Forensic Medicine and Toxicology
Biotoxicology: Snakebite and Animal Venoms
Biotoxicology for MBBS: the big four snakes, neurotoxic versus haemotoxic bites, anti-snake venom, scorpion sting and insect stings, mapped to NMC code FM11.1.
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Biotoxicology: Snakebite and Animal Venoms
Biotoxicology for MBBS: the big four snakes, neurotoxic versus haemotoxic bites, anti-snake venom, scorpion sting and insect stings, mapped to NMC code FM11.1.
Biotoxicology covers poisoning by living organisms, dominated in India by snakebite. This chapter distinguishes neurotoxic elapid bites from haemotoxic viper bites, sets out first aid and anti-snake venom treatment, and covers scorpion sting, insect stings and spider bites with their management.
High-yield: Biotoxicology: Snakebite and Animal Venoms
- The big four venomous snakes of India are the cobra, the common krait, Russell's viper and the saw-scaled viper.
- Elapid venom, as in the cobra and krait, is mainly neurotoxic and causes ptosis, bulbar palsy and respiratory paralysis.
- Viper venom is mainly haemotoxic and vasculotoxic, causing local swelling, bleeding, coagulopathy and acute kidney injury.
- Krait bites are typically painless with little local swelling, and victims may wake with abdominal pain and paralysis in the early morning.
- The 20-minute whole blood clotting test is a simple bedside marker of the coagulopathy in viper envenomation.
- Polyvalent anti-snake venom is the mainstay of treatment and is given only when there are signs of systemic or significant local envenomation.
- Pressure immobilisation and reassurance are useful first aid, while tourniquets, incision and suction are harmful and are not advised.
- Neostigmine with atropine can improve the neuromuscular block from post-synaptic neurotoxins such as cobra venom.
- Scorpion sting, especially the red scorpion, causes an autonomic storm with sweating, salivation and cardiac effects, treated with prazosin.
- Anti-scorpion venom may be used with prazosin in severe red scorpion envenomation.
- Hymenoptera stings from bees and wasps can cause local reactions or life-threatening anaphylaxis needing adrenaline.
- A bee leaves its barbed sting in the skin, which should be scraped out rather than squeezed to avoid injecting more venom.
- Spider bites such as the black widow cause muscle cramps and autonomic effects, while the brown recluse causes local necrosis.
- Anaphylaxis to any venom is treated first with intramuscular adrenaline, then oxygen, fluids and antihistamines.
- Biological samples such as the bite-site swab and blood should be preserved for identification of the offending species where relevant.
Neurotoxic versus haemotoxic snakebite
- **Elapids (cobra, krait):** Neurotoxic. Ptosis, bulbar and respiratory paralysis. Krait bite often painless. Neostigmine may help.
- **Vipers (Russell's, saw-scaled):** Haemotoxic and vasculotoxic. Local swelling, bleeding, coagulopathy, renal failure.
- **Bedside test:** 20-minute whole blood clotting test flags the coagulopathy of viper envenomation.
- **Treatment:** Polyvalent anti-snake venom for systemic or severe local envenomation, with supportive care.
NMC competencies in this chapter
- **FM11.1:** Biotoxicology: snakebite, scorpion sting, hymenoptera and spider bites, and biological specimens
Frequently Asked Questions
What are the big four venomous snakes of India?
The cobra, the common krait, Russell's viper and the saw-scaled viper. The cobra and krait are neurotoxic, while the two vipers are haemotoxic and vasculotoxic.
Why is a krait bite dangerous despite little local swelling?
Krait venom is potently neurotoxic yet causes minimal pain or swelling, so the bite is often missed until the victim develops paralysis, classically waking with abdominal pain and weakness.
What first aid is recommended for snakebite?
Reassurance, keeping the limb still with pressure immobilisation and moving to hospital. Tourniquets, cutting and suction are harmful and should not be used.
How is a severe red scorpion sting treated?
Prazosin is the mainstay to counter the autonomic storm and its cardiac effects, with anti-scorpion venom and supportive care in severe cases.
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