Snake venom toxicity: Usefulness and limitations of antivenom

Dr Aniruddha GhoseChittagong Medical College

Overview

• Composition of snake venom

• Actions of components• Phenotypic expressions• Actions of anti venom• Limitations of anti

venom• Clinical implication

Snake venom: composition

• Snake venoms are the most complex of all natural venoms and poisons– mixture of more than 100 different components

• Mostly protein– enzymes, polypeptide toxins and non-toxic proteins

• Non protein components– carbohydrates, metals, lipids, free amino acids,

nucleosides and biogenic amines (serotonin and acetylcholine)

• Evolutionary pressures have selected venom toxins that are specific for many targets in animal tissues

• The toxins of most importance in human envenoming include those that affect the nervous, cardiovascular, and haemostatic systems, and cause tissue necrosis

Venom enzymes

• These include digestive hydrolases, hyaluronidase, kininogenase.

• Most venoms contain l-amino acid oxidase, phosphomono- and diesterases, 5’-nucleotidase, DNAase, NAD-nucleosidase, phospholipase A2 and peptidases.

• Zinc metalloproteinase haemorrhagins: Damage vascular endothelium, causing bleeding

• Serine proteases and other procoagulant enzymes

Venom enzymes

• Phospholipase A2 (lecithinase)• Acetylcholinesterase• Hyaluronidase• Proteolytic enzymes (metalloproteinases,

endopeptidases or hydrolases) and polypetide cytotoxins (“cardiotoxins”)

Samson A.O., Scherf. T., Eisenstein M., Chill J., and Anglister J., “The mechanism for acetylcholine receptor inhibition by alpha-neurotoxins and species-specific resistance to alpha-bungarotoxin revealed by NMR” , 2002, Neuron, 35, 319-332.

Neurotoxicity

Neuromuscular junction showing ion channels and sites of action of presynaptic and postsynapticsnake venom neurotoxins, and three neurotoxins specifi c to mamba (Dendroaspis) venoms—ie, dendrotoxins, fasciculins, and calciseptine

Venom polypeptide toxins (“neurotoxins”)

• Postsynaptic (α) neurotoxins: α-bungarotoxin and cobrotoxin: bind to acetylcholine receptors at the motor endplate.

• Presynaptic (β) neurotoxins: β-bungarotoxin, crotoxin, and taipoxin, contain a phospholipase A subunit– These release acetylcholine at the nerve endings at

neuromuscular junctions and then damage the endings, preventing further release of transmitter

Samson A.O., Scherf. T., Eisenstein M., Chill J., and Anglister J., “The mechanism for acetylcholine receptor inhibition by alpha-neurotoxins and species-specific resistance to alpha-bungarotoxin revealed by NMR” , 2002, Neuron, 35, 319-332.

Faiz et al. Brain 2010: 133; 3181–3193

Synaptic vesicles labelled withanti-synaptophysin IgG (green)

Acth receptors labelled with TRITC-conjugated a-bungarotoxin (red).

Combined images

Neurotoxicity

• Neurotoxins bind to their receptors with high affinity, making reversal of paralysis by antivenom implausible.

• Rapid improvement in neurotoxicity has been noted when postsynaptic toxins were implicated—eg, Asian cobras and Australasian death adders (Acanthophis spp).

• Anticholinesterases sometimes reverse postsynaptic neurotoxicity in envenomed patients.

Naja kaouthia bite: neurotoxic effects

Naja kaouthia bite: neurotoxic effects

• Paralysis in envenomed people starts with ptosis, external ophthalmoplegia, and mydriasis, descending to involve muscles innervated by the other cranial and spinal nerves and leading to bulbar and respiratory paralysis and, if ventilation is supported, eventually to total flaccid paralysis

Necrotoxicity

• A range of venom myotoxic and cytolytic factors – zinc-dependent metalloproteinases and myotoxic

phospholipases A2. • Digestive hydrolases, hyaluronidase,

polypeptide cytotoxins (Elapidae)• Secondary effects of inflammation • Ischaemia, resulting from thrombosis,

intracompartmental syndrome, or application of a tight tourniquet, contributes to tissue loss.

Naja kaouthia bite: local necrosis

© DA Warrell

Myotoxicity

• Myotoxic phospholipases A2 in venoms of some species of Viperidae and Elapidae, especially sea snakes, cause generalised rhabdomyolysis that is often complicated by acute renal failure (B Niger)

Haemotoxicity

• Serine proteases, metalloproteinases, C-type lectins, disintegrins, and phospholipases: by activating or inhibiting coagulant factors or platelets, and disrupting vascular endothelium.

• Viperidae contain thrombinlike fibrinogenases and activators of prothrombin, factors V, X, and XIII, and endogenous plasminogen.

Haemotoxicity

• Toxins bind to a range of platelet receptors, inducing or inhibiting aggregation.

• Phospholipases A2 hydrolyse or bind to procoagulant phospholipids and inhibit the prothrombinase complex.

• Haemorrhagins (metalloproteinases) damage vascular endothelium: Spontaneous systemic bleeding

Haemotoxicity

• The combination of consumption coagulopathy, anticoagulant activity, impaired and few platelets, and vessel wall damage can result in severe bleeding, a common cause of death after bites by Viperidae, Australian Elapidae, and some Colubridae.

Cryptelytrops erythrusus

Cadiotoxicity

• Hypotension after snake bite – permeability factors that cause hypovolaemia

from extravasation of plasma– toxins acting directly or indirectly on cardiac

muscle, vascular smooth muscle, and on other tissues.

• Sarafotoxins potently vasoconstrict coronary and other arteries, and delay atrioventricular conduction

Clinical effects of venom action

• Neurotoxicity• Myotoxicity• Haemotoxicity• Necrotoxicity

• Cardiotoxicity

Role of antivenom

• The only specific antidote to the toxins in snake venom

• Hyperimmune globulin from an animal that has been immunised with the appropriate venom

• Albert Calmette: “Serum antivenimeuse”: 1895: quickly accepted

• Immunoglobulin antivenoms are accepted as essential drugs

• Reappraisal is needed• The limitations of antivenom treatment should

be recognized

Limitations of Anti Venom

• Patients with respiratory, circulatory, and renal failure need urgent resuscitation as well as antivenom.

Role of AV in neurotoxicity

• Pre synaptic neuro toxicity: can not be reversed especially in Krait bite

• Entubation is essential– Respiaratory failure– Impending resp failure

• Neostigmine: no effect

Low-cost, rechargeable, portable, disposable ventilator$300: typical ventilators $8,000-$60,000

• Post-synaptic paralysis: (clinical evidence confirming experimental studies) indicating AV can reverse this paralysis in at least some cases. – Naja kaouthia

SOP should be

• First ensure adequate respiratory effort– Entubation– Amboo

• Neostigmine• Antivenom

• Simultaneous approach

Role of AV in reversing coagulopathy

• Controversial– for most species there is good clinical evidence AV

can help control or reverse coagulopathy• The caveat is that if it is a consumptive

coagulopathy the response time will be longer– While AV can neutralize venom, it cannot speed

replacement of consumed coagulation factors or fibrinogen

Role of AV in reversing coagulopathy

• Controversial– for most species there is good clinical evidence AV

can help control or reverse coagulopathy• The caveat is that if it is a consumptive

coagulopathy the response time will be longer– While AV can neutralize venom, it cannot speed

replacement of consumed coagulation factors or fibrinogen

No anti venom for Pit vipers

Role of AV in myolysis

• Also uncertain• Theoretically it could be argued it won't help

much if major myolysis is already established.• Clinical experience shows cases where use of

AV was associated with a marked improvement in both symptoms and CK levels within a short time (a few hours only).

Role of AV in local tissue necrosis

• Treating local tissue injury: difficult • Evidence for using AV is muddy• Probably helps to at least some extent,

particularly if given early

Venom injection

Inflammatory reaction to envenomation

Further tissue damage

In situ injection oftoxin inhibitors or

antibody fragments

iv administration ofantivenom

NecrosisHemorrhage

ECM degradation

Blockade of deleterious effects of inflammation

Tissue repair andregeneration

Stimulus for tissueregeneration

Ancillaryinterventions

Localeffects

Local tissue destruction

© José María Gutiérrez

Role of AV in Nephrotoxicity

• Possible causes:– Hypotension– DIC– Direct nephrotoxic action

• AV even given early failed to prevent development of renal failure (Myanmar)

Treating renal failure

AV hypersensitivity

• Dependent on the dose, route, and speed of administration, and the quality of refinement, the risk of any early reaction varies from about 3% to more than 80%

• Only about 5–10% of reactions are associated with severe symptoms such as bronchospasm, angiooedema, or hypotension

• May be life threatening

• Treating physicians should actively look for early features like restlessness, urticaria

• Prompt intervention• React at the first sign e,g, single urticaria

– Adrenalin, steroid, H1 blocker: Repeat as necessary

• “Pre medication”!!!

So

• Don’t be disappointed if you don’t have anti venom

• Don’t be content when you have it

• Remain vigilant

Conclusion

• Snake venom is a complex mixture of different component

• Phenotypic presentation depends on action of these compounds on victims body

• Anti venom is the mainstay of treatment • Anti venom can not neutralize all effects of venom• Supportive treatment is crucial• Attending physician has an important role in

determining outcome

Acknowledgement

• Prof David A Warrell• Prof Jullian White

Thank You