When a snake bites and injects venom, usually its aim is to immobilise its natural (food) prey.

As humans are far too large for a venomous snake to eat, most snake bites occur when the snake is provoked into acting in self-defence.

In most cases, the snake is provoked by accident – for example, when a person accidentally steps on a snake while out walking. However, sometimes a snake bites after being deliberately threatened and frightened by someone:

• kicking it
• striking it
• trying to pick it up

Snake bites that involve foreign (exotic) snakes kept as pets usually occur when someone handles or 'plays' with them, often after drinking too much alcohol or taking recreational drugs.

Snake venom

Snake venom contains many different toxins (poisons) evolved to kill or immobilise the snake’s prey. There are four main types of snake venom toxins:

• haemotoxins – affect the circulatory system (heart and blood)
• neurotoxins – affect the nervous system at the places where nerves connect to muscles
• cytotoxins – damage and kill tissue cells (such as skin) causing blood and plasma (the clear fluid in blood) to leak into the tissue near the bite
• myotoxins – destroy muscle tissue both at the site of the bite and generally throughout the body

The four types of toxins are discussed in more detail below.

Haemotoxins

Haemotoxins destroy red, oxygen-carrying blood cells, damage the lining of blood vessels and disrupt the blood's ability to clot.

They can also cause a drop in blood pressure, which can result in tissue and organ damage, loss of consciousness and death.

Neurotoxins

Neurotoxins block or damage nerves where they connect to muscles, preventing the nerve signals getting through.

This causes paralysis and symptoms such as muscle weakness throughout the body and swallowing and breathing difficulties that can lead to lack of oxygen (respiratory failure) and death.

Cytotoxins

Cytotoxins cause swelling, bruising, blistering and gangrene (death of tissue cells) near the location of the snake bite. This may require plastic surgery or, in severe cases, amputation.

Myotoxins

Myotoxins damage muscle cells, causing pain and muscle weakness.

They may also damage your kidneys, which filter waste products from your blood, causing the flow of urine to decrease or even stop. The urine may become dark brown or black.