Pre-Clinical Assays Predict Pan-African Echis Viper Efficacy for a Species-Specific Antivenom

Abstract

BackgroundSnakebite is a significant cause of death and disability in subsistent farming populations of sub-Saharan Africa. Antivenom is the most effective treatment of envenoming and is manufactured from IgG of venom-immunised horses/sheep but, because of complex fiscal reasons, there is a paucity of antivenom in sub-Saharan Africa. To address the plight of thousands of snakebite victims in savannah Nigeria, the EchiTAb Study Group organised the production, testing and delivery of antivenoms designed to treat envenoming by the most medically-important snakes in the region. The Echis saw-scaled vipers have a wide African distribution and medical importance. In an effort to maximise the clinical utility of scarce antivenom resources in Africa, we aimed to ascertain, at the pre-clinical level, to what extent the E. ocellatus-specific EchiTAbG antivenom, which was designed specifically for Nigeria, neutralised the lethal activity of venom from two other African species, E. pyramidum leakeyi and E. coloratus.Methodology/Principal FindingsDespite apparently quite distinctive venom protein profiles, we observed extensive cross-species similarity in the immuno-reactivity profiles of Echis species-specific antisera. Using WHO standard pre-clinical in vivo tests, we determined that the monospecific EchiTAbG antivenom was as effective at neutralising the venom-induced lethal effects of E. pyramidum leakeyi and E. coloratus as it was against E. ocellatus venom. Under the restricted conditions of this assay, the antivenom was ineffective against the lethal effects of venom from the non-African Echis species, E. carinatus sochureki.Conclusions/SignificanceUsing WHO-recommended pre-clinical tests we have demonstrated that the new anti-E. ocellatus monospecific antivenom EchiTAbG, developed in response to the considerable snakebite-induced mortality and morbidity in Nigeria, neutralised the lethal effects of venoms from Echis species representing each taxonomic group of this genus in Africa. This suggests that this monospecific antivenom has potential to treat envenoming by most, perhaps all, African Echis species.

Medically important differences in snake venom composition are dictated by distinct postgenomic mechanisms

“…4 and SI Appendix, Table S7). We previously found no significant difference between the lethality of the four Echis venoms to laboratory mice (12), and here we demonstrate that all exhibit comparable lethality to the venom of C. cerastes (Fig. 4A).…”

“…Thus, the antivenom EchiTAbG (manufactured from the IgG of sheep hyperimmunized with E. ocellatus venom; ref. 4) is equally effective at neutralizing the lethal effects of venom from E. ocellatus, E. coloratus, and E. p. leakeyi in vivo (12). These results emphasize that a certain degree of venom variation does not render a monospecific antivenom ineffective, so long as there are sufficient species-common toxin epitopes present to ensure the cross-reactive efficacy of antivenom antibodies.…”

“…1). These species were selected based on their medical importance in Africa and our previous descriptions of interspecific variation in venom composition, dietary preference, and prey lethality (12,20,25,26). We used our previously constructed and assembled venom gland transcriptomes for the four Echis species (26,27) and prepared venom gland trancriptomes for B. arietans and C. cerastes by using identical protocols.…”

“…Murine in vivo lethality studies (12) were conducted to calculate the lethal dose 50 (LD 50 ; the amount of venom that kills 50% of injected mice) for B. arietans and C. cerastes venom. We used the same murine model to test the efficacy of the E. ocellatus monospecific antivenom, EchiTAbG, at neutralizing five times the venom LD 50 for B. arietans and C. cerastes in the effective dose 50 (ED 50 ) assay (12). Both sets of experiments were undertaken as previously described using venom from the four Echis species (12), with comparisons undertaken using the 95% confidence limits generated by probit analysis (40).…”

“…Importantly, the extent of this variation is not simply reflected by taxonomic distance (9)(10)(11) and, therefore, cannot be readily predicted. The consequence of venom variation is that antivenoms raised against any particular species of snake are often ineffective in treating snakebite by different, even closely related, species (5,12,13).…”