Initial Experience with F(ab')2 Antivenom Compared with Fab Antivenom for Rattlesnake Envenomations Reported to a single poison center during 2019.

BACKGROUND: There are two Food and Drug Administration (FDA)-approved antivenoms available for rattlesnake envenomations in the United States: the equine-derived F (ab')2 product sold with the brand name Anavip (F (ab')2 AV) and the ovine-derived Fab product sold with the brand name Crofab (FabAV).

OBJECTIVE: To compare the clinical outcomes of rattlesnake envenomation patients treated either with FabAV or F (ab')AV or a combination of these.

METHODS: This is a retrospective chart review of all human rattlesnake envenomations requiring antivenom reported to one regional poison control center in 2019. Patients were categorized as receiving F (ab')2 AV, FabAV, or a combination of both. Baseline characteristics included demographics, time between envenomation and administering antivenom, an abbreviated snakebite severity score (ASSS), and the presence of coagulopathy at presentation.

RESULTS: There were a total of 123 patients requiring antivenom. Of these, 57 (46.3%) received FabAV, 53 (43.1%) received F (ab')2 AV, and 13 (10.6%) received a combination of these. Those receiving F (ab')2 AV were younger, with an average age of 40.8 (±25.0) years versus 51.3 (±19.9) years (p = 0.0161) for those receiving FabAV. Time between envenomation and antivenom administration, ASSS, and the percentage of those with coagulopathy at presentation were otherwise similar. Patients treated with F (ab')2 AV or FabAV received a similar total number of vials [16.0 vials (±6.1) vs 14.5 vials (±5.4), p = 0.189], but patients treated with F (ab')2 AV were more frequently given additional doses [31 patients (58.5%) vs. 22 FabAV patients (38.6%), p = 0.0051]. In patients with outpatient follow-up for 2 weeks, fewer patients treated with F (ab')2 AV developed late coagulopathy [5 patients (11.1%) vs 22 FabAV patients (48.9%), p = 0.0004]. Adverse events were generally mild and uncommon with no difference in frequency between patients who received either antivenom (2 F (ab')2 AV patients vs 4 FabAV patients, p = 0.6637).

CONCLUSIONS: Other than patient age, we found no significant difference in the baseline demographics, time between envenomation and administering antivenom, an abbreviated snakebite severity score (ASSS), and the presence of coagulopathy at presentation between patients receiving F (ab')2 AV or FabAV. Patients receiving F (ab')2 AV were more likely to be given an additional dose beyond the minimum typical treatment course, but less likely to develop late coagulopathy. Adverse events were uncommon and generally mild whether patients received either antivenom.