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  1. Pharmacology. Although two antivenoms are still available for the treatment of crotaline envenomation in some parts of the United States, production of the older product, Crotalinae polyvalent antivenom (equine), or Antivenom Crotalinae Polyvalent (Wyeth-Ayerst), has ceased, and it has been replaced by the newer Crotalinae polyvalent immune Fab (ovine), or CroFab (Protherics). To produce the polyvalent Fab antivenom, sheep are hyperimmunized with pooled venom from four North American snakes: Crotalus adamanteus, Crotalus atrox, Crotalus scutulatus, and Agkistrodon piscivorus. Papain then is added to the pooled serum product collected from the donor animals to cleave the immunogenic Fc fragment from the IgG antibody. The result is an affinity-purified Fab fragment antivenom. After administration, the antivenom is distributed widely throughout the body, where it binds to venom. Some remaining stocks of the Wyeth product may be found in parts of the United States.

  2. Indications. Antivenom is used for treatment of significant envenomation by Crotalinae species (Table III–4 and See Snakebite).

    Table III-4 Initial Dose of Crotalinae Antivenom

  3. Contraindications. Known hypersensitivity to the antivenom or to horse serum is a relative contraindication for the Wyeth product; antivenom may still be indicated for severe envenomation despite a patient history of allergic reaction. Known hypersensitivity to sheep or sheep serum, or to papain or papayas, is a contraindication for the CroFab product.

  4. Adverse effects

    1. Immediate hypersensitivity reactions (including life-threatening anaphylaxis) may occur to both products, even in patients with no history of animal serum sensitivity and negative skin test results. Skin testing is not indicated with CroFab, and immediate hypersensitivity reactions appear to be less common.

    2. Mild flushing and wheezing can occur within the first 30 minutes of intravenous administration and often will decrease after the rate of infusion has been slowed.

    3. Delayed hypersensitivity (serum sickness) occurs in more than 75% of patients who receive more than four vials of Wyeth antivenom and virtually all patients who receive more than 12 vials. Onset occurs in 5–14 days. CroFab administration can also lead to delayed hypersensitivity reactions, but these appear to be much less common than with the Wyeth product.

    4. Use in pregnancy. FDA Category C (indeterminate; see Table III–1). There are no data on teratogenicity. Anaphylactic reaction resulting in shock or hypoxemia in the mother could conceivably adversely affect the fetus. However, severe envenomation of the mother should be treated aggressively to limit venom effects that could affect the fetus or placenta.

  5. Drug or laboratory interactions. There are no known interactions.

  6. Dosage and method of administration. The initial dose is based on the severity of symptoms, not on body weight (see Table III–4...

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