Dangerous Discussions: Part One
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By Kevin Scott
Over the last couple of months, I've had the opportunity to discuss with various people the differences between poisons, toxins and venoms a surprising number of times. Having studied chemistry, carried out research in a biochemistry and molecular biology laboratory, and having worked in the reptile industry for close to a decade, I find the topic especially interesting when it pertains to herpetofauna.
Depending on whom you ask, the precise definitions for poison, toxin, and venom will differ slightly. There are, however, major differences between these terms, and often the terms are erroneously interchanged. The following is a brief discussion of these differences. Let's start off by taking a look at and comparing definitions from The Oxford English Dictionary and Stedman's Concise Medical Dictionary.
Oxford English Dictionary Poison: A substance that, when introduced to or absorbed by a living organism causes death or injury. Toxin: An antigenic poison or venom of plant or animal origin. Venom: Poisonous fluid secreted by animals such as snakes and scorpions and typically injected into prey or aggressors by biting or stinging.
Stedman's Concise Medical Dictionary Poison: Any substance, either taken internally or applied externally, that is injurious to health or dangerous to life. Toxin: A noxious or poisonous substance that is formed or elaborated during the metabolism and growth of certain microorganisms and some higher plant and animal species. Venom: A poisonous fluid secreted by snakes, spiders, scorpions and other cold-blooded animals.
According to the way that we have defined these terms here, the word poison behaves as sort of an umbrella term for things that can harm biological systems. Poisons include substances that range from household cleaners and pesticides to large organic molecules found in frogs and protein complexes found in snake venom. While the small, brightly colored South American dart frogs are commonly called Poison Frogs, or Poison Arrow Frogs (family Dendrobatidae), it is generally considered incorrect to refer to a venomous snake as a poisonous snake (I will explain why in a moment), although this happens rather often.
TOXINS & VENOM
According to these definitions, a toxin is a type of poison that is produced through a biological pathway. Although this particular medical dictionary's definition does not make it explicit, toxinologists generally agree that toxins must be taken into the body by absorption or consumption. Venoms, in contrast, must be "injected" into the body by way a specially evolved mechanism, for instance, a stinger or fangs. (I use the word "injected" loosely here, more on this in part II).
In addition to the differences in the mode of application, toxins and venoms are comprised of substances that are inherently different from one another. Toxins tend to consist of comparatively simple organic molecules while venom is usually comprised of an array of peptides and proteins that possess enzymatic activity. In general, venoms are extremely complex mixtures of different compounds while toxins are chemically well defined, pure, and homogenous substances (Mebs 2002).
All amphibians secrete "toxic" substances through their skin that act as anti-bacterial and anti-fungal agents to help them avoid contracting illnesses (Clark 2007). Many species secrete more potent compounds as anti-predatory mechanisms. A particularly well-characterized group is the Arrow Frogs. Several species belonging to this group possess extremely potent toxins, many of which are alkaline steroids. The frogs use these compounds as a defense, and the mechanism through which they work are part of the definition of what a toxin is. Because there is no delivery system for these compounds, they must be consumed by a predator in order for them to be employed. Venom is usually secreted from specialized gland or tissue and is subsequently stored in specialized sacs until it is used. Technically venom can be referred to as being toxic or poisonous, but an animal, a snake for example, that is venomous, is not poisonous, because it wouldn't harm you to eat it.
The origin of words is a topic that I find interesting and illuminating. Sometimes you can quickly see the Latin or Greek root of a word just by looking at it, but sometimes the derivation is not immediately obvious. The origin of the word toxin for example, I found surprising.
Comes from Middle English (probably 15th century) denoting a harmful medical drink, which comes from Old French poison, a potion or poisonous drink (14th century), previously simply a drink (12th century), but originally from Latin, potare, to drink.
Comes from the Latin toxicus or toxicum, meaning poisoned or poison, respectively, from the Greek word toxikon, or (poison for) arrows, from the Greek toxon, or bow. I found it interesting that the source of the word toxin comes from poisons that were extracted from plants and invertebrates to coat the tips of arrows by ancient Greeks and Romans. The Poison Arrow Frogs obviously got their common name because their toxins were used for the same purpose.
Comes from Middle English, from the Old French venim, a variation of venin from an alteration of the Latin venenum, or poison.
In closing, I would like to point out that I am by no means offering precise definitions for any of the terms used. In fact, I would go so far as to argue that there are no watertight definitions for any of these words. For one thing, biology and biochemistry are so incredibly complex that it is near impossible to precisely define these things by placing them neatly into some well-defined little boxes, there always seem to be exceptions to every rule. Language is always changing and words usually have more than one meaning, so even a precise definition can be open to interpretation. The language discussed here is only relevant within the English language, but there are at least thirteen dialects of English, each with its own differences from modern American English.
When setting out to writing this article I had two points in mind:
To explore some of the fascinating issues that arise when language is used to describe biological systems. Language is inherently obscure and biology is inherently complex. I feel that both are important to understand and interesting to study.
To clear up some of differences between terms in an attempt to at least tighten up the definitions already in place. Even though biology always seems to offer exceptions, we can at least attempt to avoid some of the common errors in terminology.
In part I of this article I have played with the etymology of poisons, toxins, and venom. In part II of this article I will probe deeper into the biology and chemistry of toxins and venom and the evolutionary impact on delivery systems, and I will discuss various types of each.
Mebs, Dietrich. 2002. Venomous and Poisonous Animals: A Handbook for Biologists, Toxicologists and Toxinologists, Physicians and Pharmacists. Stuttgart: Medpharm.
Clark, BT. 2007. "The Natural History of Amphibian Skin Secretions, Their Normal Functioning and Potential Medical Applications." Biological Reviews. (3):365-379.