Acute adverse effects of F(ab')₂AV and FabAV use for rattlesnake Envenomations: A four-year poison center study

Rattlesnake envenomations account for many of the Crotalid envenomations in the United States annually. Two antivenoms are currently available to treat Crotalid envenomation in this country: Crotalidae-polyvalent ovine immune Fab antivenom (CroFab®; FabAV) and Crotalidae equine immune F(ab')₂ antivenom (ANAVIP®; F(ab')₂AV). Few studies have compared the adverse effect rates for each. We performed a retrospective chart review of rattlesnake envenomations called to the California Poison Control System from October 2018 to August 2022. Those treated at healthcare facilities with either antivenom were included. Those treated with both antivenoms were excluded. Records were obtained from the poison center electronic medical records system. Demographic and clinical data were abstracted. "Severe" adverse events were defined as multi-organ system involvement, swelling of the patient's airway, and/or hemodynamic instability. All others were categorized as "non-severe." A total of 481 cases were included with 360 treated with FabAV and 121 with F(ab')₂AV. The median age was 47 and 46 years, and 72 % and 73 % were male, respectively. Clinical signs and symptoms of envenomation were similar in each group. The FabAV group received a median of six vials. The F(ab')₂AV group received a median of 10 vials, based on the recommended loading doses of FabAV and F(ab')₂AV. Following antivenom administration, 18 individual acute non-severe AEs were reported in 12 FabAV-treated patients. Two acute non-severe AEs were reported in two F(ab')₂AV-treated patients. Rash or urticaria was the most commonly reported adverse effect in both groups after antivenom administration. Five patients (1.5 %) had severe adverse events reported in the poison center records following FabAV administration, and none were reported following F(ab')₂AV administration (p = 0.025). Overall, our poison center data suggests the rate of adverse events is low following the use of either antivenom. Our findings are limited by the lack of consistent timing data, a smaller F(ab')₂AV cohort, retrospective format, and use of poison center data.