CHAPTER 3-6: ANTIVENOM, MICRURUS FULVIUS (CORAL SNAKE), AND EXOTIC ANTIVENOMS

INTRODUCTION

1. Pharmacology

   1. To produce the antivenom for North American coral snake bites, horses are hyperimmunized with venom from Micrurus fulvius, the eastern coral snake. The lyophilized protein preparation from pooled equine sera contains IgG antibodies to venom fractions as well as residual serum proteins. Administered intravenously, the antibodies distribute widely throughout the body, where they bind the target venom.

   2. Exotic antivenoms. Companies outside the United States produce a variety of antivenoms for exotic snakebites. Most of these products are used to treat snakebites by elapids because this family of snakes causes the most severe envenomations worldwide. Many of these are still whole-antibody products derived from horses. A few are produced as Fab fragments, or the slightly larger F(ab)₂ molecule (cleaved with pepsin instead of papain). In both of these cases, the Fc is removed from the solution. Many foreign antivenom products are polyvalent, a mixture of antivenoms for several species.

2. Indications

   1. Envenomation by the eastern coral snake (M. fulvius) or the Texas coral snake (M. fulvius tenere).

   2. May not be effective for envenomation by the western, Arizona, or Sonora coral snake (Micrurus euryxanthus), but symptomatic bites by these small western US elapids are very rare.

3. Contraindications. Known hypersensitivity to Micrurus antivenom or to horse serum is a relative contraindication; if a patient with significant envenomation needs the antivenom, it should be given with caution. Antivenoms produced outside the United States may be made from horse or sheep serum.

4. Adverse effects

   1. Immediate hypersensitivity, including life-threatening anaphylaxis, may occur even after a negative skin test for horse serum sensitivity.

   2. Delayed hypersensitivity (serum sickness) may occur 1–3 weeks after whole-antibody antivenom administration, with the incidence and severity depending on the total quantity of antivenom administered.

   3. Use in pregnancy. FDA Category C (indeterminate). There are no data on teratogenicity. Anaphylactic reactions resulting in shock or hypoxemia in expectant mothers could conceivably affect the fetus adversely. This should be weighed against the potential detrimental effect of the venom on both the placenta and the fetus (see Table III–1).

   4. Exotic antivenoms. All whole-antibody preparations carry the same risk for immediate and delayed allergy.

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

6. Dosage and method of administration. Generally, the recommended initial dose of Micrurus antivenom is three to five vials. The drug is most effective if given before the onset of signs or symptoms of envenomation. An additional three to five vials may be given, depending on the severity of neurologic manifestations but not on body weight (children may require doses as large as or even larger than those for adults).

   The recommended dose of exotic snake antivenom will vary. With other elapids, such as cobras, the antivenom is also more effective if given early in the course of the envenomation.

   1. Treat all patients in an intensive care unit setting.

   2. Before a skin test or antivenom administration, insert at least one and preferably two secure intravenous lines.

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