Crotaline Fab antivenom reverses platelet dysfunction induced by Crotalus scutulatus venom: an in vitro study

Patients sustaining rattlesnake envenomation often develop thrombocytopenia, the etiology of which is not clear. Laboratory studies have demonstrated that venom from several species, including the Mojave rattlesnake (Crotalus scutulatus scutulatus), can inhibit platelet aggregation. In humans, administration of crotaline Fab antivenom has been shown to result in transient improvement of platelet levels; however, it is not known whether platelet aggregation also improves after antivenom administration.

The objective was to determine the effect of C. scutulatus venom on platelet aggregation in vitro in the presence and absence of crotaline Fab antivenom.

Blood was obtained from four healthy male adult volunteers not currently using aspirin, nonsteroidal anti-inflammatory drugs, or other platelet-inhibiting agents. C. scutulatus venom from a single snake with known type B (hemorrhagic) activity was obtained from the National Natural Toxins Research Center. Measurement of platelet aggregation by an aggregometer was performed using five standard concentrations of epinephrine (a known platelet aggregator) on platelet-rich plasma over time, and a mean area under the curve (AUC) was calculated. Five different sample groups were measured: 1) blood alone, 2) blood + C. scutulatus venom (0.3 mg/mL), 3) blood + crotaline Fab antivenom (100 mg/mL), 4) blood + venom + antivenom (100 mg/mL), and 5) blood + venom + antivenom (4 mg/mL). Standard errors of the mean (SEM) were calculated for each group, and paired t-tests were used to measure differences between groups.

Antivenom administration by itself (group 2) did not significantly affect platelet aggregation compared to baseline (103.8%, SEM ± 3.4%, p = 0.47). Administration of venom (group 3) decreased platelet aggregation (72.0%, SEM ± 8.5%, p < 0.05). Concentrated antivenom administration in the presence of venom (group 4) normalized platelet aggregation (101.4%, SEM ± 6.8%) and in the presence of diluted antivenom (group 5) significantly increased aggregation (133.9%, SEM ± 9.0%; p < 0.05 for both groups when compared to the venom-only group). To further assess the effects of the venom and antivenom, each was run independently in platelet-rich plasma without epinephrine; neither was found to significantly alter platelet aggregation in the absence of epinephrine.

Crotaline Fab antivenom improved platelet aggregation in an in vitro model of platelet dysfunction induced by venom from C. scutulatus. It is unclear at this time whether this improvement in platelet dysfunction translates into improved clinical outcomes in envenomated patients.