Snakebite & Venomous Animal Stings

Snakebite is a major cause of preventable rural mortality in India (the global capital of snakebite deaths) and a recurring NEET PG favourite in Forensic Medicine & Toxicology. This note integrates venom pharmacology, clinical syndromes, antivenom protocol, and the medico-legal/postmortem angle examiners love.

Definition & medico-legal importance

A venomous bite is the inoculation of toxic secretions through specialised fangs into a victim's tissues. Forensically, snakebite deaths are almost always accidental, but rare cases involve homicide (deliberate inoculation of venom, the classic "Ahmednagar snake murder" type) and even suicide. Distinguishing genuine envenomation from non-venomous bite, and identifying the offending species from clinical/autopsy features, is the core medico-legal task.

High-yield: India accounts for the largest number of snakebite deaths worldwide (~58,000/year). The "Big Four" responsible for most fatal envenomations are Russell's viper, Common krait, Indian cobra, and Saw-scaled viper.

Classification of venomous snakes

Venomous land snakes belong chiefly to three families. Sea snakes (Hydrophiidae) cause myotoxic envenomation.

Family	Examples	Fang type	Predominant venom
Elapidae	Cobra (Naja), Krait (Bungarus), King cobra, Coral snake	Short, fixed, grooved (proteroglyph)	Neurotoxic
Viperidae	Russell's viper (Daboia), Saw-scaled viper (Echis), Pit vipers	Long, mobile, canalised, retractable (solenoglyph)	Vasculotoxic / haemotoxic
Hydrophiidae	Sea snakes (Enhydrina)	Short fixed fangs	Myotoxic
Colubridae	Mostly non-venomous; rear-fanged (opisthoglyph) e.g. Boomslang	Rear fixed	Variable

High-yield — fang dentition mnemonic: "P-O-S" in increasing fang sophistication → Proteroglyph (elapids, front fixed) → Opisthoglyph (rear) → Solenoglyph (vipers, front folding, most advanced). Aglyphous = non-venomous (no specialised fangs).

Venom composition & pathophysiology

Venom is modified saliva — a complex mixture of enzymes (phospholipase A2, proteases, hyaluronidase "spreading factor", L-amino acid oxidase) and low-molecular polypeptide toxins. The clinical syndrome depends on which fraction dominates.

Neurotoxic venom (elapids): Acts at the neuromuscular junction, producing a descending flaccid paralysis.

• α-bungarotoxin / cobra α-neurotoxin → postsynaptic block (competes with acetylcholine at nicotinic receptors — curare-like, reversible by antivenom and sometimes neostigmine).
• β-bungarotoxin (krait) → presynaptic block (inhibits acetylcholine release; poorly reversible — hence krait paralysis is prolonged and antivenom/neostigmine respond poorly once established).
• Cobra venom also contains cardiotoxin and prominent local cytotoxicity/necrosis; krait venom causes minimal local reaction.

Vasculotoxic venom (vipers): Acts on the haemostatic system and vascular endothelium.

• Procoagulant enzymes activate factor X / prothrombin → consumption coagulopathy → defibrination → incoagulable blood and DIC.
• Haemorrhagins damage endothelium → spontaneous systemic bleeding.
• Russell's viper venom additionally causes capillary leak syndrome, acute kidney injury (AKI), and pituitary infarction (Sheehan-like → hypopituitarism, "Russell's viper bite induced hypopituitarism").

Myotoxic venom (sea snakes): Causes generalised rhabdomyolysis → myoglobinuria → AKI and hyperkalaemia.

High-yield: Russell's viper venom is the basis of the dilute Russell's viper venom time (dRVVT) used to detect lupus anticoagulant — a classic lab cross-link question.

Identifying the offending snake

Snake identification features

Feature	Venomous (typical viper/elapid)	Non-venomous
Head	Triangular (vipers); cobra has hood	Rounded/oval
Pupil	Vertical/elliptical (vipers, many)	Round
Belly scales	Cover full width (single row)	Smaller, not full width
Pit (loreal)	Present in pit vipers (heat sensor)	Absent
Tail	Single row of subcaudal scales (vipers)	Double row
Fang	Long folding (viper) / short fixed (elapid)	No fangs / small teeth

• Cobra — hood with spectacle (binocular) mark; krait — steel-blue/black with white paired crossbands, narrow head; Russell's viper — chain of dark rhomboid/elliptical spots, loud hiss; Saw-scaled viper — rough serrated lateral scales producing rasping sound, "**" mark on head.

Fang-mark patterns (examiner favourite)

• Two distinct puncture (fang) marks with surrounding bruising → venomous bite (esp. viper, with local swelling/necrosis).
• A single fang mark or two parallel rows of fine teeth marks without swelling → usually non-venomous.
• Krait bites may be painless with minimal/invisible marks, often occurring at night while the victim sleeps on the floor — the patient wakes with abdominal pain, ptosis and paralysis. This explains the classic "unexplained early-morning respiratory paralysis" scenario.

High-yield: A victim bitten by a krait at night may show no local signs at all — diagnosis is clinical (ptosis → bulbar → respiratory paralysis). Absence of fang marks does NOT exclude envenomation.

Clinical features by syndrome

Neurotoxic (cobra/krait) — descending paralysis flow: Ptosis & ophthalmoplegia → diplopia → bulbar palsy (dysphagia, dysphonia) → broken-neck sign (neck muscle weakness) → diaphragmatic/respiratory failure → death by asphyxia.

• Earliest reliable sign = ptosis. Pupils may be dilated and fixed.
• Abdominal pain and paraesthesia common with krait.

Vasculotoxic (Russell's/saw-scaled viper):

• Local: severe pain, swelling, blistering, regional lymphadenopathy, local necrosis.
• Systemic bleeding: gum bleeding, haematuria, haematemesis, ecchymoses, intracranial bleed.
• Incoagulable blood, AKI (oliguria), shock, capillary leak.

Myotoxic (sea snake): Muscle pain, trismus, myoglobinuria (dark urine), hyperkalaemic cardiac arrest, late respiratory paralysis.

Diagnosis & investigation of choice

The 20-minute Whole Blood Clotting Test (20WBCT) is the bedside investigation of choice for viper envenomation and the single most exam-tested test.

20WBCT method: 2 mL fresh venous blood in a clean, dry glass tube, left undisturbed and upright for 20 minutes, then tilted once.

• Non-clotting blood at 20 minutes = coagulopathy = systemic viper envenomation → indication for antivenom.

Other tests: PT/aPTT/INR, fibrinogen, D-dimer, platelet count, serum creatinine, CK and urine myoglobin (myotoxic), ECG (hyperkalaemia/cardiotoxicity), and arterial blood gas in neuroparalysis.

High-yield: A positive 20WBCT (non-clotting blood) is itself an indication for anti-snake venom even if local signs are mild.

Management & drug of choice

Stepwise approach (do-it-R.I.G.H.T.):

1. Reassure — most bites are by non-venomous snakes or are "dry bites."
2. Immobilise the limb like a fracture (splint); keep below heart level controversial — keep at neutral; remove rings/tight items.
3. Get to Hospital quickly; avoid harmful traditional first aid.
4. Tell the doctor of evolving symptoms (ptosis, bleeding).

First-aid DON'Ts (commonly asked): No tight arterial tourniquet, no incision/suction, no ice, no electric shock, no local herbal application. The previously taught pressure immobilisation bandage is now reserved mainly for neurotoxic elapid bites and is de-emphasised in Indian guidelines.

Definitive treatment:

• Polyvalent Anti-Snake Venom (ASV) is the drug/antidote of choice. Indian ASV is raised against the Big Four (Russell's viper, saw-scaled viper, cobra, krait) — it is polyvalent, not species-specific.
• Indications: systemic envenomation — non-clotting 20WBCT/coagulopathy, spontaneous systemic bleeding, neurotoxicity, cardiovascular instability, AKI, local swelling crossing a joint or rapidly progressing.
• ASV is given IV (infusion of reconstituted lyophilised or liquid ASV); never inject ASV locally into the bite site.
• Anaphylaxis to ASV is treated with adrenaline (IM 0.5 mg) — adrenaline is the drug of choice for ASV reactions; antihistamines/steroids are adjuncts.
• Neostigmine + atropine/glycopyrrolate trial for neurotoxic (especially cobra/postsynaptic) bites improves muscle power; krait (presynaptic) responds poorly.
• Supportive: mechanical ventilation for respiratory paralysis (lifesaving in krait), dialysis for AKI, blood products only after adequate ASV.

High-yield: ASV neutralises only circulating, unbound venom — give it early. Ptosis/neuroparalysis from cobra may reverse with ASV + neostigmine; established krait paralysis needs ventilatory support as the mainstay.

Complications

• Respiratory failure (neurotoxic) — leading cause of death in elapid bites.
• Acute kidney injury — hallmark of Russell's viper (acute tubular necrosis, occasionally cortical necrosis).
• DIC and intracranial haemorrhage (viper).
• Compartment syndrome / limb necrosis / gangrene — local viper effect; may need fasciotomy/amputation.
• Hypopituitarism (Russell's viper) — anterior pituitary infarction, delayed presentation.
• Serum sickness 5–10 days after ASV; ASV anaphylaxis acutely.
• Sea-snake: hyperkalaemic cardiac arrest from rhabdomyolysis.

Postmortem / autopsy findings

• External: two fang puncture marks with surrounding ecchymosis/oedema; local blistering, necrosis or gangrene (viper); marks may be absent in krait.
• Viper death: widespread haemorrhages (subcutaneous, serosal, GI, intracranial), incoagulable/fluid blood in chambers, swollen necrotic limb, congested haemorrhagic kidneys.
• Elapid death: few external signs; findings of asphyxia — congestion, petechiae, frothy airway, generalised visceral congestion (death from respiratory paralysis).
• Toxicological detection: venom can be demonstrated from the bite-site tissue and blood by ELISA (most sensitive/specific), or by precipitin and other immunoassays — important when species identification is contested medico-legally.

High-yield: In suspected snakebite homicide or contested cases, ELISA on bite-site tissue/serum is the investigation to detect and identify venom postmortem.

Key differentials

• Krait paralysis vs Guillain–Barré, myasthenic crisis, organophosphate poisoning, hypokalaemic periodic paralysis — descending paralysis with ptosis and a nocturnal history points to krait.
• Viper coagulopathy vs other causes of bleeding (sepsis-DIC, leptospirosis, dengue haemorrhagic fever) — 20WBCT and bite history clarify.
• Dry bite (no envenomation) vs envenomated bite — serial observation and 20WBCT.
• Scorpion sting vs snakebite — see below.

Other venomous animal stings (toxicology cross-links)

Animal	Key toxin/effect	Treatment
Scorpion (Mesobuthus tamulus, Indian red scorpion)	Autonomic storm — sympathetic then parasympathetic; pulmonary oedema, myocarditis	Prazosin (α-blocker) is drug of choice; scorpion antivenom adjunct
Honey bee/wasp	Histamine, melittin; anaphylaxis	Adrenaline, antihistamine, steroids
Spider (black widow)	α-latrotoxin → muscle cramps	Calcium gluconate, antivenom
Jellyfish/sea anemone	Nematocyst toxin	Vinegar (acetic acid) deactivates nematocysts

High-yield: Prazosin is the cornerstone drug for red scorpion (Mesobuthus tamulus) envenomation — frequently confused with snakebite questions.

Recently asked / exam angle

• Fang-type matching: proteroglyph = elapid, solenoglyph = viper, opisthoglyph = rear-fanged colubrid, aglyphous = non-venomous.
• Presynaptic vs postsynaptic neurotoxin: β-bungarotoxin (krait) = presynaptic, poor antivenom response; cobra α-toxin = postsynaptic, curare-like, responds to neostigmine.
• Investigation of choice in viper bite = 20-minute whole blood clotting test; venom detection postmortem = ELISA.
• Russell's viper → AKI + DIC + hypopituitarism; dRVVT detects lupus anticoagulant.
• Drug for ASV anaphylaxis = adrenaline; drug for red scorpion sting = prazosin.
• Earliest sign of neurotoxic envenomation = ptosis.
• Identification of the snake from belly scales, pupil shape, loreal pit, and subcaudal scale rows.
• First-aid: tourniquets and incisions are contraindicated; immobilisation + rapid transport is correct.
• Sea snake → myoglobinuria / rhabdomyolysis / hyperkalaemia.
• ASV in India is polyvalent against the Big Four.

Rapid revision

1. India has the world's highest snakebite mortality; Big Four = Russell's viper, krait, cobra, saw-scaled viper.
2. Elapids = neurotoxic (proteroglyph); vipers = vasculotoxic (solenoglyph); sea snakes = myotoxic.
3. Krait β-bungarotoxin = presynaptic → prolonged paralysis, poor ASV/neostigmine response → ventilate.
4. Cobra α-neurotoxin = postsynaptic (curare-like) → reversible, neostigmine helps.
5. Krait bite = painless, often no fang mark, nocturnal, wakes with ptosis & abdominal pain.
6. Ptosis is the earliest sign of neuroparalysis; death is by respiratory failure.
7. 20-minute WBCT non-clotting = systemic viper envenomation = ASV indication.
8. Russell's viper → AKI, DIC, capillary leak, hypopituitarism; venom → dRVVT for lupus anticoagulant.
9. ASV (polyvalent, IV) is the antidote; never inject locally; treat anaphylaxis with adrenaline.
10. No tourniquet, no incision, no suction; splint and transport fast.
11. Prazosin = drug of choice for Indian red scorpion (Mesobuthus tamulus) sting.
12. Postmortem venom detection of choice = ELISA on bite tissue/serum; autopsy of elapid death shows asphyxial signs, viper death shows haemorrhages and fluid blood.