When biotoxins are tools of terror
Early recognition of intentional poisoning can attenuate effects
LCDR David L. Blazes, MC, USNR; LT James V. Lawler, MC, USNR; CAPT Angeline A. Lazarus, MC, USN
VOL 112 / NO 2 / AUGUST 2002 / POSTGRADUATE MEDICINE
CME learning objectives
The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or necessarily reflecting the views of the US Department of Defense, the US Navy, or the National Naval Medical Center.
Preview: Capable of causing
disease as well as being therapeutic, toxins have been both a curse and a
blessing for millennia. Now, the threat of rogue countries or terrorist
organizations using toxins as weapons of war is very real. In this article,
Drs Blazes, Lawler, and Lazarus review the clinical and epidemiologic
features of the toxin-mediated diseases most likely to occur as a result of
a biological warfare event.
In nature, specialized bacteria, fungi, plants, and animals use toxins for defensive as well as offensive purposes. Humans occasionally become inadvertent victims of these toxins, which cause such diseases as tetanus, botulism, and common food poisoning. Today, we must also consider the threat of intentional poisoning as a method of warfare.
Some toxins have been prepared for use as weapons by various countries and possibly by terrorist organizations. For instance, before 1969, even the United States produced botulinum toxin as part of its biowarfare program (1). Iraq is thought to have thousands of liters of botulinum toxin--enough to easily kill everyone in the world (2).
Toxins are attractive as biological agents because of their extreme potency and the relative ease of producing them. They are some of the most poisonous known substances; lethal doses often are expressed in nanograms (1). Many toxins are stable under normal environmental conditions.
Toxins are a diverse group of biologically derived substances that have myriad characteristics and mechanisms of action (table 1). They range from very small, nonpeptide, organic compounds to large, complex proteins. They may act on nerves, the gastrointestinal tract, or the immune system (3). Some produce stereotypical symptoms regardless of whether they are inhaled, ingested, or absorbed; others have varied clinical presentations, depending on the route of administration.
Confusion has abounded in the literature about whether to classify toxins as biological or chemical agents. Historically, saxitoxin and ricin have been classified as chemical agents in the literature and in conventions, whereas the other toxins have been classified as biological agents under the 1972 Convention on the Prohibition of the Development, Production, and Stockpiling of Bacteriological (Biological) and Toxin Weapons and on Their Destruction (4). For the sake of clarity and space, we discuss only the three toxins thought to be most important and likely to be used as agents of biowarfare: botulinum toxin, Staphylococcus aureus enterotoxin B (SEB), and trichothecene mycotoxins.
Botulinum toxin refers to any of the seven related but distinct toxins produced by the anaerobic bacterium Clostridium botulinum. Identified by the letters A through G, the botulinum toxins are produced by different strains of bacteria but act through a similar mechanism. The toxin binds to the presynaptic nerve terminal, which prevents release of acetylcholine and results in neuromuscular transmission blockade (2,5). Most cases of botulism occur sporadically and are caused by ingestion of improperly packaged foods.
Diagnosis of inhalational botulism is difficult and must be made on clinical grounds in most circumstances. Differential diagnostic considerations are generally limited to Guillain-Barré syndrome, myasthenia gravis, poliomyelitis, and tick paralysis, although these conditions usually do not occur in epidemics in the United States. Laboratory findings are nonspecific; antibodies generally do not develop, because the dose of botulinum toxin that causes clinical illness is subimmunogenic (2). Physicians may attempt to diagnose botulism with a mouse bioassay, which is available only at the Centers for Disease Control and Prevention (CDC) and some state public health laboratories (6). Botulinum toxin also may be present in the nares of patients who inhaled it and may be detected with an enzyme-linked immunosorbent assay (ELISA) if the sample is obtained within 24 hours of exposure (3).
Treatment should not be delayed while awaiting confirmation of the diagnosis, because early therapy may limit the extent and severity of paralysis. Samples for the mouse bioassay should be collected before administration of the antitoxin, and patients should be assessed for hypersensitivity to equine antitoxin. Antibiotics have no role in the treatment of botulism, because the botulinum toxin is preformed and there are no bacteria to eliminate.
S aureus enterotoxin B
One of seven enterotoxins produced by the bacterium S aureus, SEB is widely known as the most common cause of food poisoning. SEB is a heat-stable protein toxin that also is stable in aerosol form (7). Its stability and the relative ease of acquiring it make SEB an attractive agent for biological warfare. Unlike anthrax and even botulinum toxin, SEB usually does not cause death. Rather, it is an effective incapacitating agent. As a biowarfare agent, SEB likely would be disseminated through an aerosol route, but contamination of food supplies is possible as well.
SEB acts as a superantigen, activating large numbers of T lymphocytes by binding to class II major histocompatibility complex molecules (8,9). This binding bypasses the normal mechanism of antigen recognition and leads to the nonspecific activation of polyclonal T lymphocytes. Unlike the response to botulinum toxin, the immune response to SEB and the signs and symptoms of intoxication depend on the route of exposure.
Diagnosis of SEB intoxication is difficult because serologic tests are neither sensitive nor specific and the toxin is generally absent at symptom onset. Interestingly, SEB and its metabolites can be detected in urine for several hours after exposure; thus, a urine sample should be obtained (3). As with botulinum toxin, ELISA or polymerase chain reaction tests of samples obtained by nasal swabs within 24 hours of exposure may yield a diagnosis, but a negative result does not eliminate the possibility of exposure (3). A peripheral leukocytosis may be present, but this finding is nonspecific for SEB intoxication.
Mycotoxins are products of metabolism of fungi that cause disease in humans and animals. Naturally occurring human diseases now known to be caused by mycotoxins have been described for more than 100 years. Most of these diseases are associated with the consumption of moldy cereals or grains. Aflatoxins produced by certain species of Aspergillus are linked to hepatotoxicity and carcinoma, especially in Southeast Asia (10). The T-2 trichothecene mycotoxin produced by Fusarium causes alimentary toxic aleukia, a devastating chronic disease that killed 10% of the population of Orenburg, Russia, in the 1940s (11).
Both aflatoxins and trichothecenes have been prepared as biological weapons. The former Soviet Union had an extensive program dedicated to the preparation of these agents as weapons, and evidence suggests that they killed thousands of people after they were deployed in Afghanistan and Southeast Asia (as "yellow rain"). Recently, Iraq prepared both aflatoxin and the trichothecenes--particularly T-2--for use as weapons (12).
Trichothecenes are relatively easy to produce and aerosolize. They are very hearty compounds that can survive autoclaving, and they do not degrade when exposed to light. They can be ingested, inhaled, or absorbed through skin or mucous membranes. They can irritate or incapacitate at low doses and can kill in minutes at higher doses. As an aerosol, trichothecenes are roughly equipotent to mustard gases, although they are much more readily absorbed through the skin (5,11).
Trichothecene toxicity occurs through several mechanisms. The most important is probably interference of ribosomal peptidyl transferase, which inhibits protein elongation (11). T-2 also is known to increase lipid peroxidation of cell membranes and to disrupt mitochondrial electron transport (5).
Diagnosis of trichothecene poisoning can be difficult. It should be suspected if a large number of people have signs and symptoms consistent with toxicity--especially if they report being exposed to yellow or otherwise colored mist or smoke. Trichothecenes can be detected in environmental samples through gas and liquid chromatography, mass spectrometry, or electrochemical sensors (14). Studies have found that gas chromatography, mass spectrometry, ELISA, and radioimmunoassay can detect trichothecenes or metabolites in urine (15,16). These tests appear to be relatively sensitive and specific, but they are not commercially available.
Summary and conclusion
Toxin-mediated diseases have made humans ill for millennia. They also have been used in beneficial ways. Unfortunately, the use of biological agents as weapons of terror has now been realized, and separating naturally occurring disease from bioterroristic events has become an important public health goal. The key to timely identification of such attacks relies on education of primary care physicians, first responders, and public health officials. We must remain vigilant to unusual case presentations or clusters of similar cases and report them immediately to public health authorities.
Dr Blazes is an attending physician, and Dr Lawler is a fellow, infectious diseases service, National Naval Medical Center, Bethesda. Dr Lazarus is program director, internal medicine service, National Naval Medical Center. Correspondence: LCDR David L. Blazes, MC, USNR, Infectious Diseases Service, National Naval Medical Center, 8901 Wisconsin Ave, Bethesda, MD 20889. E-mail: firstname.lastname@example.org.