Black widow spider bite

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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]
  • Aptly named because of their sexual cannibalism (but only occurs approximately 2% of the time)[3]
  • Most venomous spiders in North America[4]
    • Venom several times more potent than rattlesnake venom
  • Most people who are envenomated have no serious damage due to the low volume of venom delivered.[5]
  • Rarely bites can be fatal
    • High risk are the young, very old, and the infirm.[6]


  • 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 (category C), elderly

Not Indicated

  • Routine antibiotic use is not indicated
  • IV calcium gluconate to reduce muscle spasm is an outdated therapy not favored by toxicologists[7]


  • 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. Hack, J. (2021). Spider's Secret. ACEP NOW, 40(12), 16–18.
  4. Hack, J. (2021). Spider's Secret. ACEP NOW, 40(12), 16–18.
  5. Hack, J. (2021). Spider's Secret. ACEP NOW, 40(12), 16–18.
  6. Hack, J. (2021). Spider's Secret. ACEP NOW, 40(12), 16–18.
  7. 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.