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Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.

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Acute Allergic Reactions Associated With CroFab

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In the April 2008 issue of Annals of Emergency Medicine appears an article entitled "Acute Hypersensitivity Reactions Associated with Administration of Crotalidae Polyvalent Immune Fab Antivenom," by Robert Cannon, Anne-Michelle Ruha, and John Kashani. These doctors performed a chart review of all patients that had been admitted with the diagnosis of rattlesnake bite to Phoenix Children's Hospital and Banner Good Samaritan Medical Center in Phoenix, Arizona between July 2000 and June 2004.

The chart review yielded 93 patients who were treated with crotalidae polyvalent immune Fab (ovine [sheep]) antivenom (commercially known as CroFab; FabAV, Protherics, Inc., Brentwood, TN). This antivenom is made of "small" antibody fragments in a manner designed to make the antivenom more effective in humans at neutralizing snake venom, while resulting in fewer allergic (adverse) reactions. These 93 patients included 72 males and 21 females. Their mean age was 34.5 years (range 16 months to 91 years), and the mean dose of antivenom administered was 12 vials (range 4 to 32 vials). The incidence of acute hypersensitivity (allergic) reactions was 5 of 93, or 5.4%. In four of the cases, the reaction was mild and easily treated, such that the full course of antivenom could be administered. One patient developed a reaction that necessitated discontinuation of antivenom administration.

These data are even better than the previously reported data with respect to incidence of allergic reactions. Before CroFab came on the scene in 2001, doctors used Antivenin (Crotalidae) Polyvalent from Wyeth Laboratories, which is made from a different process, and was notorious for causing frequent serious allergic reactions. I can recall spending long hours working in the intensive care unit administering this older product to severely envenomed victims, essentially having to infuse a few drops of the antivenin alternated with intravenous epinephrine to counteract the allergic manifestations. It would take many hours to administer the requisite doses of antivenom, and many patients suffered both immediate and delayed allergic reactions. With CroFab, it has become much easier to manage snakebite victims.

As the authors explain, FabAV is produced by immunizing individual flocks of sheep with one of 4 poisonous snake venoms: Eastern Diamondback rattlesnake, Western Diamondback rattlesnake, Mojave rattlesnake, and cottonmouth. Because the sheep are immunized, they develop antibodies, which circulate in their bloodstreams. The serum from their blood is digested with papain, which breaks down the antibodies into fragments, from which the Fab fragments are isolated. Other fragments and extraneous proteins are discarded. The four different types of Fab fragments (one type from each flock of sheep) are combined to create a highly purified product that lacks the immunogenic larger antibody fragments and proteins fround in Antivenin (Crotalidae) Polyvalent. Thus, the final product is much less prone to induce an adverse allergic reaction.

Using the newer antivenom product, fewer patients should have allergic reactions necessitating therapies such as infusion of epinephrine, which will make their care easier and less dangerous.

Preview the 25th Anniversary & Annual Meeting of the Wilderness Medical Society, which will be held in Snowmass, Colorado July 25-30, 2008.

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Dr. Paul S. Auerbach is the world’s leading authority on wilderness medicine.

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