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For decades Wyeth-Ayerst (Marietta, PA) manufactured Antivenin Crotalidae Polyvalent (ACP) for treatment of crotaline snakebites in the United States. It was a poorly purified whole IgG product derived from horse serum with significant risk for acute and delayed allergic reactions.10 ACP was formulated as a freeze-dried powder that required reconstitution before use. Wyeth has since stopped production of ACP, though expired supplies may still be available. Previous editions of this textbook contain details about Wyeth ACP.

In October 2000, the US Food and Drug Administration approved Crotalidae polyvalent immune Fab, manufactured by Protherics Inc (Savage Laboratories, Brentwood, TN). This antivenom is derived from sheep serum and formulated specifically to treat crotaline snakebites found in the United States. It is an effective and less allergenic alternative to the previously manufactured horse serum product and is currently the only crotaline antivenom available in the United States.10,14

A limited number of bites from exotic nonnative snakes, kept in zoos and by amateur collectors, occur in North America each year. Numerous antivenoms exist to treat bites from these exotic snakes, but they are of limited availability and difficult to obtain. Poison centers have access to the online Antivenom Index, which is useful when attempting to locate antivenom for a nonnative snake envenomation. Local zoos may also be useful resources when attempting to locate an exotic antivenom. Recommendations for these diverse antivenoms are difficult to make, but any available package instructions should be followed, and preparations should be made to treat life-threatening hypersensitivity reactions when using these unfamiliar products.

Crotalidae polyvalent immune Fab (ovine), marketed under the brand name CroFab™, is produced by inoculating sheep with the venom of the eastern diamondback rattlesnake (Crotalus adamanteus), western diamondback rattlesnake (Crotalus atrox), cottonmouth (Agkistrodon piscivorus), and Mojave rattlesnake (Crotalus scutulatus). The refining process begins with papain digestion of isolated IgG antibodies to eliminate the Fc portion of the immunoglobulin, and then isolation of the antibody fragments [Fab and F(ab)2]. This is followed by affinity purification and lyophilization. The resultant Fab fragments have a smaller molecular weight, are less immunogenic, and have increased tissue penetration compared with whole IgG antivenoms used previously.

Clinical Use

After an envenomation, the major indications for administration of antivenom are (1) hemodynamic compromise, (2) significant coagulopathy or thrombocytopenia, (3) neuromuscular toxicity, or (4) progression of swelling. These indications are vague and allow clinical interpretation of the need for antivenom under varying circumstances. Antivenom should not be given prophylactically to patients with minimal symptoms, or in the absence of evidence of envenomation. Patients with only minimal local tissue swelling that is not progressing should not be given antivenom.

When indicated, antivenom should be given as soon as possible to neutralize circulating venom. Animal studies report decreased mortality when antivenom is given immediately after envenomation.5 Antivenom will not ...

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