Several pathophysiologic mechanisms cause the clinical symptoms seen with arthropod bites and stings. Mechanical injury during the bite or sting results in local pain and swelling. Even when other symptoms are minimal, breakage of the skin can serve as a portal of entry for bacteria and viruses that can result in a secondary infection. Arthropod saliva often aids in digestion and inhibition of blood coagulation. It may also increase blood flow to the region and anesthetize the bite site. Most responses to insect bites and stings result from the victim’s immune response to these secretions.¹ Some insects, such as bees, inject venom through a stinger that can cause allergic reactions in the skin, the respiratory tract, the cardiovascular system, and the gastrointestinal system. These are due to the presence of multiple protein allergens that possess enzymatic activity. Arthropods can also transmit pathogenic microorganisms to their vertebrate host through a bite or with contaminated feces. These, in turn, can cause a vector-borne disease.³

Arthropod bites and stings typically cause cutaneous traumatic injury or local inflammatory and hypersensitivity reactions to the arthropod saliva or stinger venom. Clinically, bites and stings typically manifest as erythematous and edematous papules and urticaria. When associated with toxic venom, significant allergic and systemic reactions can follow, including autonomic instability, neurotoxicity, angioedema, anaphylaxis, organ failure, or circulatory collapse.¹ Arthropod bites can also serve as vectors for bacterial, viral, and protozoal diseases.

Ophthalmic manifestations from arthropod bites or stings are varied, depending on the type of organism and the location of the injury. Stings and bites to the eyelids and periorbital skin are usually associated with pain, erythema, edema, and pruritus (Figure 51.1).⁴,⁵,⁶ Eyelid necrosis,⁷ reactive blepharospasm,⁸ and orbital inflammatory syndrome⁹ have all been associated with ocular or eyelid insect bites and stings. Conjunctival arthropod bites and stings are uncommon but have been described from bee stings, wasps, and ants. Symptoms include pain, chemosis, hyperemia, epiphora, photophobia, and foreign body sensation.¹⁰,¹¹,¹²,¹³

Centipede bites inject a venom containing histamine, serotonin, enzymes, acid, alkaline phosphatase, and naphthylamidase.¹ Contact with skin most commonly results in intense localized pain, erythema, and edema that produces two hemorrhagic punctures accompanied by pain.¹⁴,¹⁵ Millipedes inflict damage through the secretion of a toxic liquid from glands on the sides of their body segments. These secretions contain hydrogen cyanide, organic acids, cresol, phenol, benzoquinones, and hydroquinones.¹ The toxin produces a localized caustic effect resulting in intense burning accompanied by erythema and occasionally vesicle formation.¹⁶

Caterpillars contain tiny hairs (setae) covered by microscopic barbs or spines on their body.¹⁷ The setae can blow in the wind and land on the skin, in the eyes, and on clothing, sometimes causing allergic reactions.¹⁶ Several caterpillar species contain a toxin and can cause poisoning. When the setae or spines contact human skin, they can cause pain, pruritus, and hemorrhagic purpuric papules at the site of contact. When they land on the eyelids or conjunctiva (Figure 51.2), setae can result in severe ocular surface inflammation.¹⁸,¹⁹,²⁰,²¹,²²,²³ Rarely, they may become impacted in the meibomian gland orifices (Figure 51.3),²⁴ or even penetrate the cornea (Figure 51.4) or extend through it into the anterior chamber.²⁵ Iritis, vitritis, and papillitis are uncommon but well-documented ocular complications from caterpillar hairs.²⁰,²¹,²⁶,²⁷

Spiders are carnivorous arthropods that use venom to immobilize their prey. The two spiders with the greatest potential to cause significant morbidity in humans in the
United States are the brown recluse (Loxosceles reclusa) and black widow (Latrodectus mactans).²⁸ Brown recluse spiders have a yellow to brown cephalothorax and a tan abdomen and possess a violin-shaped marking on their dorsal cephalothorax. They are found predominantly in the South and the Central United States, where they reside in dark, dry places. They usually bite on the extremities when disturbed and bites often cause a small painless, erythematous lesion. Brown recluse venom contains hemolytic enzymes that can cause ischemia with extensive tissue destruction and full-thickness skin necrosis (Figure 51.5).¹⁴,²⁹,³⁰ Black widow spiders are dark brown or black with a rounded, shiny abdomen and have a red or orange hourglass on the ventral surface of their abdomen. With a bite, there is usually a dull ache or numbness at the puncture site and two puncture wounds are surrounded by erythema that may appear within 60 minutes. Black widow venom contains the potent neurotoxin, alpha-latrotoxin, which causes the release of excessive amounts of neurotransmitters, including acetylcholine, norepinephrine, glutamate, and dopamine from presynaptic nerve terminals. It can cause significant muscle pain, diaphoresis, tachycardia, flushing, hypertension, ptosis, nausea, vomiting, dyspnea, and rarely death.¹,³¹ Unlike the brown recluse, the venom of the black widow does not cause local necrosis.

Wasps, bees, yellow jackets, hornets, and fire ants are included in the insect order of Hymenoptera. They all have a stinger that can deliver venom containing bradykinin, acetylcholine, histamine, and serotonin that are responsible for most of the clinical toxicity.¹ Local reaction to stings typically presents with the immediate onset of pain, erythema, and edema. Severe reactions can occur with pruritus, facial flushing, and urticaria progressing to respiratory distress, angioedema, vomiting, abdominal cramping, and syncope. Some honeybees and bumblebees possess a stinger with curved barbs that remain in the injection site following a sting. Wasps and yellow jackets do not lose their stinger during the sting so they can sting multiple times. Ocular bee
stings most often involve the cornea, and less frequently the conjunctiva or the eyelid.¹²,³²,³³,³⁴ Symptoms are usually mild but rarely can result in severe visual symptoms including intraocular inflammation, marked loss of vision, and optic neuritis.³⁵,³⁶,³⁷,³⁸ Rare cases of anaphylaxis, renal failure, and death have been reported.³⁹

Mosquitoes are members of the fly family and live in grass and bushes located close to areas where humans live and work. They are considered the deadliest animals on Earth because they harbor and can transmit a highly diverse microbiota, including bacteria, fungi, and viruses.⁴⁰ Male mosquitos do not bite humans, but females do. They have a long tubular proboscis for biting to feed on blood. During the bite, saliva is injected while siphoning the blood. The saliva contains proteins that cause an allergic reaction in most individuals.⁴¹ The most common symptoms are urticarial wheals and pruritus. The greatest danger from mosquitos is their ability to transmit several serious diseases including malaria, filariasis, yellow fever, dengue fever, West Nile virus, Zika virus, and chikungunya.⁴²,⁴³

Fleas are ectoparasites that feed on mammals and birds. They do not have wings but jump from one organism to another. The life cycle of the flea consists of egg, larva, pupa, and biting adult, and the adult flea feeds on blood. Fleas can transmit several infections such as rickettsiosis, bartonellosis, typhus, and tungiasis.⁴⁴,⁴⁵ Flea bites typically present as erythematous papules, or urticarial vesicles or bullae, sometimes with a hemorrhagic center and associated with severe pruritus. Scratching can result in secondary bacterial infection. A case of orbital inflammatory disease from a flea beneath the eyelid was reported.⁹

Ticks feed by cutting a hole in the epidermis and injecting an anticoagulant agent to inhibit platelet aggregation.¹ Tick bites are usually painless, presenting as an erythematous papule or a pruritic urticarial rash. The most important possible consequence of tick bites is their ability to serve as vectors for diseases such as Rocky Mountain spotted fever, endemic typhus, Q-fever, hemorrhagic fever, Lyme disease, relapsing fever, babesiosis, ehrlichiosis, and tularemia.⁴⁶

Lice are parasitic insects that also feed on human blood, injecting their saliva into the skin. Three types of lice feed on humans: the head louse, the body louse, and the pubic or crab louse (Pthirus pubis). Head and pubic lice do not transmit disease, but body lice can transmit typhus and louse-borne relapsing fever.¹ Head lice occur most frequently in younger children, and because these insects are unable to hop, jump, or fly, they are spread through direct head-to-head contact. Transmission on environmental surfaces such as combs and towels is less common since lice do not survive off the scalp for more than about 1 day.⁴⁷ They are typically seen on the head, eyebrows, and eyelashes and exist as adults, nymphs, and eggs (or nits) clinging to hair shafts. Symptoms can result from hypersensitivity reactions manifesting as diffuse eczematous dermatitis or diffuse skin-colored papules.⁴⁸,⁴⁹ Secondary bacterial infection with regional lymphadenopathy can complicate the clinical picture.⁵⁰ Pubic lice are generally limited to areas with short hair, such as pubic hair, but may occasionally be found on eyelashes, eyebrows, and axillary and beard hair.¹ Transmission is usually through sexual contact and occurs most frequently in adults. Body lice live and lay their eggs in seams of clothing or bedding, moving to the skin only to feed. They produce more severe pruritus from an allergic response to the lice saliva.