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Introduction

  1. Pharmacology. To produce the antivenom, horses are hyperimmunized with Latrodectus mactans (black widow spider) venom. The lyophilized protein product from pooled equine sera contains whole-IgG antibodies specific to certain venom fractions as well as residual serum proteins such as albumin and globulins. After intravenous administration, the antivenom distributes widely throughout the body, where it binds to and neutralizes venom. A new F(ab)2 antivenom for black widow spider envenomation has been developed but is not yet approved for use in the United States. The new product may be safer than the presently approved whole-IgG product.
  2. Indications
    1. Black widow envenomation-induced severe hypertension, muscle pain, or cramping unrelieved by muscle relaxants, analgesics, or sedation; particularly in patients at the extremes of age (ie, younger than 1 year or older than 65 years).
    2. Black widow envenomation in pregnancy may cause abdominal muscle spasms severe enough to threaten spontaneous abortion or early onset of labor.
  3. Contraindications. Known hypersensitivity to horse serum.
  4. Adverse effects
    1. Immediate hypersensitivity may rarely occur, including life-threatening anaphylaxis.
    2. Delayed-onset serum sickness may occur after 7-14 days but is rare owing to the small volume of antivenom used in most cases.
    3. Use in pregnancy. FDA Category C (indeterminate). There are no data on teratogenicity. An anaphylactic reaction resulting in shock or hypoxemia in the mother could conceivably adversely affect the fetus (see Table III-1).
  5. Drug or laboratory interactions. No known interactions.
  6. Dosage and method of administration. In most cases, one vial of antivenom is sufficient to treat black widow envenomation in adults or children. The antivenom is dosed based on symptoms, rather than patient weight.
    1. Treat all patients in a monitored setting, such as an emergency department.
    2. Before a skin test or antivenom administration, insert at least one and preferably two secure intravenous lines.
    3. Perform a skin test for horse serum sensitivity by using a 1:10 dilution of antivenom (some experts prefer this method) or the sample of horse serum provided in the antivenom kit (according to package instructions). Do not perform the skin test unless signs of envenomation are present and imminent antivenom therapy is anticipated. If the skin test is positive, consider the need for antivenom versus the adequacy of simple supportive care. Even if the skin test is negative, anaphylaxis may still occur unpredictably.
    4. If antivenom is used in a patient with horse serum sensitivity, pretreat with intravenous diphenhydramine and famotidine or another H2 blocker and have ready at the bedside a preloaded syringe containing epinephrine (1:10,000 for intravenous use) in case of anaphylaxis. Dilute the antivenom 1:10 to 1:1,000 and administer it very slowly.
    5. Reconstitute the lyophilized product to 2.5 mL with the supplied diluent, using gentle swirling for 15-30 minutes to avoid shaking and destroying the immunoglobulins (as indicated by the formation of foam).
    6. Dilute this solution to a total volume of 10-50 mL with normal saline.
    7. Administer the diluted antivenom slowly over 15-30 minutes. One or two vials are sufficient in most cases.