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Arthropod Bites and Stings

Basics

  • Arthropods include insects (bees, wasps, mosquitoes, lice) and arachnids (spiders, scorpions, ticks).
  • Transmission: venom, microorganisms, irritants through bites or stings; invasion of tissue; or allergic reactions.
  • Sequelae: local redness, itching, pain, swelling; large local reactions; systemic reactions (anaphylaxis, neurotoxicity, organ damage); tissue necrosis; secondary infection.

Epidemiology

  • ~1 million ED visits annually in US.
  • Wide regional and seasonal variation.

Etiology and Pathophysiology

  • Pathophysiologic effects:
  • Toxic (local and systemic venom effects).
  • Allergic (immune-mediated inflammation, anaphylaxis).
  • Infectious (arthropod-borne diseases).
  • Traumatic (mechanical injury).
  • Family history of atopy may predispose to severe allergies.

Risk Factors

  • Previous sensitization.
  • Activities or occupations with arthropod exposure.
  • Young, elderly, immunocompromised, chronic cardiac/respiratory disease.
  • Mastocytosis increases risk of severe Hymenoptera sting reactions.

Prevention

  • Avoid arthropod habitats.
  • Use repellents:
  • DEET (20-35%): broad spectrum, safe >2 months age and pregnancy.
  • Picaridin 20% spray: comparable to DEET.
  • PMD (lemon eucalyptus) 30%: 4-5 hrs protection, not for <3 years.
  • Barrier methods: light-colored clothing, permethrin-treated clothing, bed nets.
  • Tick removal within 24 hrs reduces disease risk.

Diagnosis

History

  • Sudden pain or itching after bite/sting or exposure.
  • Unknown exposure common in bed bugs, lice, scabies, ticks.
  • Consider travel, occupational, social history.

Physical Exam

  • Remove stinger if present (flick/scrape).
  • Anaphylaxis signs: flushing, urticaria, angioedema, respiratory distress, hypotension.
  • Local findings: erythema, swelling, wheals, papules, bullae.
  • Assess for infestation (lice, scabies).
  • Signs of secondary infection: erythema, pain, fever, lymphangitis, abscess.

Differential Diagnosis

  • Contact dermatitis, drug eruption, mastocytosis.
  • Bullous diseases, viral exanthem, cellulitis, abscess.
  • Anaphylaxis, angioedema, urticarial vasculitis.

Diagnostic Tests

  • Usually not needed.
  • Labs in severe envenomation: CBC, CMP, coagulation studies.
  • Monitor for arthropod-borne diseases depending on exposure.
  • Allergy testing and serum tryptase in anaphylaxis follow-up.

Treatment

General Measures

  • Symptomatic relief: local wound care, ice, topical steroids, antihistamines, analgesics.

First Line

  • Anaphylaxis:
  • Epinephrine IM (0.3-0.5 mg adults, 0.01 mg/kg pediatrics).
  • Oxygen, IV fluids.
  • Antihistamines and steroids as adjuncts (not substitutes).
  • Non-anaphylaxis bites/stings:
  • Wound cleansing.
  • Antibiotics if infected.
  • Tetanus prophylaxis.
  • Oral antihistamines (diphenhydramine, cetirizine).
  • Short course oral steroids for severe pruritus.
  • Topical steroids 3-5 days.

Specific Therapies

  • Scorpion stings: manage catecholamine excess (prazosin, nitroprusside, Ξ²-blockers), atropine for hypersalivation, antivenom in severe cases.
  • Black widow bites: muscle spasm control (benzodiazepines, opioids), antivenom for severe symptoms.
  • Fire ants: sterile pustules; do not open or drain.
  • Brown recluse spider: local care, tetanus prophylaxis, pain control, surgical consult if needed.
  • Ticks: early removal to reduce disease.
  • Pediculosis: permethrin 1%, alternatives include pyrethrins, ivermectin.
  • Scabies: permethrin 5% cream, ivermectin orally.

Issues for Referral

  • Anaphylaxis or systemic symptoms require allergist referral for testing.

Surgery/Procedures

  • Debridement and delayed skin grafting for severe brown recluse bites.

Complementary & Alternative Medicine

  • Ice, cool compresses.
  • Calamine lotion common but unproven.
  • Baking soda paste (3 tsp baking soda + 1 tsp water) may soothe bites.

Admission Criteria

  • Anaphylaxis.
  • Vascular instability.
  • Neuromuscular or GI symptoms.
  • Renal damage/failure.

Ongoing Care

  • Venom immunotherapy is highly effective for Hymenoptera sting anaphylaxis prevention.
  • Monitor for delayed infectious or immune-mediated complications.

Patient Education

  • Symptomatic care and avoidance of arthropods.
  • Instruction on epinephrine use and anaphylaxis action plan.
  • Vector-borne disease surveillance.

Prognosis

  • Excellent for local/mild reactions.
  • Immediate epinephrine reduces mortality in anaphylaxis.

Complications

  • Anaphylaxis.
  • Secondary bacterial infection.
  • Arthropod-transmitted infections.
  • Scarring, phobias.

ICD10 Codes

  • T63.481A Toxic effect of arthropod venom, accidental initial encounter
  • T63.301A Toxic effect of unspider venom, accidental initial encounter
  • T63.484A Toxic effect of other arthropod venom, undetermined initial encounter

Clinical Pearls

  • Urgent epinephrine administration is key to treating anaphylaxis.
  • Local treatment suffices for most bites and stings.
  • Tick-borne illnesses are increasing in the US; early tick removal is critical.