Pirates of the coagulase: Clinical implications of dramatic ontogenetic shifts in Yellow Beard (Bothrops atrox) lancehead pitviper Factor Va-mediated venom activation of blood clotting factors

Bothrops atrox (Yellow Beard Lancehead Pitviper) is responsible for the majority of snakebite morbidity in the Amazon, yet the biochemical basis and clinical consequences of its ontogenetic venom variation remain incompletely resolved. We compared Colombian neonate and adult B. atrox venoms across plasma and fibrinogen clotting assays, thromboelastography, antivenom neutralisation, and clotting factor zymogen activation with defined cofactor conditions. Neonate venom clotting was faster on plasma, whereas adult venom showed stronger thrombin-like (pseudo-procoagulant) action on fibrinogen. All three regional antivenoms neutralised both age classes to varying degrees, with Butantan outperforming ICP and Antivipmyn-Tri. For all antivenoms, neutralisation was consistently better for adult venom. Factor-activation assays revealed activation of prothrombin and Factor VII by both age classes, with a stronger neonate signal. Prothrombin activation strictly required Factor Va as an obligate cofactor; neonate venom could generate usable FVa from FV in addition to using endogenous (thrombin-produced) FVa, while adult venom depended on endogenous FVa. Strikingly, we demonstrate Factor VII activation by B. atrox for the first time and show that FVa markedly potentiates this reaction, including FVa produced by venom cleavage of FV, with age-class differences in the efficiency of venom-produced FVa utilisation. Unlike prothrombin, the neonate venom was able to activate FVII in the absence of FVa, but at much lower levels. Metalloproteases being responsible for prothrombin activation was confirmed with the selective inhibitor prinomastat. These data resolve the mechanistic drivers of ontogenetic potency shifts and explain antivenom performance differences, with immediate implications for antivenom formulation and adjunctive inhibitor use. • Neonate Bothrops atrox venom is far more procoagulant than adult venom on plasma and fibrinogen. • Neonate venom activates prothrombin and Factor VII via FVa-dependent pathways more strongly than adults. • We provide the first evidence that Bothrops atrox venom directly activates human Factor VII. • Clinically used antivenoms neutralise adult venom better than neonate venom; Butantan performs best. • Findings indicate high coagulopathy risk after neonate bites and support adding neonate venom to antivenom immunizing mixture.