You've received antivenom following a North American crotalid envenomation, but you're still not out of the woods yet!

There are approximately 100,000 deaths worldwide from snakebites each year. In North America, venomous snakebites are often the result of envenomation from the genus Crotalidae, which includes The Western Diamondback (Crotalus atrox), the Eastern Diamondback (Crotalus adamanteus), and the Mojave rattlesnake (Crotalus scutulatus). Bites from these snakes are usually treated with Crotalidae Polyvalent Immune Fab (ovine)[FabAV]. For many years Antivenin (Crotalidae) Polvalent [Equine] (ACP) was the sole treatment for crotalid envenomation in North America. It is/was composed of purified horse sera containing IgG from horses immunized with crotalid snake venom. ACP was known for causing possible hypersensitivity reactions, including anaphylaxis and serum sickness. FabAV is made from sheep (ovine) and is composed of Fab fragments and is less immunogenic. FabAV seems to work well, but patients need to be closely monitored to determine how many repeated doses are required. A case report from 2008 published in Clinical Toxicology describes a 24-year old man who was intoxicated and fell into his western diamondback rattlesnake enclosure and was bitten on both hands and the right side torso by two diamondbacks. When he arrived at the hospital he was hypotensive, tachycardic, and thrombocytopenic with a platelet count of 17/nl. Several hours after administration of current FabAV protocol the patient's hematocrit continued to drop. The patients blood pressure upon arrival was 83/48mmHg and heart rate was 104 bpm. The patient received 5 vials over a 1 hour period. 5 additional vials were given over the next hour. Blood pressure improved to 114/86 mmHg and platelet count improved to 206/nl. He was then given 2 vials every six hours for the next 18 hours. The next morning the patient had a platelet count of 15/nl and hematocrit decreased from 37.8% to 21.6%. The patient was given packed red blood cells and platelets over the next 12 hours and his hematocrit and platelet counts stabilized. The patient remained in the hospital for eleven days while doctors worked hard at maintaining adequate blood stats. It took the patient 14 days for his own body to keep his hematocrit and platelet counts within physiological range. So, FabAV may work well at minimizing tissue damage and possible death from crotalid envenomation, but extensive patient monitoring and repeated FabAV doses are very important.

Clinical Toxicology (2008) 46, 823-826