Spider Envenomation

Few of the many spider species harm humans. The medically important groups are Latrodectus (widow spiders, including the black widow, redback and the button spider of southern and eastern Africa), Loxosceles (recluse spiders, including those of southern Africa and the Americas), Atrax and Hadronyche (Australian funnel-web spiders), Phoneutria (Brazilian wandering spiders) and Sicarius (sand spiders).

Mechanisms

• Latrotoxin: pore formation and massive presynaptic release of acetylcholine and catecholamines, causing muscle spasm and autonomic effects
• Sphingomyelinase D (recluse venom): dermonecrosis through inflammatory and complement activation, and systemic hemolysis
• Funnel-web venom: prolongs sodium channel opening and causes autonomic storm
• Wandering spider venom: sodium channel toxins causing severe pain and priapism

Widow bites rarely kill (under 1%), with deaths mostly in the very young, the elderly and those with hypertension. Recluse bites heal in most cases, but about 1 to 5% cause systemic loxoscelism, more often in children. Funnel-web deaths stopped after antivenom was introduced.

Complications: hypertensive crisis and pulmonary edema (widow), necrotic ulcer, hemolytic anemia, disseminated intravascular coagulation and renal failure (recluse), and cardiorespiratory collapse (funnel-web).

• Widow: a pinprick followed by spreading pain over 30 to 120 minutes, cramping of the abdomen, back and chest with a board-like abdomen that mimics an acute abdomen, sweating at the bite site, piloerection, hypertension, tachycardia, restlessness and headache
• Recluse: a painless bite, then at 2 to 8 hours erythema, blistering and a "red, white and blue" pattern, with an eschar and ulcer forming over 3 to 7 days. Systemic: fever, rash, hemoglobinuria, jaundice and renal failure over 24 to 72 hours
• Funnel-web: immediate severe pain with fang marks, fasciculation, sweating, drooling, hypertension then hypotension, pulmonary edema and coma within 10 to 30 minutes
• Wandering spider: severe pain, sweating, drooling and priapism
• Red flags: children, severe hypertension, dark urine, respiratory distress and rapidly spreading necrosis

1. Clinical diagnosis; identify the spider when brought in.
2. Widow and funnel-web: blood pressure, ECG, creatine kinase, electrolytes and troponin.
3. Recluse with systemic features: full blood count, hemolysis markers (LDH, bilirubin, haptoglobin, direct antiglobulin test), creatinine, urinalysis for hemoglobinuria and coagulation studies.
4. Wound culture if secondary infection is suspected.

Differentials: acute abdomen, pancreatitis, renal colic, tetanus, strychnine poisoning, methicillin-resistant Staphylococcus aureus abscess (often mislabelled as a spider bite), cellulitis, pyoderma gangrenosum, anthrax.

1. First aid: wash the wound, apply a cold pack for 10 minutes at a time, and elevate. For funnel-web bites apply a pressure immobilization bandage and keep the patient still. Do not use it for widow or recluse bites. Tetanus toxoid 0.5 mL IM if not immunized.
2. Widow bites:
   • Morphine 0.05 to 0.1 mg/kg IV titrated (adults 2.5 to 5 mg every 10 to 15 minutes), or fentanyl 0.5 to 1 microgram/kg IV.
   • Diazepam 5 to 10 mg IV (children 0.1 to 0.2 mg/kg) for spasm.
   • Antivenom for severe pain, hypertension, pregnancy, children or the elderly: Latrodectus equine antivenom 1 vial (6000 units) IV in 100 mL of saline over 30 minutes, or redback antivenom 2 vials (500 units each) IM, repeated after 2 hours if needed. Have epinephrine 0.5 mg IM ready.
   • Hypertension: nifedipine 10 mg orally, or sodium nitroprusside 0.5 to 10 micrograms/kg/min IV.
3. Recluse bites:
   • Rest, elevation, cold compress and paracetamol 1 g orally every 6 hours.
   • Cefalexin 500 mg orally every 6 hours for 7 days (children 12.5 mg/kg every 6 hours) only for secondary infection.
   • Avoid early excision, and debride after the lesion demarcates at 2 to 6 weeks, with skin grafting if needed.
   • Systemic loxoscelism: IV crystalloid 20 mL/kg, urine output over 1 mL/kg/h, red cell transfusion for severe hemolysis, methylprednisolone 1 mg/kg IV daily for 3 days in the first 24 hours (limited evidence), recluse antivenom (5 ampoules IV) where available and early, and haemodialysis for renal failure. Avoid dapsone, which causes hemolysis in G6PD deficiency.
4. Funnel-web bites: funnel-web antivenom 2 ampoules IV over 15 minutes (125 mg each), repeated after 15 minutes if no improvement, and 4 ampoules for severe envenomation. Intubation and ventilation for pulmonary edema.
5. Wandering spider bites: lidocaine 1% 3 mL infiltrated for pain, antivenom 2 to 4 ampoules IV for severe features, and monitoring.

Observation: 6 hours for widow and wandering spider bites, 24 hours for funnel-web, and 48 to 72 hours for recluse bites with systemic features. Follow-up for wounds at 1 week and 6 weeks.