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Polyvalent Anti-Snake Venom (4-Snake / Anti-Venom)

What Is Polyvalent Antivenom?

Antivenom (anti-snake venom, ASV) is the mainstay of therapy for venomous snakebites. It is composed of heterologous antibodies derived from the serum of animals (horses or sheep) immunized with one or more snake venoms. The antibodies bind and neutralize venom molecules.

Monovalent antivenom: effective against one specific snake species
Polyvalent antivenom: effective against venom from multiple species — used when the snake identity is unknown or when bites from several species are possible in a given region

"If the identity of the animal is uncertain, a polyvalent antivenom or mixture of antivenoms is preferred." — Goldman-Cecil Medicine

4-Snake Polyvalent Antivenom (India/South Asia)

The most well-known "4-snake" or "polyvalent" ASV is produced in India and covers the "Big Four" dangerous snakes responsible for the majority of snakebite deaths on the subcontinent:

Snake	Type	Primary Toxicity
Russell's viper (Daboia russelii)	Viperidae	Coagulopathy, nephrotoxicity, neurotoxicity
Saw-scaled viper (Echis carinatus)	Viperidae	Coagulopathy (consumptive), haemorrhage
Indian cobra (Naja naja)	Elapidae	Neurotoxicity, cytotoxicity
Common krait (Bungarus caeruleus)	Elapidae	Neurotoxicity (presynaptic), respiratory paralysis

Production

Animals (horses) are immunized with venoms from the four species above
Immune serum is harvested and purified into Fab or F(ab')₂ antibody fragments
- Fab (papain-digested): shorter half-life, more likely to have recurrent coagulopathy
- F(ab')₂ (pepsin-digested): longer serum half-life, reduced recurrence of coagulopathy
The immunogenic Fc portion is removed during purification to reduce hypersensitivity reactions

"Antivenom can be monovalent, effective against one type of venom, or polyvalent and effective against venom from multiple species. An antivenom binds and neutralizes a toxin. Early administration after injury is key." — Goodman & Gilman's

Indications for Administration

Give antivenom immediately when any of the following are present:

Category	Signs/Symptoms
Local	Progressive swelling beyond the wrist/ankle, necrosis
Haematological	Coagulopathy, thrombocytopenia, spontaneous bleeding (20-minute whole blood clotting test positive — blood fails to clot)
Neurological	Ptosis, ophthalmoplegia, respiratory paralysis, dysarthria
Cardiovascular	Hypotension, ECG abnormalities
Systemic	Generalised rhabdomyolysis, haemolysis, oliguria/AKF

"Pregnancy is not a contraindication to antivenom therapy." — Tintinalli's Emergency Medicine

Dosing

Route: IV infusion (preferred over IM)
Dose: Determined by severity of envenomation, NOT by patient weight — children receive the same dose as adults (the dose neutralizes venom, not patient body mass)
Reconstitution: Typically diluted in 100–250 mL normal saline, infused over 30–60 minutes
Repeat dosing: If signs of envenomation persist (especially coagulopathy), repeat doses are given
Locally available polyvalent ASV is effective against envenoming by multiple or unknown snakes

Adverse Effects / Hypersensitivity

Antivenom carries a significant risk of hypersensitivity reactions (since it is a foreign protein):

Reaction Type	Features	Management
Early anaphylactic (within 10–180 min)	Urticaria, bronchospasm, hypotension	Adrenaline (epinephrine) IM, antihistamines, corticosteroids
Pyrogenic	Fever, rigors (often due to endotoxin contamination)	Paracetamol, slow infusion rate
Late serum sickness (5–14 days)	Fever, rash, arthralgia, lymphadenopathy	Corticosteroids (prednisolone)

"Immediate and delayed allergic reactions to Crotalidae polyvalent immune Fab antivenom" are well-documented. — Mulholland & Greenfield's Surgery

Premedication: Subcutaneous or IV adrenaline (0.25 mg SC) is recommended by some authorities before antivenom infusion, though this is debated. Have resuscitation equipment at the bedside.

Contraindications / Cautions

No absolute contraindications when life-threatening envenomation is present
Prior hypersensitivity to equine products increases risk — but is NOT an absolute contraindication
Use with caution in atopic individuals/asthmatics (higher risk of anaphylaxis)

Specific Toxin Profiles Covered

Viperidae (Russell's viper, Saw-scaled viper)

Venom type: Haemotoxic + cytotoxic
Causes: Disseminated intravascular coagulation (DIC), thrombocytopenia, fibrinogen consumption, local necrosis
Lab: Elevated PT/INR, low fibrinogen, thrombocytopenia

Elapidae (Cobra, Krait)

Venom type: Neurotoxic (pre- and post-synaptic)
Krait: Presynaptic — blocks ACh release; less responsive to antivenom alone
Cobra: Post-synaptic — blocks nicotinic receptors; may respond to neostigmine
Causes: Ptosis, bulbar palsy, respiratory failure
Adjunct: Neostigmine (35 µg/kg IV) may reverse post-synaptic neurotoxicity; anticholinesterase effect at NMJ

Storage & Shelf Life

Store at 2–8°C (refrigerated, not frozen)
Lyophilised preparations have a longer shelf life than liquid preparations
Check expiry date — expired antivenom loses potency

Key Points Summary

Feature	Detail
Composition	Horse/sheep-derived Fab or F(ab')₂ antibody fragments
Coverage	Russell's viper, Saw-scaled viper, Indian cobra, Common krait
Route	IV infusion (preferred)
Dose basis	Severity of envenomation (not body weight)
Major ADR	Anaphylaxis (early), serum sickness (late)
Key adjunct	Neostigmine for post-synaptic neurotoxicity
Contraindication	None absolute in life-threatening envenomation

Sources: Tintinalli's Emergency Medicine (A Comprehensive Study) | Goldman-Cecil Medicine | Goodman & Gilman's Pharmacological Basis of Therapeutics | Comprehensive Clinical Nephrology, 7th Ed.

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