Venomous Landmines: Clinical Implications of Extreme Coagulotoxic Diversification and Differential Neutralization by Antivenom of Venoms within the Viperid Snake Genus Bitis

Abstract

The genus Bitis comprises 17 snake species that inhabit Africa and the Arabian Peninsula. They are responsible for a significant proportion of snakebites in the region. The venoms of the two independent lineages of giant Bitis (B. arietans and again in the common ancestor of the clade consisting of B. gabonica, B. nasicornis, B. parviocula and B. rhinoceros) induce an array of debilitating effects including anticoagulation, hemorrhagic shock and cytotoxicity, whilst the dwarf species B. atropos is known to have strong neurotoxic effects. However, the venom effects of the other species within the genus have not been explored in detail. A series of coagulation assays were implemented to assess the coagulotoxic venom effects of fourteen species within the genus. This study identified procoagulant venom as the ancestral condition, retained only by the basal dwarf species B. worthingtoni, suggesting anticoagulant venom is a derived trait within the Bitis genus and has been secondarily amplified on at least four occasions. A wide range of anticoagulant mechanisms were identified, such as coagulant and destructive activities upon fibrinogen in both giant and dwarf Bitis and the action of inhibiting the prothrombinase complex, which is present in a clade of dwarf Bitis. Antivenom studies revealed that while the procoagulant effects of B. worthingtoni were poorly neutralized, and thus a cause for concern, the differential mechanisms of anticoagulation in other species were all well neutralized. Thus, this study concludes there is a wide range of coagulotoxic mechanisms which have evolved within the Bitis genus and that clinical management strategies are limited for the procoagulant effects of B. worthingtoni, but that anticoagulant effects of other species are readily treated by the South African polyvalent antivenom. These results therefore have direct, real-work implications for the treatment of envenomed patients.

Characterising venom variation and antivenom effectiveness within the “True” vipers (Viperinae: Viperidae)

“…All venoms in present study hypothesis, the sister genus Macrovipera has been documented as having Factor X and Factor V activation activity similar to that of Daboia [186]. This is further corroborated by the widespread procoagulant activity within the Viperinae subfamily, including members of Atheris, Bitis, Cerastes, Echis, Eristicophis, Proatheris, and Pseudocerastes, as each genus possesses either majority members or a basal member that clot plasma and/or activate factor X or prothrombin [186][187][188][189]. This indicates that the expression of procoagulant toxins may be the ancestral condition for the true vipers.…”

“…In contrast, newer functional approaches to testing antivenom efficacy focus on assessing the ability of an antivenom to neutralise toxins that induce a specific in vitro effect, such as coagulant activity (e.g. [187,205]).…”

“…The antivenoms were first tested for their ability to neutralise the procoagulant activity of the venoms on plasma by adopting a technique that has been employed in a number of "functional activity neutralisation" studies [187,205,209]. Briefly, venoms were added to human plasma over a series of venom concentrations and time until clot formation was measured.…”

Intra-Specific Venom Variation in the Australian Coastal Taipan Oxyuranus scutellatus

“…This approach has resulted in a lack of data on the extent of intra-specific/intra-population venom variation in snakes, and may have contributed to an incomplete understanding of snake venom evolution. A small number of recent studies have examined inter- and intra-population venom variation, although these have been almost entirely restricted to New World crotalines [ 12 , 13 , 14 , 15 , 16 ]; or solely comparing the haemotoxic activity [ 17 , 18 , 19 ]. The results of these studies indicate that individual (intra-population) venom variation is genetically inherited and is not altered ontogenetically by environmental factors, seasonal variation, or diet [ 14 , 15 , 20 , 21 , 22 ].…”

Effects of Australian snake venoms on coagulation: differential potency, biochemistry, and antivenom efficacy

“…Some snake venom PLA2s inhibit the coagulation cascade, thereby disrupting haemostasis and promoting bleeding (Sunagar et al, 2015a;Sunagar et al, 2015b). Such toxic actions have been characterised for both Group I and Group II, such as: Group I PLA2s in the Australian elapid snake genus Denisonia which inhibit the prothrombinase complex, with D. devisi doing so in a manner specific to amphibian prey (Youngman et al, 2018); Group I PLA2s in the venoms of African spitting cobras (Naja species) inducing anticoagulation by inhibiting Factor Xa (Bittenbinder et al, 2018;Kini and Evans, 1995); and Group II PLA2s in Bitis venoms that inhibit the prothrombinase complex (Kerns et al, 1999;Youngman et al, 2019). Within the genus Pseudechis, a potent anticoagulant PLA2 (PA11) has been purified from P. australis venom, but its target in the coagulation cascade remains unknown (Du et al, 2016).…”