Black widow spider bite


Black widow spider
Classic "hourglass" marking shape on ventral side of L. hesperus.
  • Identification: red hourglass on otherwise black spider. Females tend to be larger and deliver more venom [1]
  • Genus: Latrodectus
  • Black widows live in southern Canada and every U.S. state except Alaska. Bites are more common in warmer months [2]


  • 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; may develop characteristic halo or targetoid lesion
  • 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
  • For high-risk patients (pregnancy, children, older adults w/ cardiovascular issues), rule out peritonitis, UTI, and rhabdomyolysis
    • CBC
    • Creatinine phosphokinase
    • Lactic acid
    • UA


  • 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  • IV calcium gluconate to reduce muscle spasm is an outdated therapy not favored by toxicologists[3]


  • 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. Maretić Z. Latrodectism: variations in clinical manifestations provoked by Latrodectus species of spiders. Toxicon. 1983;21(4):457-466. doi:10.1016/0041-0101(83)90123-x
  2. Isbister GK, Fan HW. Spider bite. Lancet. 2011;378(9808):2039-2047. doi:10.1016/S0140-6736(10)62230-1
  3. 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.