Early Experience with Crotalidae Immune F(ab')₂ Antivenom to Treat Arizona Rattlesnake Envenomations

Crotalidae immune F(ab')₂ (Fab2AV) became available in the USA in 2019 for treatment of rattlesnake envenomation. In the clinical trial comparing Fab2AV to crotalidae immune polyvalent fab (FabAV), Fab2AV was associated with less late hemotoxicity. The purpose of this study was to describe outcomes following use of Fab2AV in patients with rattlesnake envenomation in Arizona.

This is an observational study of patients admitted to a medical toxicology service at two hospitals in Arizona between January 1, 2019 and December 31, 2020. Patients with rattlesnake envenomation who received Fab2AV were included. Patients who received FabAV, alone or in combination with Fab2AV, were excluded. The main outcomes of interest were antivenom dose, adverse reactions, late hemotoxicity, and hospital readmission or retreatment.

Forty-six patients were included. The mean age was 40 years, with 15% under 12 years of age. All exhibited swelling, 20% thrombocytopenia, and 35% coagulopathy. Median time to treatment was 3 h and median total Fab2AV dose was 20 vials. Three patients had an acute reaction to Fab2AV which was non-life-threatening and resolved with antihistamines and/or steroids. In the follow-up period, one case of delayed thrombocytopenia (platelets = 108 K/mm³) and one case of recurrent thrombocytopenia (platelets = 111 K/mm³) were identified. There was no late coagulopathy. Five patients reported symptoms consistent with mild serum sickness.

In this series of patients with rattlesnake envenomation in Arizona who were treated with Fab2AV, there were no cases of clinically significant late hemotoxicity, and no patients required late retreatment with antivenom. Acute and delayed reactions did occur in some patients but were mild and easily treated.