Snake Bites & the Science Behind Antivenoms

The aim of first in the case of snake bite is to: - Delay the systemic absorption of venom from the bite - Control distressing or dangerous early symptoms of envenomation - Transport the patient to a place where they can obtain medical care as quickly as possible

There are certain practices associated with snake bites especially in Africa due to old traditional practices. These should be avoided. I call them the DONT'S of first aid and they include:

• Local knife/razor cuts at the site of the bite
• Attempts to suck out the venom from the wound
• Use of the black snake stone
• Use of tight tourniquets
• Electric shocks
• Application/Ingestion of herbs

What then should be done as First Aid?

• Reassure the patient - 70% of all snake bites are from non-venomous species and only 50% of bites by venomous species actually envonomate the patient.
• Immobilise the affected part with splints using bandages to stabilize. Do not make it tight so it doesn't affect the blood supply. THIS IS VERY IMPORTANT.
• Get to the hospital immediately.
• Tell the doctor of all symptoms experienced.

Pressure Immobilization

Pressure immobilization is very important but it must be done rightly or else it might cause more serious complications.

Pressure Immobilization is not appropriate for cytotoxic bites but may be effective against neurotoxic venom.
Its aim is to contain venom within a bitten limb and prevent it from moving through the lymphatic system to the vital organs.

There are two components:

• Pressure to prevent lymphatic drainage
• Immobilization of the bitten limb to prevent pumping action of skeletal muscle

An elastic bandage should be used but any cloth can be used in an emergency The bandage should be as tight as when strapping a sprained ankle. It must NOT cut off blood flow or be uncomfortable.

The location of the bite should be clearly marked on the outside of the bandages and it should be done as quickly as possible. Once applied,don’t remove till you get to a hospital. It may contain the symptoms effectively, but once removed, the venom is released into the circulation.

Identify the snake

If the snake is available, its identification can be extremely helpful.
If not available, characteristic descriptions may help eg. Cobras may rear up and spread a characteristic hood and hiss, Puff adders make a loud blowing sound,saw-scaled vipers produce a rasping sound.

---

Specific Therapy

The specific therapy for snake bites is the use of antivenom.

The use of antivenoms are not indicated in all situations. Antivenoms are indicated if there is:

• Hypotension
• Shock or other signs of cardiotoxicity
• Non-clotting blood
• Rhabdomyolysis
• Impaired consciousness
• Neurotoxicity – ptosis, muscle weakness,depressed respiration
• Known necrotic venom eg. Bitis arietans (puff adder)
• Swelling involving more than half of the bitten limb
• Rapid progression of swelling
• Bites on digits and into other tight fascial compartments

Antivenoms are however contraindicated in:

• Patients with an atopic history (severe asthma, hay fever etc)
• Patients with a history of previous reactions to equine antisera (eg. ATS seum) have an increased risk of severe reactions In these cases, pretreatment with SC Adrenaline and hydrocortisone is justified to prevent or diminish the reaction.

---

Antivenom should be given as soon as possible once signs of local or severe local envenoming are evident.
It is most effective if given within 4 hours and less effective after 8 hours. In cases of severe envenomation, it can be given even after 5 days.

Children should be given the same dose of antivenom as adults – antivenom is given to neutralize a specific dose of venom.

When patients arrive at hospital with a touniquet in place, antivenom treatment if thought necessary should be started before loosening it as there is a risk of severe envenomation when the venom in the occluded limb is released into the general circulation.

Antivenom acts only against circulating toxin, not toxin fixed to tissue and so specific measures should be taken eg. Atropine,neostigmine for neurotoxic envenomation.

There are two types of antivenom that can be used:

1. Monovalent/monospecific antivenom - this is specific to only one kind of venom. It can be used if the exact species of snake that caused the bite can be reliably identified.
2. Polyvalent/polyspecific - this antivenom is effective against a wide range of venoms. It is used if the species can’t be reliably identified.

---

Available Antivenoms in Nigeria

1. South African Institute of medical Research(SAIMR) – monospecific for Echis carinatus (10-20ml)
2. Fab Afrique polyvalent (40mls)
3. Echilab monospecific antivenom raised against venom of Nigerian Echis occelatus
4. Clomidro Costa Rican antivenom raised against venoms of 3 Nigerian snakes – Bitis arietans, Echis occelatus, Naja nigricollis (30-40ml)

Antivenom Reactions

There are some unwanted reactions or side effects associated with antivenom use. These can be classified as early or late reactions depending on how soon they occur after administration.

Early Reactions – Occur 3 to 60 minutes after administration. They include:

• Cough
• Fever
• Tachycardia
• Urticaria
• Systemic anaphylaxis
• Angioedema

Late Reactions – They occur 5 to 24 days after administration (average of 7 days). They include:

• Itching
• Urticaria
• Fever
• Arthralgia
• Proteinuria - protein in urine
• Neurological symptoms

Supportive Therapy

• Administer Tetanus toxoid to prevent Tetanus
• Use antibiotics to prevent wound infections
• Care of the bitten limb – Examine frequently for evidence of necrosis and clean with antiseptics
• Debridement of necrotic tissue – To reduce the risk of secondary infection and to promote eventual healing
• Muscle haematomas – Conservative treatment
• Amputation of doomed digits and limbs
• If there is venomous spit into the eyes, flush with large volumes of water

---

Prevention of Snake Bites

1. Avoid walking through grass at dark when most snakes are out
2. Wear heavy boots and clothes in hazardous zones
3. Avoid swimming in overgrown rivers
4. Avoid keeping domestic animals like chickens close to the house and ensure adequate pest control
5. Avoid collecting firewood and dislodging rocks etc with bare hands
6. Never put your hand into burrows or holes
7. Snakes rarely attack without provocation so, avoid snakes – do not attack or disturb them.

Conclusion