Chapter 17. Introduction to Biotoxins

Biotoxins occupy a middle ground in the spectrum between chemical and biological agents (Table 17–1). Neither a living entity, such as bacteria, nor a laboratory invention, such as nerve gases, biotoxins are products of metabolic pathways that are toxic to humans. Some are made by single-celled organisms, whereas others are derived from multicellular organisms. Naturally occurring biotoxins may also be reproduced synthetically. Biotoxins can be extremely potent, but they are not communicable and do not replicate within the host. For this reason, properly decontaminated patients pose no risk to health care workers (HCWs) and others with whom they may come into contact. The clinical impact of biotoxins is arguably more akin to chemical weapons than to biological weapons; however their biological origins tie them in readily with biological weapons.

Exposure to biotoxins occurs through ingestion, dermal absorbtion, or as an aerosol. Militarily or as a weapon of terror, aerosol forms of biotoxins pose the greatest risk not only in terms of numbers of people exposed, but the rapidity of symptom onset. With some of the biotoxins, inhalational effects are understood only from animal studies, leaving many open questions about how an inhalational exposure might present. Not all biotoxins are likely candidates for use as weapons of mass destruction or terrorist weapons. Due to their biological and physiochemical properties, Army biowarfare experts consider botulinum toxin and Staphylococcal Enterotoxin B (SEB) to be of most concern from the point of view of battlefield exposures. SEB, a so-called superantigen, causes hyperactivation of the immune system and prompt incapacitation (see Chapter 19). As a Category A agent, botulinum is discussed in its own chapter (Chapter 18).

Since terrorists may have more limited goals—for example, sowing fear, as was seen with anthrax—wider options in regard to potential biotoxin weapons, including ricin, abrin and mycotoxins, exist. This, too, is discussed briefly.

The CDC classifies the biotoxins as biological weapons. Others make biotoxins a category unto itself, or classify them as chemical agents. For the purposes of this text, we have chosen to follow the CDC categorization, thereby providing discussion of particular biotoxins according to their Category A and Category B biological weapons designation. The only exception is ricin, which although listed with Category B agents by the CDC is included among the Category A agents because of a number of features, including its use already, indicating it to be a threat comparable to other Category A agents.

The other deviation from the CDC categories is our having included four biotoxins that the CDC leaves off of their lists: abrin, a toxin made from the seeds of the commonly found rosary pea, aflatoxin, a mycotoxin usually produced by species of fungus that have a proclivity for peanuts; T-2 trichothecenes, another mycotoxin, but one that is believed to have been used previously as a weapon; and saxitoxin, a potent neurotoxin produced by a commonly found algae species. These agents were selected for inclusion because each possesses various features that make them legitimate threats as biological weapons. They are listed at the end of the section on biological weapons, because though legitimate, they are less likely to be employed than are the biological agents that precede them.