Black widow spider bite


  • Identification: red hourglass on otherwise black spider
  • Genus: Latrodectus
Black widow spider


  • α-Latrotoxin envenomation causes massive release of neurotransmitters (norepinephrine, dopamine, acetylcholine, glutamate and GABA) from the nerve terminals → muscle cramps, tachycardia, hypertension
  • This clinical envenomation syndrome is known as latrodectism

Clinical Features


  • Pinprick sensation; then increasing local pain that may spread to entire extremity
  • Erythema appears 20-60 min after the bite
  • Pain begins to abate after several hours and disappears by 2-3d


Grading System

  • Grade 1: Range from no symptoms to local pain at the envenomation site with normal vital signs
  • Grade 2: Involve muscular pain at the envenomation site to migration to the trunk, diaphoresis at bite site and normal vital signs
  • Grade 3: Include grade 2 with abnormal vital signs; diaphoresis distant to envenomation site, generalized myalgia to back, chest and abdomen, nausea vomiting and headaches.

Differential Diagnosis

Envenomations, bites and stings


  • Generally clinical diagnosis


  • Local wound care, analgesia and tetanus prophylaxis are mainstays of treatment
  • Antivenin - indicated for systemic illness only
    • Dosage: Usual dose is one to two vials diluted in 50 to 100 mL 5% dextrose or 0.9% sodium chloride, then infused over 1 hour
    • Use is controversial - Morbidity (pain, cramping and autonomic disturbances) are high, but mortality is low. Antivenin is horse serum derived, and can lead to anaphylaxis and serum sickness
    • Consider for: Pediatric patients, pregnant women, elderly

Not Indicated

  • Routine antibiotic use is not indicated
  • IV calcium gluconate to reduce muscle spasm is an outdated therapy not favored by toxicologists[1]


  • Generally may be discharged
  • Consider admission for:
    • Symptoms of moderate or severe envenomation
    • Pregnant women
    • Children
    • Preexisting cardiovascular disease or hypertension

See Also

External Links


  1. Clark RF et al. Clinical presentation and treatment of black widow spider envenomation: a review of 163 cases. Ann Emerg Med. 1992 Jul;21(7):782-7.