Arthropod Bites and Stings

Pathophysiology & Venom Dynamics

Bites vs. Stings

• Bites (blood-feeding arthropods): mosquitoes, fleas, assassin bugs, bedbugs and ticks pierce the skin with their mouthparts (proboscis/chelicerae) to feed on blood.
  • How they cause harm: their saliva contains anticoagulant enzymes, vasodilators and foreign proteins that trigger type I (IgE-mediated) and type IV (T cell-mediated) hypersensitivity.
• Stings (venom-injecting arthropods): bees and wasps (Hymenoptera), spiders, scorpions and centipedes use a stinger or fangs to inject venom directly into soft tissue.
  • How they cause harm: venom is a mix of toxic peptides (melittin, apamin, mastoparan), digestive enzymes (phospholipase A2, hyaluronidase) and chemical mediators (histamine, serotonin) that destroy cells directly and cause intense burning pain and vessel spasm.

Immune and Toxic Reactions in the Skin

• Immediate reaction (early response): within minutes, mast cells release histamine, causing vessels to dilate and leak fluid and producing a short-lived, very itchy wheal.
• Delayed reaction (late-phase response): after 2–6 hours, neutrophils, eosinophils and T cells move in, forming firm red indurated papules that last for days or weeks.
• Direct toxic reaction (direct cytotoxic response): enzymes in brown recluse spider venom (sphingomyelinase D) or centipede venom damage the lining of blood vessels, blocking small vessels and leading to local ischemia and skin necrosis.

Clinical Patterns of Skin Injury

Papular Urticaria (Lichen Urticatus)

• Most common in children and people with an allergic (atopic) tendency.
• Typical lesion: raised red bumps 0.5–2 cm across, often with a tiny central puncture mark (central punctum).
• Distribution: clusters or lines on exposed skin (arms, legs, neck). Intense itching leads to scratching, crusting and chronic lichenified bumps.

Blistering and Necrotic Lesions

• Bullous arthropod reaction: a strong hypersensitivity reaction causes marked swelling of the upper dermis, lifting the epidermis into tense, clear-fluid blisters on inflamed red skin.
• Skin necrosis (necrotic arachnidism / centipede bites): spider or centipede bites can produce a dusky purple center and rapidly spreading swelling, followed by blood-filled blisters and then a sharply demarcated black area of dead skin (a deep, slow-healing ulcer).

Systemic Anaphylaxis

• Most often follows stings from bees and wasps (Vespidae/Apidae) or fire ants (Solenopsis).
• Warning signs: widespread hives, angioedema (swelling of the lips, eyes or throat), wheezing and difficulty breathing from bronchospasm, low blood pressure, sweating, confusion and anaphylactic shock.

Telling Apart Reactions to Common Arthropods

Emergency Care and Management at Haute-Derma

Step 1: Initial Care and Removing the Source

• Removing the stinger (bee stings): lift it out with medical tweezers or scrape it off gently with the edge of a plastic card. Do not squeeze the venom sac, as this pushes more venom into the skin.
• Cleaning the wound: wash the area with a mild antiseptic (chlorhexidine or povidone-iodine) and saline.
• Cold compress: apply a cold pack or wrapped ice for 15–20 minutes every hour to constrict blood vessels, reduce swelling and dull pain signals.

Step 2: Assessing Severity

• We take an allergy history, count the bites or stings and check vital signs (blood pressure, pulse, SpO2, breathing rate).
• We screen for signs of acute anaphylaxis: shortness of breath, hoarseness, difficulty swallowing, dizziness and low blood pressure.

Step 3: Local and Systemic Medical Treatment

• Treatment is matched to the severity of the reaction to control inflammation and itch and to prevent tissue necrosis.

Medical Treatment and Managing Complications

Topical and Oral Medications

• Topical treatments:
  • Medium- to very high-potency topical corticosteroids: hydrocortisone butyrate, betamethasone dipropionate or clobetasol propionate twice daily for 5–7 days to quickly settle inflamed wheals.
  • Soothing lotions: calamine lotion or zinc oxide to help relieve itching.
• Oral medications:
  • Second-generation H1 antihistamines: levocetirizine (5 mg/day), fexofenadine (180 mg/day) or desloratadine (5 mg/day) to control flares of itching and reduce hive-like bumps.
  • Oral corticosteroids: prednisolone (0.5–1 mg/kg/day for 3–5 days) for extensive local reactions, swelling of an entire limb segment, or severe blistering.

Emergency Treatment of Anaphylaxis

• Epinephrine (adrenaline): the first-line, essential drug:
  • Dosing: deep intramuscular injection into the outer mid-thigh.
    • Adults: 0.3–0.5 mg (1:1000 solution).
    • Children: 0.01 mg/kg (maximum 0.3 mg).
  • Repeat after 5–15 minutes if symptoms do not improve.
• Breathing and circulation support: lay the patient flat with legs raised, give oxygen by mask, run Ringer's lactate or 0.9% saline rapidly, and add intravenous methylprednisolone and diphenhydramine.

Recovery and Preventing Dark Marks (PIH)

• Antibiotics for secondary infection: if scratched areas become infected (impetigo, cellulitis), we prescribe mupirocin or fusidic acid cream, or oral cefalexin or amoxicillin-clavulanate.
• Skin repair and fading dark marks: once the acute inflammation has settled, a repair cream with panthenol and madecassoside, combined with brightening actives (niacinamide, azelaic acid), helps prevent post-inflammatory hyperpigmentation (PIH).