Venomous snakes are found throughout the world; they are even lurking in our oceans. Said to cause over 3 million bites a year worldwide, they pose a significant health problem both in mortality and morbidity, causing amputations and renal failure, in addition to over 100,000 deaths a year. The major culprits are vipers, elapids, colubrids and sea snakes. Of these, elapids such as cobras and kraits have developed the most potent toxins. Snake venoms contain a multitude of biologically active toxins that work together for the capture of prey (take a look at the Table listing the variety of toxins found in the venom of the many-banded krait). Their effects include pro- and anti-blood coagulation, neurotoxicity, mycotoxicity, nephrotoxicity, cardiotoxicity and necrotoxicity (local tissue damage). Amongst these, the neurotoxins play a key role in immobilising prey through paralysis, disorientation and depressed respiration.
Venoms often contain different neurotoxins that work synergistically to cripple the nervous system. Neurotoxins can be classified according to their site of action: pre-synaptic neurotoxins block neurotransmission by affecting acetylcholine transmitter release; post-synaptic neurotoxins are antagonists of the acetylcholine receptor. Together these neurotoxins effectively block skeletal neuromuscular transmission by crippling receptors, while at the same time acting to destroy any neurotransmitter that might compete with the toxin for receptor binding. Venoms often contain several post-synaptic neurotoxins, each with a high affinity for a nicotinic receptor subtype - in this way the venom can cripple as many receptors as possible. The post-synaptic neurotoxins are found only in elapids and sea snakes (Hydrophiidae). In the many-banded krait pictured above, a pre-synaptic toxin is b-bungarotoxin, while post-synaptic toxins are a‑ and k-bungarotoxins.
Milking a snake
Photographs courtesy of B. G. Fry, Australian Venom Research Unit, Melbourne, Australia