Utilising venom activity to infer dietary composition of the Kenyan horned viper (Bitis worthingtoni)

Sudden death after a bite by a lowland swamp viper (Proatheris superciliaris) in Malawi: Dying of fright?

Proatheris superciliaris, the lowland swamp viper, has a limited distribution along lakeshores and rivers in Malawi, Southern Tanzania, and central Mozambique. Its venom is known to be procoagulant. Only five P. superciliaris bites have been reported, all inflicted by captive snakes, and none was fatal. Here we present a case of sudden death following a bite by Proatheris superciliaris in rural Malawi that cannot be attributed to envenoming. A healthy 32-year-old woman was planting rice in a flooded rice paddy field when she suddenly told her sister in a quiet voice that she had been bitten by a snake. She then collapsed face-upwards into the ankle-deep water. She remained motionless while her sister and uncle carried her out of the rice paddy onto dry land a few meters away. The victim did not regain consciousness. Her uncle heard one exhalation but no further breathing. The snake responsible was killed by a friend. Although the venom of this species can cause life-threatening coagulopathy, this woman's death occurred too rapidly to be attributable to envenoming. Only two explanations seem plausible: anaphylaxis, or vasovagal shock triggered by fear. In the present case, the victim died within minutes of the bite, closely observed by her anxious relatives, but showed no features of anaphylaxis. In Malawi, as in much of sub-Saharan Africa, many people are reportedly terrified of snakes, believing that bites by almost any species can cause rapid death. In this case, death occurred less than 2 min after a bite from Proatheris superciliaris. We believe that the cause of death was most likely a severe vasovagal attack, in response to the fear and pain of the snakebite that triggered vasodilatation, bradycardia, and hypotension leading to cardiac arrest.

Extreme Procoagulant Potency in Human Plasma of Venoms from the African Viperid Genera Atheris, Cerastes, and Proatheris and the Relative Efficacy of Antivenoms and Synthetic Enzyme-Inhibitors

The African viperid snake genera Atheris, Cerastes, and Proatheris are closely related, similar in size, but occupy extremely divergent ecological niches (arboreal in tropical rainforests, fossorial in deserts, and swamp-dwelling, respectively). Their venoms have not previously been subjected to comparative analyses for their action upon the coagulation of blood, most notably with significant data deficiencies from Atheris and Proatheris. In contrast, the closely related genus Echis is well-documented as capable of producing potent procoagulant effects. In light of this, we set out to compare the coagulotoxic actions of Atheris ceratophora, A. chlorechis, A. desaixi, A. nitschei, A. squamigera, C. cerastes, C. cerastes gasperettii, C. vipera, and Proatheris superciliaris and explore potential pharmacological interventions to reestablish normal blood coagulation. All venoms displayed extremely potent procoagulant effects, over twice as fast as the most potent Echis reported to date. Although Cerastes is used in the immunising mixture of two different regionally available antivenoms (Inoserp-MENA with C. cerastes, C. cerastes gasperettii, C. vipera and Saudi Arabian polyvalent with C. cerastes), none of the other species in this study are included in the immunising mixture of any antivenom. Notably, all the Cerastes species were only neutralised by the Inoserp-MENA antivenom. C. cerastes venom was not neutralised well by the Saudi Arabian antivenom, with the low levels of recognition for any of the Cerastes venoms suggesting a strong regional variation in the venom of this species, as the C. cerastes venom tested was of African (Tunisian) origin versus Saudi locality used in that antivenom's production. The other antivenoms (Micropharm EchiTAbG, ICP EchiTAb-Plus-ICP, Inosan Inoserp Pan-Africa, Premium Serums PANAF Sub-Sahara Africa, South African Vaccine Producers Echis, South African Vaccine Producers Polyvalent) all displayed trivial-to-no ability to neutralise the procoagulant toxicity of any of the Atheris, Cerastes, or Proatheris venoms. Comparative testing of the enzyme inhibitors DMPS, marimastat, and prinomastat, revealed a very potent neutralising capacity of marimastat, with prinomastat showing lower but still significant potency at the same molar concentration, while a 5× molar concentration of DMPS had no apparent effect on procoagulant venom effects normalized by the other inhibitors. These results and methods contribute to the body of knowledge of potential clinical effects and data necessary for evidence-based advancement of clinical management strategies.

Viscoelastic Coagulation Testing in Exotic Animals

Whole blood viscoelastic coagulation testing (VCT) allows global assessment of hemostasis and fibrinolysis. Although not widely used in exotic animal practice, VCT has been used in exotic animal research settings. Differences in patient demographics and analytical variables can result in dramatically different results with the same analyzer. To improve the utility of VCT in exotic animal medicine, standardization of protocols is necessary to facilitate the establishment of reference intervals. Despite these challenges, the quantitative/qualitative nature of VCT has already proved its real-world value to some clinicians.

Untangling interactions between Bitis vipers and their prey using coagulotoxicity against diverse vertebrate plasmas

Venom is a key evolutionary innovation which plays a primary role in prey subjugation by venomous snakes. However, while there is a growing body of literature indicating the composition and activity of snake venoms is under strong natural selection driven by differences in prey physiology, the majority of studies have historically focussed on the activity of snake venoms with regards only towards human or mammalian physiologies. This study aimed to use clotting assays measuring both time and strength of clotting to characterise the coagulotoxic activity of venoms from a taxonomically, morphologically, and ecologically diverse range of Bitis spp. of viperid snakes upon the plasma of model species: amphibian (Cane Toad, Rhinella marina); lizard (Blue-tongue Skink, Tiliqua scincoides); avian (Domestic Chicken, Gallus gallus); and rodent (Brown Rat, Rattus norvegicus). Significant variation in coagulotoxic activity across the different plasmas was observed between species and compared to the known affects upon human plasma. Bitis caudalis was notable in being active on all four plasmas, but in extremely divergent manners: accelerating clotting times and producing strong, stable clots upon amphibian plasma (consistent with true procoagulation); accelerating clotting time but producing weak, unstable clots upon lizard plasma (consistent with pseudo-procoagulation); delaying avian clotting time beyond machine maximum reading time (strong anticoagulation consistent with either inhibition of clotting enzymes or total destruction of fibrinogen, or both); and delaying clotting of rodent plasma (consistent with inhibition of clotting enzymes) and with only weak clots formed (consistent with destruction of fibrinogen). In contrast, the sister species B. peringueyi and B. schneideri displayed activity only upon the lizard plasma, slightly accelerating the clotting times to produce weak, unstable clots (consistent with pseudo-procoagulation). The other dwarf species, B. cornuta, displayed strong anticoagulation upon avian and rodent plasmas, delaying clotting beyond the machine maximum reading time (strong anticoagulation consistent with either inhibition of clotting enzymes or total destruction of fibrinogen, or both). In contrast, the giant species studied (B. gabonica) showed only a very weak pseudo-procoagulant activity upon lizard plasma. The wide range of variation seen within this study highlights the importance of studying venom activity on relevant models when making conclusions about the ecological role of venoms and the extreme limitation in extrapolating animal results to predict potential human clinical effects.

Evidence for Resistance to Coagulotoxic Effects of Australian Elapid Snake Venoms by Sympatric Prey (Blue Tongue Skinks) but Not by Predators (Monitor Lizards)

Some Australian elapids possess potently procoagulant coagulotoxic venoms which activate the zymogen prothrombin into the functional enzyme thrombin. Although the activity of Australian elapid prothrombin-activators has been heavily investigated with respect to the mammalian, and in particular, human clotting cascades, very few studies have investigated the activity of their venom upon reptile plasmas. This is despite lizards representing both the primary diet of most Australian elapids and also representing natural predators. This study investigated the procoagulant actions of a diverse range of Australian elapid species upon plasma from known prey species within the genera Tiliqua (blue tongue skinks) as well as known predator species within the genera Varanus (monitor lizards). In addition to identifying significant variation in the natural responses of the coagulation cascade between species from the genera Tiliqua and Varanus relative to each other, as well as other vertebrate lineages, notable differences in venom activity were also observed. Within the genus Tiliqua, both T. rugosa and T. scincoides plasma displayed significant resistance to the procoagulant activity of Pseudechis porphyriacus venom, despite being susceptible to all other procoagulant elapid venoms. These results indicate that T. rugosa and T. scincoides have evolved resistance within their plasma to the coagulotoxic venom activity of the sympatric species P. porphyriacus. Other venoms were able to activate Tiliqua prothrombin, which suggests that the lessened activity of P. porphyriacus venom is not due to modifications of the prothrombin and may instead be due to a serum factor that specifically binds to P. porphyriacus toxins, as has been previously seen for squirrels resistant to rattlesnake venom. In contrast, none of the predatory lizards studied (Varanus giganteus, V. mertensi and V. varius) demonstrated resistance to the venom. This suggests that the mechanical protection afforded by thick osteodermic scales, and prey handling behaviour, removes a selection pressure for the evolution of resistance in these large predatory lizards. These results therefore reveal differential interactions between venoms of snakes with sympatric lizards that are on opposite sides of the predator-prey arms race.

The Dragon's Paralysing Spell: Evidence of Sodium and Calcium Ion Channel Binding Neurotoxins in Helodermatid and Varanid Lizard Venoms

Bites from helodermatid lizards can cause pain, paresthesia, paralysis, and tachycardia, as well as other symptoms consistent with neurotoxicity. Furthermore, in vitro studies have shown that Heloderma horridum venom inhibits ion flux and blocks the electrical stimulation of skeletal muscles. Helodermatids have long been considered the only venomous lizards, but a large body of robust evidence has demonstrated venom to be a basal trait of Anguimorpha. This clade includes varanid lizards, whose bites have been reported to cause anticoagulation, pain, and occasionally paralysis and tachycardia. Despite the evolutionary novelty of these lizard venoms, their neuromuscular targets have yet to be identified, even for the iconic helodermatid lizards. Therefore, to fill this knowledge gap, the venoms of three Heloderma species (H. exasperatum, H. horridum and H. suspectum) and two Varanus species (V. salvadorii and V. varius) were investigated using Gallus gallus chick biventer cervicis nerve–muscle preparations and biolayer interferometry assays for binding to mammalian ion channels. Incubation with Heloderma venoms caused the reduction in nerve-mediated muscle twitches post initial response of avian skeletal muscle tissue preparation assays suggesting voltage-gated sodium (Na_V) channel binding. Congruent with the flaccid paralysis inducing blockage of electrical stimulation in the skeletal muscle preparations, the biolayer interferometry tests with Heloderma suspectum venom revealed binding to the S3–S4 loop within voltage-sensing domain IV of the skeletal muscle channel subtype, Na_V1.4. Consistent with tachycardia reported in clinical cases, the venom also bound to voltage-sensing domain IV of the cardiac smooth muscle calcium channel, Ca_V1.2. While Varanus varius venom did not have discernable effects in the avian tissue preparation assay at the concentration tested, in the biointerferometry assay both V. varius and V. salvadorii bound to voltage-sensing domain IV of both Na_V1.4 and Ca_V1.2, similar to H. suspectum venom. The ability of varanid venoms to bind to mammalian ion channels but not to the avian tissue preparation suggests prey-selective actions, as did the differential potency within the Heloderma venoms for avian versus mammalian pathophysiological targets. This study thus presents the detailed characterization of Heloderma venom ion channel neurotoxicity and offers the first evidence of varanid lizard venom neurotoxicity. In addition, the data not only provide information useful to understanding the clinical effects produced by envenomations, but also reveal their utility as physiological probes, and underscore the potential utility of neglected venomous lineages in the drug design and development pipeline.

Venom-Induced Blood Disturbances by Palearctic Viperid Snakes, and Their Relative Neutralization by Antivenoms and Enzyme-Inhibitors

Palearctic vipers are medically significant snakes in the genera Daboia, Macrovipera, Montivipera, and Vipera which occur throughout Europe, Central Asia, Near and Middle East. While the ancestral condition is that of a small-bodied, lowland species, extensive diversification has occurred in body size, and niche specialization. Using 27 venom samples and a panel of in vitro coagulation assays, we evaluated the relative coagulotoxic potency of Palearctic viper venoms and compared their neutralization by three antivenoms (Insoserp Europe, VIPERFAV and ViperaTAb) and two metalloprotease inhibitors (prinomastat and DMPS). We show that variation in morphology parallels variation in the Factor X activating procoagulant toxicity, with the three convergent evolutions of larger body sizes (Daboia genus, Macrovipera genus, and Vipera ammodytes uniquely within the Vipera genus) were each accompanied by a significant increase in procoagulant potency. In contrast, the two convergent evolutions of high altitude specialization (the Montivipera genus and Vipera latastei uniquely within the Vipera genus) were each accompanied by a shift away from procoagulant action, with the Montivipera species being particularly potently anticoagulant. Inoserp Europe and VIPERFAV antivenoms were both effective against a broad range of Vipera species, with Inoserp able to neutralize additional species relative to VIPERFAV, reflective of its more complex antivenom immunization mixture. In contrast, ViperaTAb was extremely potent in neutralizing V. berus but, reflective of this being a monovalent antivenom, it was not effective against other Vipera species. The enzyme inhibitor prinomastat efficiently neutralized the metalloprotease-driven Factor X activation of the procoagulant venoms. In contrast, DMPS (2,3-dimercapto-1-propanesulfonic acid), which as been suggested as another potential treatment option in the absence of antivenom, DMPS failed against all venoms tested. Overall, our results highlight the evolutionary variations within Palearctic vipers and help to inform clinical management of viper envenomation.

A Clot Twist: Extreme Variation in Coagulotoxicity Mechanisms in Mexican Neotropical Rattlesnake Venoms

Rattlesnakes are a diverse clade of pit vipers (snake family Viperidae, subfamily Crotalinae) that consists of numerous medically significant species. We used validated in vitro assays measuring venom-induced clotting time and strength of any clots formed in human plasma and fibrinogen to assess the coagulotoxic activity of the four medically relevant Mexican rattlesnake species Crotalus culminatus, C. mictlantecuhtli, C. molossus, and C. tzabcan. We report the first evidence of true procoagulant activity by Neotropical rattlesnake venom in Crotalus culminatus. This species presented a strong ontogenetic coagulotoxicity dichotomy: neonates were strongly procoagulant via Factor X activation, whereas adults were pseudo-procoagulant in that they converted fibrinogen into weak, unstable fibrin clots that rapidly broke down, thereby likely contributing to net anticoagulation through fibrinogen depletion. The other species did not activate clotting factors or display an ontogenetic dichotomy, but depleted fibrinogen levels by cleaving fibrinogen either in a destructive (non-clotting) manner or via a pseudo-procoagulant mechanism. We also assessed the neutralization of these venoms by available antivenom and enzyme-inhibitors to provide knowledge for the design of evidence-based treatment strategies for envenomated patients. One of the most frequently used Mexican antivenoms (Bioclon Antivipmyn®) failed to neutralize the potent procoagulant toxic action of neonate C. culminatus venom, highlighting limitations in snakebite treatment for this species. However, the metalloprotease inhibitor Prinomastat substantially thwarted the procoagulant venom activity, while 2,3-dimercapto-1-propanesulfonic acid (DMPS) was much less effective. These results confirm that venom-induced Factor X activation (a procoagulant action) is driven by metalloproteases, while also suggesting Prinomastat as a more promising potential adjunct treatment than DMPS for this species (with the caveat that in vivo studies are necessary to confirm this potential clinical use). Conversely, the serine protease inhibitor 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF) inhibited the direct fibrinogen cleaving actions of C. mictlantecuhtli venom, thereby revealing that the pseudo-procoagulant action is driven by kallikrein-type serine proteases. Thus, this differential ontogenetic variation in coagulotoxicity patterns poses intriguing questions. Our results underscore the need for further research into Mexican rattlesnake venom activity, and also highlights potential limitations of current antivenom treatments.