Snake bite
Keep the patient still, do a 20-minute whole blood clotting test, give anti-snake venom only if there are signs of envenoming, and have adrenaline ready.
A 30-year-old farmer was bitten on the foot while cutting crops 2 hours ago. His foot and lower leg are swollen and his gums are bleeding. A tight rope is tied around his thigh. His blood has not clotted in a glass tube after 20 minutes.
Does he need anti-snake venom, and how much?
- Keep the patient and the limb stillImmobilise the limb with a splint. Reassure: fear makes everything worse.
- Do the 20WBCT now2 mL of blood in a clean, dry glass tube, left untouched for 20 minutes. Not clotted = venom-induced coagulopathy.
- Look for envenomingBleeding, swelling spreading, ptosis, double vision, difficulty swallowing or breathing, low BP, dark urine.
- Remove tourniquets carefullyWith ASV and adrenaline ready, as venom may be released.
- ASV only if envenomedPolyvalent ASV IV. Adrenaline drawn up before starting.
- Observe every bitten patient for 24 hoursSigns can appear late, especially after krait bites.
- Tetanus, paracetamolNo NSAIDs, no IM injections in viper bites.
- Repeat 20WBCT every 6 hoursUntil it clots normally.
- Blood that does not clot at 20 minutes
- Ptosis, double vision, difficulty swallowing or breathing
- Low BP or shock
- Little or dark urine
- Swelling spreading fast
- Reaction to ASV
- Respiratory paralysis (krait, cobra)
- Bleeding and kidney failure (vipers)
- Krait bite with no fang marks
- Compartment syndrome mimic
- ASV anaphylaxis
| 20WBCT | Not clotted at 20 minutes = venom coagulopathy: give ASV |
| ASV indication | Signs of envenoming only, not for every bite. Dose in the ASV section |
| Before ASV | Adrenaline 0.5 mg IM drawn up and ready |
| Viper | Repeat ASV if 20WBCT still not clotting at 6 hours |
| Neurotoxic | Repeat ASV after 1–2 hours if getting worse |
Check every dose against a current drug reference and your hospital protocol.
On this page
The snakes in Pakistan
- Painful swelling, blisters
- Bleeding from gums, bite site, urine
- 20WBCT does not clot
- Kidney injury (Russell's viper)
- Bite at night while sleeping on the floor
- Often painless, no swelling, fang marks may be invisible
- Belly pain, then ptosis and paralysis
- Painful swelling, skin may blacken
- Ptosis, double vision, weakness, breathing failure
First aid: myths and facts
- No tight tourniquets or ropes. They do not stop venom and can cause gangrene.
- No cutting, sucking or burning the bite.
- No herbal remedies or traditional healers before hospital. They waste time.
- No ice on the bite.
- Do: keep the patient calm and still, splint the limb, remove rings and watches, and get to hospital fast.
- If a snake was killed, bring it (carefully) or a photo. Never try to catch it.
The 20-minute whole blood clotting test
- Take 2 mL of venous bloodUse a fresh venepuncture.
- Put it in a new, clean, dry glass tubeGlass, not plastic. Plastic tubes do not activate clotting.
- Leave it undisturbed for 20 minutesUpright, at room temperature.
- Tip the tube once gentlyIf the blood is still liquid and runs down, it has not clotted.
- Not clotted = venom coagulopathyGive ASV.
- Repeat every 6 hoursUntil it clots. Repeat 6 hours after each ASV dose.
Signs of envenoming: who needs ASV
- Blood not clotting on 20WBCT, or spontaneous bleeding (gums, bite site, urine, vomit).
- Neurotoxic signs: ptosis, double vision, difficulty swallowing or speaking, weakness, breathing difficulty.
- Low BP or shock.
- Dark urine (myoglobin or haemoglobin) or falling urine output.
- Local swelling spreading quickly, involving more than half the bitten limb, or within 48 hours of a bite on fingers or toes.
Most bites need no ASV. ASV is precious and can cause serious reactions. Never give it to "be safe" when there are no signs.
Anti-snake venom (ASV)
- Repeat 20WBCT 6 hours after the first dose
- Still not clotting: repeat 10 vials
- Continue until it clots
- Reassess at 1–2 hours
- Getting worse: repeat 10 vials
- Ventilation is the main treatment if breathing fails
Itching, hives, fever, wheeze, low BP, usually in the first hour.
- Stop the infusion.
- IM adrenaline 0.5 mg for anything more than a mild rash.
- Antihistamine for itching once stable.
- Restart ASV slowly once settled. The patient still needs it.
Neurotoxic bites
- Watch for ptosis (ask the patient to look up), double vision, nasal voice, difficulty swallowing, drooling, weak neck flexion, and shallow breathing.
- Breathing failure can come fast. Intubate and ventilate early. Patients can recover fully with ventilation, even after days.
- Neostigmine trial: works best in cobra bites. Give atropine first to block side effects.
Viper bites: the kidneys and the limb
- Kidney injury is common after Russell's viper bites. Measure urine output, creatinine and potassium daily. Dialysis may be needed.
- Swelling: severe swelling can look like compartment syndrome. True compartment syndrome is uncommon. Correct the clotting with ASV first. Surgery (fasciotomy) only after measuring compartment pressure and once the blood clots, on senior and surgical advice.
- Avoid NSAIDs and intramuscular injections while the blood is not clotting.
Supportive care
- Antibiotics only if the wound is infected or has been cut or tampered with.
- Clean the wound. Elevate the limb once ASV has been given.
Investigations
| Test | Why |
|---|---|
| 20WBCT | Coagulopathy at the bedside, every 6 hours |
| CBC | Low platelets, anaemia from bleeding, haemolysis |
| PT/INR, APTT, fibrinogen (if available) | Coagulopathy |
| Urea, creatinine, potassium | Kidney injury |
| Urine dipstick | Blood or myoglobin |
| ECG | Arrhythmias, high potassium |
| CK | Muscle breakdown |
Common mistakes
- Giving ASV to every bite "to be safe".
- Using a plastic tube for the 20WBCT.
- Discharging a "dry" bite after 2 hours: krait signs can appear many hours later.
- Missing a krait bite because there are no fang marks and no pain.
- Starting ASV without adrenaline ready.
- Rushing to fasciotomy before the clotting is corrected.
Frequently asked questions
What is the dose of anti-snake venom?
10 vials of polyvalent ASV IV in 0.9% sodium chloride over 1 hour. Children get the same dose as adults. Repeat according to the 20WBCT (vipers) or clinical response (neurotoxic bites).
How is the 20-minute whole blood clotting test done?
Put 2 mL of fresh venous blood in a new, clean, dry glass tube and leave it undisturbed for 20 minutes. If it has not clotted when the tube is tipped, there is venom-induced coagulopathy.
Does every snake bite need ASV?
No. ASV is only given when there are signs of envenoming: abnormal clotting or bleeding, neurotoxic signs, shock, dark urine or rapidly spreading swelling.
Why is a krait bite dangerous?
It is often painless, happens at night, may leave no visible mark, and can cause paralysis and breathing failure hours later.
What is the 20 minute test for snake bite?
The 20-minute whole blood clotting test (20WBCT): 2 mL of fresh blood is left undisturbed in a clean dry glass tube for 20 minutes. If it has not clotted, the patient has venom-induced coagulopathy and needs anti-snake venom.
Which injection is given for snake bite?
Polyvalent anti-snake venom (ASV), given IV only when there are signs of envenoming, with adrenaline ready for reactions. Tetanus toxoid is also given.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A man was bitten by a snake 3 hours ago. There is no swelling, the 20-minute whole blood clotting test (20WBCT) shows a clot, and he has no neurological signs. What is the best management?
Answer: Observe with repeat 20WBCT. Many bites are dry. ASV is given only when there is evidence of systemic envenoming (non-clotting blood, bleeding, neurotoxicity, shock) or severe local signs. Observe for at least 24 hours.
- Give tetanus immunoglobulin and ASV: ASV is not indicated yet.
- Apply a tight tourniquet: Harmful.
- Discharge home: Envenoming can appear later.
- Give 10 vials of anti-snake venom now: ASV is given only for signs of envenoming.
2. Ten minutes into an anti-snake venom infusion, the patient develops wheeze and BP 85/50. What should you do?
Answer: Stop ASV, IM adrenaline, then restart. Early anaphylactic reactions to ASV are common. Stop, treat with IM adrenaline, then restart cautiously because the envenoming still needs treatment.
- Give atropine: Not for anaphylaxis.
- Stop ASV permanently: ASV is still needed; restart once the reaction is treated.
- Continue the infusion faster: This is an anaphylactic reaction.
- Give hydrocortisone only: Adrenaline is the key treatment.
3. A child woke at night with abdominal pain and now has drooping eyelids and difficulty swallowing. No bite mark is seen. What is the most likely diagnosis?
Answer: Krait bite. Kraits bite at night, often while sleeping on the floor, with little local mark. Ptosis, bulbar weakness and abdominal pain suggest neurotoxic envenoming: ASV and preparation for ventilation.
- Botulism: Possible but the nocturnal onset and region make krait bite likely.
- Viper bite: Vipers cause swelling and bleeding.
- Myasthenic crisis: Not with this sudden nocturnal story.
- Guillain-Barré syndrome: Usually ascending weakness over days.
4. A farmer in Sindh is bitten on the foot by a snake at night. Two hours later he has bleeding gums and his 20-minute whole blood clotting test shows unclotted blood. Which snake is most likely?
Answer: Saw-scaled viper. Viper bites (Russell’s viper, saw-scaled viper) cause local swelling and haemotoxicity: consumption coagulopathy, bleeding, AKI. The 20-minute whole blood clotting test (20WBCT) is a bedside test — unclotted blood indicates coagulopathy and need for polyvalent antivenom (repeat every 6 h if still unclotted). Krait: painless night bite, descending paralysis; cobra: local necrosis and paralysis. Avoid tourniquets and incisions.
- Common krait: Krait bites are neurotoxic and painless.
- Cobra: Cobra bites are neurotoxic with local necrosis.
- Sea snake: Sea snakes are myotoxic and coastal.
5. A man bitten while sleeping on the floor in rural Sindh wakes with abdominal pain, ptosis, difficulty swallowing and progressive weakness. There is minimal local swelling. 20WBCT is normal. What is most important?
Answer: Antivenom + ventilation. Common krait bites occur at night on people sleeping on the floor; often painless with no local signs. Neurotoxic envenoming: ptosis, ophthalmoplegia, bulbar palsy, descending paralysis, respiratory failure; abdominal pain. Give polyvalent antivenom, airway protection and mechanical ventilation as needed; neostigmine trial (more effective for cobra). Prevention: sleep on cots/under bed nets.
- Tourniquet and incision: Tourniquets and incisions are harmful.
- Vitamin K and FFP: This is a neurotoxic bite, not haemotoxic.
- Antibiotics and tetanus only: Antibiotics and tetanus cover do not treat neurotoxicity.
6. Why is neostigmine given with glycopyrrolate at the end of anaesthesia?
Answer: To block muscarinic side effects. Neostigmine inhibits acetylcholinesterase, raising ACh at the neuromuscular junction to reverse non-depolarising blockers. It also raises ACh at muscarinic receptors, causing bradycardia, bronchospasm and secretions; glycopyrrolate (or atropine) blocks these. Neostigmine is a quaternary amine and does not cross the blood–brain barrier.
- To prolong the neuromuscular block: It reverses the block.
- To reverse opioid effects: Naloxone reverses opioids.
- To treat malignant hyperthermia: Dantrolene.
- To reduce postoperative nausea: Not the purpose.
7. Which enzyme in snake venom is mainly responsible for tissue destruction and haemolysis?
Answer: Phospholipase A2. Snake venoms contain phospholipase A2 (membrane damage, haemolysis, myotoxicity, neurotoxicity), hyaluronidase (spreading factor), proteases and metalloproteinases (tissue damage, bleeding), and procoagulant enzymes causing consumption coagulopathy (vipers). Neurotoxins (cobra, krait) block neuromuscular transmission. The treatment is antivenom.
- Amylase: Not a venom toxin.
- Lactase: Not a venom toxin.
- Pepsin: Not a venom toxin.
- Trypsinogen: Not a venom toxin.
References and dose sources
- WHO fact sheet: snakebite envenoming
- Anti-snake venom (ASV) dosing: WHO: Guidelines for the management of snakebites (see also our anti-snake venom (asv) dose page)
- Adrenaline (epinephrine) dosing: Drugs.com: Epinephrine dosage (see also our adrenaline (epinephrine) dose page)
- Atropine dosing: Drugs.com: Atropine dosage (see also our atropine dose page)
- Neostigmine dosing: Drugs.com: Neostigmine dosage (see also our neostigmine dose page)
- Tetanus toxoid and immunoglobulin dosing: Drugs.com: Tetanus toxoid dosage (see also our tetanus toxoid and immunoglobulin dose page)
- Paracetamol dosing: Drugs.com: Acetaminophen (paracetamol) dosage (see also our paracetamol dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.