LAUSR

Abstract Introduction Late hemotoxicity is common following rattlesnake envenomation treated with crotalidae immune polyvalent Fab (ovine) (FabAV). Initial clinical trials showed crotalidae immune F(ab')2 (equine) (Fab2AV) to be superior to FabAV in preventing late hemotoxicity, but this effect has not been demonstrated in broader populations. This study investigated late hemotoxicity in patients receiving Fab2AV or FabAV after rattlesnake envenomation. Methods This is a retrospective analysis of prospectively collected data from patients with snakebite reported to the ToxIC North American Snakebite Registry (NASBR) between January 1, 2019, and December 31, 2020. Inclusion criteria were rattlesnake envenomation and administration of antivenom. Patients were excluded if they received more than one type of antivenom. The primary outcome was occurrence of late hemotoxicity (platelets ≤120 k/mm3 or fibrinogen ≤170 mg/dL) in patients receiving Fab2AV and FabAV. Data collected included demographics, envenomation characteristics, laboratory values, and treatment administered. Statistics including t-test and Fisher’s exact test were used. Results A total of 201 rattlesnake envenomated patients receiving antivenom were reported to the NASBR in the study period; 144 were included. 49 received Fab2AV alone, 45 received FabAV alone and 50 received both antivenoms. Baseline patient and envenomation characteristics were similar between the groups. Late hemotoxicity occurred in 2/49 patients in the Fab2AV group (4% (95% CI 0.7–12.6)) and in 19/45 patients in the FabAV group (42% (95% CI 28.4–59.0); absolute risk reduction 39.1% (95% CI 21.2–46.2) (p = 0.001). On follow up, 0 patients (0%) receiving Fab2AV were retreated with antivenom; 4 patients (9%) receiving FabAV were retreated (p = 0.049). Conclusions In the North American Snakebite Registry, late hemotoxicity was less common in rattlesnake envenomated patients treated with Fab2AV compared to FabAV.