Good News Regarding Fab Antivenom for Snakebite

The current standard of care with regard to using antivenom for pit viper snakes (e.g., rattlesnakes, copperheads, cottonmouths) in North America is to use Crotalidae polyvalent immune Fab antivenom (CroFab;FabAV). This antivenom is created using a process that obtains antibodies “raised” by immunizing sheep and then harvesting and purifying the product. One of the major benefits of using this product is to take advantage of a lesser incidence of allergic reactions (than have traditionally been observed with previous products). It appears that the situation regarding the incidence of allergic reactions may be better than previously thought.

In the article, “Short Term Outcomes After Fab Antivenom Therapy for Severe Crotaline Snakebite,” Eric Lavonas, MD and colleagues (Ann Emerg Med 2011;57:128-137) examined the short-term outcomes associated with the use of Crotalidae polyvalent immune Fab therapy and, in particular, the incidence of hypersensitivity (allergic) reactions associated with FabAV use. The primary outcome for response to therapy was the change in overall envenomation severity after FabAV administration, as well as the rates of immediate hypersensitivity reactions and serum sickness (a slightly delayed, different type of allergic reaction).

All severely envenomed patients improved after receiving FabAV. Immediate allergic reactions were reported in 6.1 percent of antivenom recipients, and serum sickness reported in five percent. The authors concluded that FabAV therapy is associated with clinical improvement in severe crotaline snake envenomation, and that immediate hypersensitivity and serum sickness rates may be less than half that described in the FabAV prescribing information. This is good news for snakebite patients and for the clinicians who treat them.