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Introduction

  1. Pharmacology. To produce the antivenom, horses are hyperimmunized with venom from four species of Centruroides scorpions (C. noxius, C.l. limpidus, C.l. tecomanus, and C.s. suffuses). The equine scorpion antibodies are cleaved with pepsin to form F(ab')2 fragments. After intravenous administration, the antivenom distributes widely throughout the body, where it binds to venom.
  2. Indications. Clinical signs of serious Centruroides scorpion envenomation, such as loss of muscle control, severe pain, roving or abnormal eye movements, slurred speech, respiratory distress, excessive salivation, or intractable vomiting.
  3. Contraindications. Although the package insert does not list any contraindications, known hypersensitivity to horse serum or horses may predispose patients to anaphylaxis after administration of equine-derived antivenom. Previous exposure to horse serum products such as antivenom could also potentially predispose to allergic reactions.
  4. Adverse effects
    1. Immediate hypersensitivity may rarely occur, including life-threatening anaphylaxis.
    2. Delayed-onset serum sickness may occur but is less likely than with whole-IgG antivenoms.
    3. The most commonly reported adverse effects include vomiting, fever, rash, and itching. Each vial of the scorpion antivenom contains a small amount of cresol, and localized reactions and myalgias have occurred with the use of cresol as an excipient.
    4. 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. The starting dose of Centruroides scorpion antivenom is three vials. Dose is based on symptoms rather than patient weight. If additional doses of antivenom are required, they should be administered one vial at a time.
    1. Treat all patients in an emergency department or intensive care setting.
    2. No skin testing is required prior to administration.
    3. If antivenom is used in a patient with known or suspected horse serum sensitivity, consider pretreatment 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.
    4. Reconstitute each vial of the lyophilized product with 5 mL of normal saline, using gentle swirling to avoid shaking and destroying the immunoglobulins (as indicated by the formation of foam).
    5. Dilute the starting dose of three vials to a total volume of 50 mL with normal saline.
    6. Administer the diluted antivenom intravenously over 10 minutes. When needed, administer additional doses one vial at a time at 30-60 minute intervals. Three vials are sufficient in most cases.