Understanding the Threat
The following is a theoretical discussion based on an understanding of physical and biochemical characteristics of toxins. It is not an intelligence assessment of the threat.
TOXINS COMPARED TO CHEMICAL WARFARE AGENTS
Toxins differ from classical chemical agents by source and physical characteristics. When considering how to protect soldiers from toxins, physical characteristics are much more important than source.
TABLE 1: Comparison of Chemical Agents and Toxins Toxins Chemical Agents Natural Origin
Difficult, small-scale production
Large-scale industrial production
Many are more toxic
Less toxic than many toxins
Not dermally active*
Legitimate medical use
No use other than mony toxins
Noticeable odor or taste
Odorless and tasteless Noticeable odor or taste
Diverse toxic effects
Many are effective immunogens**
Fewer types of effects
* Exceptions are trichothecene mycotoxins, lyngbyatoxin and some of the blue-green algal toxins. The latter two cause dermal injury to swimmers in contaminated waters, but are generally unavailable in large quantities and have low toxicity, respectively.
** The human body recognizes them as foreign material and makes protective antibodies against them.
The most important differences to understand are volatility and dermal activity. Toxins also differ from bacterial agents (e.g.: those causing anthrax or plague) and viral agents (such as those that cause VEE, smallpox, flu, etc.), in that toxins do not reproduce themselves.
TOXINS ON THE BATTLEFIELD
Because toxins are not volatile, as are chemical agents, and with rare exceptions, do not directly affect the skin, an aggressor would have to present toxins to target populations in the form of respirable aerosols, which allow contact with the more vulnerable inner surfaces of the lung. This, fortunately, complicates an aggressor's task by limiting the number of toxins available for an arsenal. Aerosol particles between 0.5 and 5 m in diameter are typically retained within the lung. Smaller particles can be inhaled, but most are exhaled. Particles larger than 5-15 gm lodge in the nasal passages or trachea and do not reach the lung. A large percentage of aerosol particles larger than 15-20 m simply drop harmlessly to the ground. Because they are not volatile, they are no longer a threat, even to unprotected troops. Although there are few data on aerosolized toxins, it is unlikely that secondary aerosol formation caused by vehicular or troop movement over ground previously exposed to a toxin aerosol would generate a significant threat; this may not be true with certain chemicals or with very heavy contamination with infectious agents such as anthrax spores.
TOXICITY, EASE OF PRODUCTION AND STABILITY
Because they must be delivered as respirable aerosols, toxins' utility as effective MCBW are limited by their toxicities and ease of production. The laws of physics dictate how much toxin of a given toxicity is needed to fill a given area of space with a small-particle aerosol. Figure 1 presents a theoretical calculation of the approximate quantities of toxins of varying toxicities required to intoxicate people exposed in large open areas on the battlefield under optimal meteorological conditions. The figure is based on a mathematical model that has been field tested and found to be valid. It shows that a toxin with an aerosol toxicity of 0.025 g/kg would require 80 kg of toxin to cover 100 km2 with an effective cloud exposing individuals to approximately a lethal dose 50 (LD50). LD50 means, for example, that a person weighing 70 kg would have a 50% chance of surviving after receiving a 70 1lg dose of a toxin with an LD50 of 1.0 11g/kg. Note that for toxins less toxic than botulinum or the staphylococcal enterotoxins, hundreds of kilograms or even ton quantities would be need to cover an area of 10x10 km (100 km2) with an effective aerosol. Assuming this to be true, the number of toxins which can be used as MCBW is very limited; most of the less toxic agents either cannot be produced in quantity with current technology, or delivered to cover large areas of the battlefield. That could change, however, especially for the peptide toxins, as techniques for generating genetic recombinants improve. Stability of toxins after aerosolization is also an important factor, because it further limits toxin weapon effectiveness.
It is readily apparent that, ignoring other characteristics, if a toxin is not adequately toxic, sufficient quantities cannot be produced to make even one weapon. Because of low toxicity. hundreds of toxins can be eliminated as ineffective for use in MCBWs. Certain plant toxins, with marginal toxicity, could be produced in large (ton) quantities. These toxins could possibly be weaponized. At the other extreme, several bacterial toxins are so lethal that MCBW quantities are measured not in tons, but in kilograms-quantities more easily produced. Such toxins are potential threats to our soldiers on the battlefield.
Figure 1. Toxicity in LD50 (see Table 2) vs. quantity of toxin required to provide a theoretically effective open-air exposure. under ideal meteorological conditions. to an area 100 km2. (Patrick and Spertzel, 1992: based on Calder K.L., BWL Tech Study #3, Mathematical models for dosage and casualty coverage resulting from single point and line source release of aerosol near ground level, DTIC# AD3 10-361, Dec. 1957.) Ricin, saxitoxin and botulinum toxins kill at the concentrations depicted; the staphylococcal enterotoxins incapacitate.
TABLE 2: Comparative Lethality Of Selected Toxins And Chemical Agents In Laboratory Mice AGENT LD50(µ/kg) MOLECULAR WEIGHT SOURCE Botulinum Toxin 0.001 150,000 Bacterium Shiga Toxin 0.002 55,000 Bacterium Tetanus Toxin 0.002 150,000 Bacterium Abrin 0.04 65,000 Plant (Rosary Pea) Diphtheria Toxin 0.10 62,000 Bacterium Maitotoxin 0.10 3,400 Marine Dinoflagellate Palytoxin 0.15 2,700 Marine Soft Coral Ciguatoxin 0.40 1,000 Fish/Marine Dinoflagellate Textilotoxin 0.60 80,000 Elapid Snake C. perfringens toxins 0.1-5.0 35,000-40,000 Bacterium Batrachotoxin 2.0 539 Arrow-Poison Frog Ricin 3.0 64,000 Plant (Castor Bean) -Conotoxin 5.0 1,500 Cone Snail Taipoxin 5.0 46,000 Elapid Snake Tetrodotoxin 8.0 319 Puffer Fish -Tityustoxin 9.0 8,000 Scorpion Saxitoxin 10.0 (Inhal;2.0) 299 Marine Dinoflagellate VX 15.0 267 Chemical Agent SEB (Rhesus/Aerosol) 27.0 (ED50~pg) 28,494 Bacterium Anatoxin-A(s) 50.0 500 Blue-Green Alga Microcystin 50.0 994 Blue-Green Alga Soman (GD) 64.0 182 Chemical Agent Sarin (GB) 100.0 140 Chemical Agent Aconitine 100.0 647 Plant (Monkshood) T-2 Toxin 1,210.0 466 Fungal Mycotoxin
Incapacitation, as well as lethality, to humans must be considered. A few toxins cause illness at levels many times less than the concentration needed to kill. For example, toxins that directly affect membranes and/or fluid balance within the lung may greatly reduce gas transport without causing death. Less potent toxins could also be significant threats as aerosols in a confined space, such as the air-handling system of a building. Finally, breakthroughs in delivery vehicle efficiency or toxin "packaging" by an aggressor might alter the relationship between toxicity and quantity, as depicted in Figure 1; but even at best, quantities needed could likely be reduced only by one-half for a given toxicity. For now, however, the figure provides a reasonable and valid way of sorting toxins.
Some toxins are adequately toxic and can be produced in sufficient quantities for weapons, but are too unstable in the atmosphere to be candidates for weaponization. Although stabilization of naturally unstable toxins and enhanced production of those toxins now difficult to produce are possible ties for the future, there exists no evidence at this time for successful amplification of toxicity of a naturally occurring toxin. Militarily significant weapons need not be MCBW From 18 January to 28 February 1991, some 39 Iraqi-modified Scud missiles reached Israel. Although many were off target or malfunctioned, some of them landed in and around Tel Aviv. Approximately 1,000 people were treated as a result of missile attacks, but only two died. Anxiety was listed as the reason for admitting 544 patients and atropine overdose for hospitalization of 230 patients. (Karsenty et al., Medical Aspects of the Iraqi Missile Attacks on Israel, Isr J Med Sci 1991: 27: 603-607). Clearly, these Scuds were not effective mass casualty weapons, yet they caused significant disruption to the population of Tel Aviv. Approximately 75% of the casualties resulted from inappropriate actions or reactions on the part of the victims. Had one of the warheads contained a toxin which killed or intoxicated a few people, the "terror effect" would have been even greater. Therefore, many toxins that are not sufficiently toxic for use in an open-air MCBW could probably be used to produce a militarily significant weapon. However, the likelihood of such a toxin weapon causing panic among military personnel decreases when the leaders and troops become better educated regarding toxins.
CLASSES AND EXAMPLES OF TOXINS
The most toxic biological materials known are protein toxins produced by bacteria. They are generally more difficult to produce on a large scale than are the plant toxins, but are many, many times more toxic. Botulinum toxins (seven related toxins), the staphylococcal enterotoxins (also seven different toxins), diphtheria and tetanus toxin are well-known examples of bacterial toxins. The botulinum toxins are so very toxic that lethal aerosol MCBW weapons could be produced with quantities of toxin that are attainable relatively easily with present technology. They cause death through paralysis of respiratory muscles. Staphylococcal enterotoxins, when inhaled, cause fever, headache, diarrhea, nausea, vomiting, muscle aches, shortness of breath, and a nonproductive cough within 2-12 hours after exposure; they can also kill, but only at much higher doses. Staphylococcal enterotoxin B (SEB) can incapacitate at levels at least one hundred times lower than the lethal level. These too would likely be delivered as a respirable aerosol.
Other bacterial toxins, classified generally as membrane-damaging, are derived from Escherichia coli (hemolysins), Aeromonas, Pseudomonas and Staphylococcus alpha, (cytolysins and phospholipases), and are moderately easy to produce, but vary a great deal in stability. Many of these toxins affect body functions or even kill by forming pores in cell membranes. In general, their lower toxicities make them less likely battlefield threats.
A number of the toxins produced by marine organisms or by bacteria that live in marine organisms might be used to produce terrorist biological weapons. Saxitoxin, the best known example of this group, is a sodium-channel blocker and is more toxic by inhalation than by other routes of exposure. Unlike oral intoxication with saxitoxin (paralytic shellfish poisoning), which has a relatively slow onset, saxitoxin can be lethal in a few minutes by inhalation. Saxitoxin could be used against our troops as an antipersonnel weapon, but because it cannot currently be chemically synthesized efficiently, or produced easily in large quantities from natural sources, it is unlikely to be seen as an area aerosol weapon on the battlefield. Tetrodotoxin is much like saxitoxin in mechanism of action, toxicity and physical characteristics. Palytoxin, from a soft coral, is extremely toxic and quite stable in impure form, but difficulty of production or harvest from nature reduces the likelihood of an aggressor using it as an MCBW. The brevetoxins, produced by dinoflagellates, and the bluegreen algal toxins like the hepatotoxin, microcystin, have limited toxicity. For many of these toxins, either difficulty of production or lack of sufficient toxicity limits the likelihood of their use as MCBW.
The trichothecene mycotoxins, produced by various species of fungi, are also examples of low molecular weight toxins (molecular weight: 400-700 daltons). The yellow rain incidents in Southeast Asia in the early 1980s are believed to have demonstrated the utility of T-2 mycotoxin as a biological warfare agent. T-2 is one of the more stable toxins, retaining its bioactivity even when heated to high temperatures. High concentrations of sodium hydroxide and sodium hypochlorite are required to detoxify it. Aerosol toxicities are generally too low to make this class of toxins useful to an aggressor as an MCBW as defined in Figure 1; however, unlike most toxins, these are dermally active. Clinical presentation includes nausea, vomiting, weakness, low blood pressure, and burns in exposed areas.
Toxins derived from plants are generally very easy to produce in large quantities at minimal cost in a low-tech environment. A typical plant toxin is ricin, a protein derived from the bean of the castor plant. Approximately 1 million tons of castor beans are processed annually worldwide in the production of castor oil. The resulting waste mash is 3-5% ricin by weight. Because of its marginal toxicity, at least a ton of the toxin would be necessary to produce an MCBW (as defined in Figure 1). Unfortunately, the precursor raw materials are available in those quantities throughout the world.
Animal venoms often contain a number of protein toxins as well as nontoxic proteins. Until recently, it would have been practically impossible to collect enough of these materials to develop them as biological weapons. However, many of the venom toxins have now been sequenced (their molecular structure is known) and some have been cloned and expressed (produced by molecular biological techniques). Some of the smaller ones could also be produced by relatively simple chemical synthesis methods. Examples of the venom toxins are 1) the ion channel (cationic) toxins such as those found in the venoms of the rattlesnake, scorpion and cone snail; 2) the presynaptic phospholipase A2 neurotoxins of the banded krait. Moiave rattler and Australian taipan snake; 3) the postsynaptic (curare-like alpha toxin) neurotoxins of the coral, mamba, cobra, sea snake and cone snail; 4) the membrane damaging toxins of the Formosan cobra and rattlesnake and 5) the coagulation/antlcoagulation toxins of the Malayan pit viper and carpet viper. Some of the toxins in this group must be considered potential future threats to our soldiers as large-scale production of peptides becomes more efficient; however, difficulty of production in large quantity presently may limit the threat potential of many of them.
HOW TOXINS WORK
Unlike chemical agents, there are many classes of toxins, and they differ widely in their mechanisms of action. makes the job of medically protecting soldiers difficult, as there are seldom instances where one vaccine or therapy would be effective against more than one toxin. Time from exposure to onset of clinical signs may also vary greatly among toxins.
(Note that, unlike a terrorist threat, one can prepare for a battlefield threat through development of specific medical countermeasures. Vaccines and other prophylactic measures can be given before combat, and therapies kept at the ready.)
Some neurotoxins, such as saxitoxin, can kill an individual very quickly (minutes) after inhalation of a lethal dose. This toxin acts by blocking nerve conduction directly and causes death by paralyzing muscles of respiration. Yet, at just less than a lethal dose, the exposed individual may not even feel ill or just dizzy. Because of the rapid onset of signs after inhalation, prophylaxis (immunization or pretreatment with drugs) would be required to protect soldiers from these rapidly acting neurotoxins. Unprotected soldiers inhaling a lethal dose would likely die before they could be helped, unless they could be intubated (a breathing tube placed in the airway) and artificially ventilated immediately. A1though the mechanism of death after inhalation of saxitoxin is believed to be the same as when the toxin is administered intravenously, it is more toxic (a smaller dose will kill) if inhaled.
Other neurotoxins, such as the botulinum toxins, must enter nerve terminals before they can block the release of neurotransmitters which normally cause muscle contraction. Botulinum neurotoxins generally kill by relatively slow onset (hours to days) respiratory failure. The intoxicated individual may not show signs of disease for 24-72 hours. The toxin blocks biochemical action in the nerves that activate the muscles necessary for respiration, leading to suffocation. Intoxications such as this can be treated with antitoxin (a preparation of antibodies from humans or animals) that can be injected hours (up to 24 hours in monkeys, and probably humans) after exposure to a lethal dose of toxin, and still prevent illness and death. Although the mechanisms of toxicity of the botulinum toxins appear to be the same after any route of exposure, unlike saxitoxin, the actual toxicity is less by inhalation (i.e., the lethal dose of botulinum toxin is slightly greater by inhalation).
While neurotoxins effectively stop nerve and muscle function without causing microscopic damage to the tissues, other toxins destroy or damage tissue directly. For these, prophylaxis (pretreatment of some kind) is important because the point at which the pathological change becomes irreversible often occurs within minutes or a few hours after exposure. An example of this type of toxin is microcystin, produced by blue-green algae, which binds very specifically to an important enzyme inside liver cells; this toxin does not damage other cells of the body. Unless uptake of the toxin by the liver is blocked, irreversible damage to the organ occurs within 15-60 minutes after exposure to a lethal dose. In this case, the tissue damage to a critical organ, the liver, is so severe that therapy may have little or no value. For this toxin, unlike most, the toxicity is the same, no matter what the route of exposure.
The consequences of intoxication may vary widely with route of exposure, even with the same toxin. The plant toxin, ricin, kills by blocking protein synthesis in many cells of the body, but no lung damage occurs with any exposure route except inhalation. If ricin is inhaled, as would be expected during a biological attack, microscopic damage is limited primarily to the lung, and that damage may be caused by a mechanism different from that which occurs if the toxin is injected. Furthermore, when equivalent doses of toxin are used, much more protective antibody must be injected to protect from inhalation exposure compared to intravenous injection of the toxin. Finally, although signs of intoxication may not be noted for 12-24 hours, microscopic damage to lung tissue begins within 8-12 hours or less. Irreversible biochemical changes may occur in 6090 minutes after exposure, again making therapy difficult.
The toxicities of some bacterial toxins are too low to make them effective lethal MCBWs, according to the standards described in Figure 1. However, some cause incapacitating illness at extremely low levels. Therefore, lethality alone is not an appropriate criterion on which to base a toxin's potential as a threat. The staphylococcal enterotoxins are examples. They can cause illness at extremely low doses, but relatively high doses are required to kill. These toxins are unusual, in that they act by causing the body to release abnormally high levels of certain of its own chemicals, which, in very small amounts, are beneficial and necessary for life, but at higher levels are harmful.
Only one class of easily produced toxins, the trichothecene mycotoxins, is dermally active. Therefore, trichothecenes must be considered by different standards than all other toxins. They can cause skin lesions and systemic illness without being inhaled and absorbed through the respiratory system. Skin exposure or ingestion of contaminated food are the two likely routes of exposure of soldiers; oral intoxication is unlikely in modern, welltrained armies. Nanogram (one billionth of a gram) quantities per square centimeter of skin cause irritation, and microgram (one millionth of a gram) quantities cause necrosis (destruction of skin cells). If the eye is exposed, microgram doses can cause irreversible injury to the cornea (clear outer surface of the eye). The aerosol toxicity of even the most toxic member of this group is low enough that large-quantity production (approximately 80 metric tons to expose a 10 km2 area with respirable aerosol) makes an inhalation threat unlikely on the battlefield. These toxins, therefore, might be dispersed as larger particles, probably visible in the air and on the ground and foliage. In contrast to treatment for exposure to any of the other toxins, simply washing the skin with soap and water within 1-3 hours after an exposure would eliminate or greatly reduce the risk of illness or injury.
MANY TOXINS, BUT NOT AN OVERWHELMING PROBLEM
Because there are hundreds of toxins available in nature, the job of protecting troops against them seems overwhelming. One would think that an aggressor would need only to discover the toxins against which we can protect our troops and pick a different one to weaponize. In reality, it is not quite that simple. The utility of toxins as MCBWs is limited by toxicity (Figure 1). This criterion alone reduces the list of potential open-air weaponizable toxins for MCBWs from hundreds to fewer than 20. Issues related to stability and weaponization will not be addressed here, but would only further reduce the list and make the aggressor's job more difficult.
POPULATIONS AT RISK
An armored or infantry division in the field is not at great risk of exposure to a marine toxin whose toxicity is such that 80 tons are needed to produce an MCBW covering 10 km2. Most marine toxins are simply too difficult to produce in such quantities. Military leaders on today's battlefield should be concerned first about the most toxic bacterial toxins and possibly some of the plant toxins that are slightly less toxic but available in large quantities in nature. The more confined the military or terrorist target (e.g., inside shelters, buildings, ships or vehicles) the greater the list of potential toxin threats which might be effective. This concern is countered, however, by the fact that toxins are not volatile like the chemical agents and are thus more easily removed from air-handling systems than are volatile agents. It is probably most cost-effective to protect our personnel from these toxins through the use of collective filtration systems.
Nonetheless, we must consider subpopulations of troops and areas within which they operate when we estimate vulnerability to a given toxin threat. Situations could well occur in which different populations of troops require protection from different toxins, because of differences in operational environments. To protect them effectively, decision makers and leaders must understand the nature of the threat and the physical and medical defense solutions.
Table 3 lists the approximate number of known toxins by toxicity level and source. To simplify our approach to development of medical countermeasures, we have divided them into "Most Toxic," "Highly Toxic" and "Moderately Toxic" categories (also see Figure 1). The Most Toxic toxins could probably be used in an MCBW; it is feasible to develop individual medical countermeasures against them. The Highly Toxic toxins could probably be used in closed spaces such as the air-handling system of a building or as ineffective terror weapons in the open; collective filtration would be effective against these toxin aerosols targeted to enclosed spaces. The Moderately Toxic toxins would likely be useful only as assassination weapons which would require direct attack against an individual; it is not feasible to develop medical countermeasures against all of the toxins in this group. Such reasoning allows us to use limited resources most effectively and maximize protection, and thus effectiveness, of our fighting force.
SOURCE Most Toxic Highly Toxic Moderately Toxic Total (Number of toxins in each category) Bacterium 17 12 >20 >49 Plant 5 >31 >36 Fungus >26 >26 Marine Organism >46 >65 >111 Snake 8 >116 >124 Alga 2 >20 >22 Insect >22 >22 Amphibian >5 >5 Total 17 >73 >305 >395 Table 3. Approximate number of toxins arbitrarily categorized as Most Toxic (LD50 <0.025 µg/kg), Highly Toxic (LD50, 0.025-2.5 µg/kg) and Moderately Toxic (LDso >2.5 g/kg). From DNA-TR-92-116, Technical Ramifications of Inclusion of Toxins in the Chemical Weapons Convention (CWC).
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