for pruritic lesions. Antibiotics are indicated if secondary infection occurs. Ticks should be removed using fine-tipped tweezers, grasping the tick as close to the skin as possible and pulling upward with steady, gentle pressure. Methods such as applying fingernail polish, alcohol, or a hot extinguished match are not recommended, as they do not facilitate detachment and may provoke regurgitation, increasing the risk of pathogen transmission.[7]
Mites: Scabies is a skin infestation caused by the 8-legged human itch mite, Sarcoptes scabiei. The mite measures approximately 0.4 mm and burrows into the stratum corneum, where it deposits its eggs. Scabies has a worldwide distribution and is usually transmitted through prolonged direct contact, often sexual.
Initial infections are frequently asymptomatic for 2 to 6 weeks. Symptom development results from an allergic response to mite proteins deposited under the skin. The most common manifestations include intense pruritus, which is often worse at night or upon exposure to warm temperatures, and the presence of erythematous papules or nodules. Tiny burrows created by the tunneling of female mites may appear as raised, linear, or curved grayish-white lines, occasionally containing a black speck representing the female mite. Cutaneous vesicles may also occur, particularly in infants and young children.
Dermatologic findings are most commonly observed in the web spaces of the fingers and toes, the ventral surface of the wrist, the elbows, the back, the buttocks, and the external genitalia. Diagnosis is primarily based on intense pruritus accompanied by a characteristic rash or burrows. Confirmation may be obtained by dermoscopy, confocal microscopy, or microscopic evaluation of skin scrapings for mites, eggs, or fecal material.
Permethrin 5% cream is the drug of choice for scabies. The cream is applied from the neck down at bedtime and washed off the following morning. Two treatments, administered 1 week apart, are recommended. All close contacts should be treated, regardless of whether they have symptoms.
Oral ivermectin, although not approved by the US Food and Drug Administration (FDA) for scabies, is frequently used in patients who have failed topical therapy or cannot tolerate topical medications. Pruritus associated with scabies infestation is managed with systemic antihistamine treatment. All clothing, bedding, and linens should be washed in hot water and dried on a high-heat setting.
Norwegian scabies, also called "crusted scabies," is a severe form of infestation that most often affects older, debilitated, or immunocompromised patients. Unlike in typical scabies cases, these patients present with thick, hyperkeratotic skin, sometimes described as “dirty” in appearance. The Centers for Disease Control and Prevention (CDC) recommends treating crusted scabies with a combination of oral ivermectin, topical permethrin, and a topical keratolytic cream. Although no consensus exists on the optimal frequency and duration of treatment, the CDC provides example regimens commonly followed in clinical practice.
Scorpions: Scorpions are large arachnids with a pair of anterior legs ending in pinchers. The tail-like metasoma contains a stinger and 2 venom glands. In the United States, only the bark scorpion (Centruroides exilicauda) possesses venom capable of causing systemic toxicity.[8]
Most stings produce localized pain similar to Hymenoptera stings. A diagnostic clue is increased sensitivity to touch or tapping on the affected area. Systemic symptoms are uncommon. However, bark scorpion venom can induce autonomic and motor effects, including hypertension, tachycardia, tachydysrhythmias, myoclonus, and fasciculations. These effects can be particularly severe in children.
Diagnosis of scorpion envenomation generally relies on a history of a scorpion sting, residence in or exposure to an endemic region, and characteristic clinical findings. Laboratory studies are usually unnecessary for mild envenomation. In moderate-to-severe cases, evaluation should include serum electrolytes, liver enzymes, creatine kinase, and urinalysis. Additional studies, such as serum lipase, complete blood count, coagulation studies, and electrocardiography, may be required depending on the clinical presentation.
Except in children, most scorpion stings manifest similarly to Hymenoptera stings and are managed with supportive care. Management includes cleaning the site with soap and water, applying ice, removing the stinger if present, and initiating pain control with acetaminophen. Agitation, muscle spasms, and myoclonus are treated with benzodiazepines, while tachyarrhythmias and hypertension are managed with intravenous β-blocker therapy. An FDA-approved antivenom specific for Centruroides is available, but reserved for cases of severe systemic toxicity.
Spiders: Spiders are carnivorous arthropods that use venom to immobilize and, in some cases, digest prey. In North America, the 2 species with the greatest potential to cause significant morbidity are the black widow and brown recluse.
Brown recluse spiders (Loxosceles reclusa) measure approximately 1 to 1.5 cm in length, with a leg span exceeding 2.5 cm. The cephalothorax color ranges from yellow to brown, and the tan abdomen possesses a violin-shaped marking on the dorsal cephalothorax, giving rise to the nickname “fiddle-back” spider. Brown recluse spiders are predominantly found in the Central and Southern US and inhabit dark, dry locations such as woodpiles, sheds, closets, and garages.
Bites typically occur on extremities when the spider’s dwelling is disturbed or the organism feels threatened. Most bites are minor and may present as small erythematous lesions, sometimes perceived as a sharp, stinging sensation, but often painless. Some bites produce cyanosis or pallor, occasionally with hemorrhagic blisters due to tissue ischemia. Severe envenomations may result in full-thickness skin necrosis, requiring extensive debridement and skin grafting.
Brown recluse venom is highly complex and contains hemolytic enzymes that can cause tissue destruction and necrosis. Diagnosis of uncomplicated bites can be challenging because the initial bite is often painless, and the spider may not be observed. Diagnosis is typically based on patient history, particularly if the spider was identified, along with characteristic dermatologic findings.
Management of brown recluse envenomation depends on clinical presentation. Uncomplicated bites are treated with routine wound care, assessment of tetanus immunization status, and local ice application, which may reduce the activity of tissue-damaging enzymes in the venom. Early surgical excision is not recommended in cases of necrotic ulceration, as it can lead to recurrent wound breakdown, delayed healing, scarring, and long-term distal extremity dysfunction.
Black widow spiders belong to the genus Latrodectus, with the North American black widow (Latrodectus mactans) being the most well-known. Five Latrodectus species are found in North America, of which 3 are black. These spiders measure approximately 1.5 cm in body length, with a leg span up to 4 cm. Black widows are dark brown or black with a rounded, shiny abdomen and are most recognizable by the red or orange hourglass marking on the ventral surface of the abdomen.
Black widows inhabit the United States and typically build webs in dark, confined spaces such as woodpiles, basements, crawl spaces, attics, and stored boxes. Most bites occur defensively, when a female spider perceives a threat to herself or her eggs, or an arthropod is unintentionally disturbed. Although black widow venom has the potential to cause serious toxicity, many bites produce only minimal symptoms and rarely result in severe damage.
A black widow bite is typically accompanied by a sharp, pinprick-like sensation, which may progress to a dull ache or numbness at the site. Two red puncta may be visible, and surrounding erythema often develops within 60 minutes. Severe envenomation may manifest as intense muscle spasms and pain in the chest, abdomen, and lower back. Additional clinical findings can include hypertension, diaphoresis, salivation, restlessness, fasciculations, ptosis, nausea, vomiting, and dyspnea. Severe symptoms generally occur within 1 to 6 hours and persist for 12 to 48 hours.
Black widow venom contains the potent neurotoxin α-latrotoxin, which, unlike brown recluse venom, does not cause local tissue necrosis. Management of bites without systemic involvement consists of supportive care, including wound cleansing, ice application, tetanus immunization updates, and pain control with acetaminophen. Muscle spasms and severe pain are treated with benzodiazepines and opioids as needed. Latrodectus-specific antivenom, derived from horse serum, is available but reserved for patients with significant systemic toxicity.
Chiggers: Chiggers are tiny red mite larvae, measuring 0.3 to 1.0 mm in length, that belong to the Thrombiculidae family. Encounters typically occur in tall grasses, weeds, and woodlands. Infestations arise when larvae feed on human skin, predominantly in areas constrained by clothing, such as the ankles, thighs, or waist.
Bites are usually not felt initially. An allergic reaction to mite saliva produces intensely pruritic red papules 3 to 14 hours after exposure. Mites appear as tiny red dots on the skin and often crawl until reaching a clothing-skin interface, such as the top of a sock or the belt line of pants. If present, chiggers may be removed during bathing by scrubbing the skin with soap and water.
Management focuses on controlling pruritus with topical calamine lotion, corticosteroids, and systemic antihistamines. Chiggers serve as vectors for Orientia tsutsugamushi, the bacterial cause of scrub typhus in parts of Asia, Russia, and islands of the Indian and Pacific Oceans. Although scrub typhus transmission from chiggers has not been documented in the United States, Orientia bacteria were identified in chiggers in North Carolina in 2022.
