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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, frothing at the mouth, and 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.

  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 affect the fetus adversely (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, not on 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 before scorpion antivenom administration.

    3. If antivenom is used in a patient with known or suspected horse serum sensitivity, it may be helpful to pretreat with intravenous diphenhydramine and ranitidine 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.

  7. Formulations

    1. Lyophilized antivenom (Centruroides), each vial contains no more than 120 mg of protein (>85% F(ab)2, <7% Fab, and <5% intact immunoglobulin); sodium chloride, sucrose, and glycine are used as stabilizers, and trace amounts of cresol, pepsin, borates, and sulfates may be present from the production process.

    2. Suggested minimum stocking levels to treat a 100-kg adult for the first 8 hours and 24 hours: Centruroides (Scorpion) Immune F(ab)2 (Equine), first 8 hours: three vials; first 24 hours: three vials.

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