Snake Venom and Antivenin

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Dr. Dharmakanta Kumbhakar
Snakebite is a com mon life-threatening condition in many temperate and tropical regions. The venomous snakes of the world are grouped into the families -Viperidae (e.g. subfamily Viperinae: the Old World vipers, Russell’s viper, European adder; subfamily Crotalinae: the New World and Asian pit vipers, rattle snakes, moccasins), Elapidae (e.g. cobras, coral snakes, krait, mambas and all Australian venomous snakes), Hydrophiidae (the sea snakes), Atractaspididae (e.g. burrowing asps, stiletto snakes, natal black snake), and Colubridae (e.g. boomslang, Australian brown tree snake).

Snake venom is produced in modified parotid glands normally responsible for secreting saliva. It is stored in structures called alveoli behind the animal’s eyes, and ejected voluntarily through its hollow tubular fangs. It is a complex mixture of enzymes, low molecular weight polypeptides, glycoproteins and metal ions. Among the deleterious components are hematotoxins (hemorrhagins and hemolysins promote vascular leaking and cause both local and systemic bleeding).Various proteolytic enzymes (myotoxins) cause local tissue necrosis, affect the coagulation pathway at various steps causes consumptive coagulopathy, impair organ functions.

Myocardial depressant factors (cardiotoxins) cause coronary vasoconstriction, AV block and reduce cardiac output; and neurotoxins act either pre- or postsynaptically to inhibit peripheral nerve impulses cause muscular paralysis and respiratory failure. Vasodilators like kallikrein causes hypotension and hyaluronidase spreads the venom. Most snake venoms have multisystem effects in their victims. Spitting cobras can spit venom in person’s eyes. This results in immediate pain, ophthalmoparesis, and sometimes blindness.

Snake venom detection kits are available in some countries. The venom can be identified from a dry swab of the bite site, wound aspirate, victim’s blood or urine using monoclonal antibody techniques. The 20- minute whole blood clotting test is a useful bedside tool in remote areas; a 2-3 ml sample of venous blood from the victim is left undisturbed at ambient temperature for at least 20 minutes. The vessel containing the blood is then tipped once and compared with a normal control. If it has not clotted, there is hemostatic disturbance from systemic envenoming.

The most appropriate therapy for envenoming is timely administration of the species appropriate antivenin. Modern antivenins are usually polyvalent, making them effective against the venom of numerous snake species. In India Polyvalent Haffkine antivenin is available. The newest available antivenin in the United States (CroFab) is an ovine antivenin produced by techniques that yield purified Fab antibodies. This product reduces the risk of allergic sequelae. Antivenin is injected into the person intravenously, and works by binding to and neutralizing venom enzymes. It cannot undo damage already caused by venom, so antivenin treatment should be sought as soon as possible, preferably within 4 hours of the bite.

Indications for antivenin administration are – cardiogenic shock, spontaneous systemic bleeding, incoagulable blood, neurotoxicity, and hematuria, other evidence of hemolysis /rhabdomyolysis, rapidly progressive excessive swelling and bites on digits by snake with known necrotic venom. Although some people may develop serious adverse reactions to antivenin, such as anaphylaxis in emergency situations this is usually treatable and hence the benefit outweighs the potential consequences of not using antivenin. Some manufacturers recommend a skin-testing procedure for potential allergy; but it doesn’t, however, reliably predict which patients will have an allergic reaction to antivenin.
Giving adrenaline to prevent adverse effect to antivenin before they occur might be reasonable where they occur commonly. A syringe containing 0.5 ml 1: 1000 adrenaline must be available whenever antivenin is administered. The rate of administration of antivenin should be based on the severity of the case and the patient’s tolerance to the antivenin.

Bites by large snakes may need relatively high antivenin doses, particularly in children or small adults.