A medical classification of venom activities is based on specific clinical
effects and a single venom toxin may have activities in several different areas
(ie some PLA2 toxins have both presynaptic neurotoxic and myotoxic activity).
It is also important to consider the way the different activities may affect
timing of onset of clinical effects.
In recent years it has been possible to measure venom or individual toxins over
time in both experimental animals and human snakebite victims. Though this has
so far only been performed for a very limited range of species, there is a greater
understanding of the toxicodynamics of envenoming . In most cases, venom is
injected fairly superficially, usually subcutaneously. Locally acting toxins
causing tissue injury will already be at their target site, so will commence
exerting their clinical effects immediately. A significant proportion of the
venom, in some species perhaps most venom, will not be absorbed directly into
the circulation. Instead it will be transported first via the lymphatic system,
entering the circulation via the thoracic duct. This helps explain the common
clinical finding of enlarged or tender lymph nodes draining the bite area and
also the high concentration of venom in these nodes at autopsy. Transport via
the lymphatic system may be rapid or sometimes delayed and there is a potential
for sequestration of venom locally, with prolonged release over hours or days.
Once in the circulation, those components affecting haemostasis or acting as
haemorrhagins will have reached their target site and will quickly exert their
effect. Similarly, nephrotoxins will quickly damage the kidneys. However, those
toxins seeking extravascular targets, particularly the neurotoxins and myotoxins,
will need to exit the circulation in sufficient concentration to exert their
effect clinically, thus these toxins are most likely to have a delayed onset
of clinically detectable actions. Some venoms are quickly cleared from the circulation,
but others remain detectable for days or even weeks without antivenom therapy.
Knowledge of such variations is clearly relevant in determining antivenom therapy.
Bite Site - local effects
Haemostatic System Toxins
Broad medical classification of snake venom activities.
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