Snake bite: Pit vipers, part 2 (Proceedings)

Apr 01, 2008

Although many first-aid measures have been advocated for pit viper bite victims, none has been shown to prevent morbidity or mortality6 Long-term follow-up of pit viper envenomated human patients has revealed that the majority of individuals with permanent loss of motion of the affected area or other negative sequelae had received some form of traditional first aid (e.g., cut and suck, constriction bands, and so on).7 The primary rule of first aid is to do no additional harm.

First-aid measures to be avoided include ice, incision and suction, tourniquets (which constriction bands quickly become with progressive limb swelling), and hot packs. Electroshock has been definitively shown to be ineffective in the treatment of pit viper envenomations and should not be attempted.8,9 Current recommendations for first aid in the field are to keep the victim calm, keep the bite site below heart level if possible, and transport the victim to a veterinary medical facility for primary medical intervention. Although two early abstracts reported that suction devices, capable of asserting two atmospheres of pressure, when applied over the bite wounds within 3 minutes after the bite and left in place for 30 minutes, may remove up to 30% of the injected venom volume subsequent research has indicated these devices are ineffective.

The patient should be hospitalized and monitored closely for a minimum of 8 hours for the onset of signs of envenomation. A severity score sheet should be recorded upon entry and at 6 hours post hospitalization at a minimum for every suspected snakebite victim. This advice cannot be overemphasized because the onset of clinical signs may be significantly delayed. The effects of snake venom are time dependent, any delay in initiating medical treatment is deleterious to the patient and may result in complications that cannot be corrected.

The initial medical response to a snake-bitten patient is to collect the appropriate pretreatment laboratory samples and make circumferential measurements at, above, and below the bite site to allow quantitative monitoring of the progression of swelling. An intravenous catheter should be placed and a crystalloid fluid drip started.

The patient can be pretreated with diphenhydramine given IV or subcutaneously (SQ) (small dogs and cats: 10 mg; large dog: 25–50 mg). Antihistamines have no effect on the venom or the course of the envenomation itself; however, they can calm the fractious patient to facilitate intravenous catheterization. Earlier beliefs that patients pretreated with antihistamines would be more refractory to allergic reactions have been put in doubt with specific clinical observations addressing this assertion.

The only proven specific therapy against pit viper envenomation is the administration of antivenin.10,11,12,13 Coagulation deficits, fluid loss, changes in neurologic status, cardiac conduction abnormalities, and the necrotizing effect of the venom can be dramatically reversed when antivenin treatment is initiated appropriately.

In North America polyvalent antivenin, which is effective against the venoms of all endemic pit viper species, is used. This polyvalent equine origin antivenin (Crotalidae) is made and marketed to the veterinary community by Fort Dodge Laboratories (Fort Dodge, IA). The antivenin is produced by inoculating horses with the venoms collected from Crotalus atrox (western diamondback rattlesnake), Crotalus adamenteus (eastern diamondback rattlesnake), Crotalus terrificus (South American rattlesnake), and Brothrops atrox (fer-de-lance). Once a horse attains a certain titer, serum is harvested and processed to concentrate the antivenom immunoglobulins. The processes used to extract these proteins result in a final product that, though rich in antibodies, has a very high equine protein contaminants and albumin component, often in the range of 50%.

A new antivenom (Crotalidae polyvalent immune Fab Ovine, Protherics Inc, Brentwood, TN) was approved for the human use by the U.S. Food and Drug Administration in late 2000. The new antivenom is a purified and lyophilized preparation of ovine Fab immunoglobin fragments. The ovine IgG molecules are cleaved to discard the inflammatory stimulating Fc portion of the antibody, retaining only the Fab molecules. The product is affinity purified and contains negligible amounts of extraneous proteins such as albumin. Advantages of this product include a high affinity for venom antigens, improved penetration into tissues (due to the smaller size of Fab compared to whole IgG), and decreased antigenic potential, thereby decreasing the risk of possible allergic reactions. A disadvantage may be the potential need for repeated administration because the smaller Fab molecules may be more rapidly cleared from the body.