The use of pathogenic agents as weapons has been documented for more than two millennia. The ancient Romans carried out biological warfare by putting carrion into the wells of their enemies to poison the water supply Such crude biowarfare tactics continued well into the 20th century.3'4 During the 14th-century siege of Kaffa (now Feodossia, Ukraine), the attacking Tartar soldiers experienced an epidemic of bubonic plague. The Tartars capitalized on the devastating disease and used the bodies of plague victims as weapons. They catapulted their own soldiers' diseased bodies into the walled city of Kaffa in an attempt to inflict harm on their enemy and likely contributed to the European plague epidemic during the Middle Ages.5-7 Smallpox and measles contained within blankets and clothing were used by the Conquistadors and British as biological weapons against Native Americans. It is believed that such tactics influenced the outcome of the French and Indian Wars.5'7 Plague was used as a weapon by the Japanese during World War II. Laboratory-raised fleas were allowed to feed on plague-infected rats. The fleas were then disseminated from airplanes over China.7'8 In 1993, the Aum Shinrikyo Cult (known for its sarin nerve gas attack in a Tokyo subway) attempted to spread anthrax in Tokyo from the rooftop of an eight-story building owned by the Cult. The purported attack did not cause any illness from the anthrax agent, because the strain Sterne (a vaccine strain) was used. It is believed that the Aum Shinrikyo Cult also dispersed botulinum toxin, Q fever, and spores from a poisonous mushroom, but all attacks failed.9-11 In 1984, cult followers of Baghwan Sri Rajneesh attempted to affect the outcome of a local election in Dalles, Oregon. They attempted to incapacitate the population and prevent them from voting by successfully contaminating salad bars in 10 restaurants with Salmonella typhimurium; 751 people developed food poisoning. A Salmonella typhimurium strain found in a laboratory at the commune of the religious cult was indistinguishable from the strain from the outbreak.12 In 1996 in Dallas, Texas, there was a documented case of intentional Shigellosis contamination. Twelve people of the laboratory staff of St. Paul Medical Center hospital developed severe, acute diarrheal illness. Shigella dysenteriae type 2 was isolated from stool samples of some of the sick workers. All outbreak patients reported eating muffins or doughnuts in a break room. Shigella dysenteriae type 2, recovered from the patients, from an uneaten muffin, and from the medical laboratory's stock strain were indistinguishable by pulsed-field gel electrophoresis. A criminal investigation ensued focusing on a laboratory technician who was subsequently convicted on five felony assaults and falsifying laboratory documents.1013
The few examples described above focus mostly on those agents that could cause harm to human targets (see CDC list).5 However, disruption of food sources and great economic upheaval could arise by targeting agriculture. U.S. commercial animal agriculture revolves around three commodities: cattle, swine, and poultry (i.e., chickens and turkeys). Sheep, goats, ducks, and other species may be considered commercially insignificant, but an attack on these could hurt smaller economic sectors and affect public confidence. While horses are not considered to be a domestic livestock industry by the United States Department of Agriculture (USDA), they do fulfill personal and recreational needs. Deer, elk, bison, and other wildlife species are also not considered domestic livestock but could become reservoirs for pathogens, and attacks on wildlife could affect the ecosystem. Fish and shellfish, which form a notable industry, are often not thought of as targets, but could be targets for an attack.
International lists of livestock diseases that pose the greatest threats are maintained by the International Organization of Epizootics (OIE).14 There are about 15 diseases on the OIE List A and a much larger number in List B. Blue-tongue, Newcastle disease, and vesicular stomatitis, which are on List A, occur naturally in the U.S. Terrorists and criminals have easier access to these and other animal pathogens, because they occur naturally. Some animal diseases are caused by zoonotic pathogens (i.e., causing disease in animals and humans). These include: Rift Valley Fever (Bunyaviridae), Glanders (Burkholderia mallei), and Q fever (Coxiella burnetii). Pathogens that reside in a broad range of host species can be more difficult to eradicate and enable access from a variety of reservoirs.
Livestock are highly vulnerable. One need only consider the most recent outbreak of Foot and Mouth disease in England to appreciate the impact of just a natural outbreak. The same or greater effect can be imagined with the use of a livestock-directed bioweapon. Animal pathogens are often highly contagious and can be dispersed without advanced technology; in addition, disease can spread rapidly, animal to animal. In the U.S., disease spread can be exacerbated because livestock are often transported in large numbers throughout the U.S.15 Historically, attacks have occurred against livestock. For example, during World War I, Glanders (Burkholderia mallei) was used by the Germans to infect Allied horses and mules. In fact, a laboratory was established in Chevy Chase, Maryland in 1916 to culture Glanders (and anthrax) and the live organisms were then provided to German agents to infect U.S. draft animals. They successfully infected a number of animals in the U.S., as well as in Europe.7,10,16 The former Soviet Union also used Glanders in the early 1980s during its war in Afghanistan.17,18 In 1952, the Mau Mau poisoned and killed some cattle at a British mission station in Kenya using a local toxic plant, the African milk bush (Synadenium compactum "Ruby"). They purportedly placed plant latex material into incisions made in the animals' skin.10,17,18
There are no major plant pathogens that also pose a significant threat to human public health. However, the plant agriculture industry is a large part of the U.S. economy and a major source of foodstuffs. A significant proportion of the global crops of soybeans, wheat, and corn are grown in the U.S. A successful attack on any of these commodities would have obvious economic impact locally, if not globally, and could affect food supplies. In addition, many other crops such as sorghum, rice, barley, cotton, and tree crops such as citrus, apples, and stone fruits are economically important in regions of the U.S. An attack on a regional crop, such as rice in the U.S., may not be catastrophic economically or significantly affect food supplies, but public concern would intensify. However, an attack on the rice crop in some Asian countries, such as India or China, could be economically devastating and/or create a famine. Also, one needs to consider forest plant life as a significant target that can impact on the ecosystem, economy (e.g., lumber), recreation, and security.
The most significant plant pathogen threats for U.S. agriculture are fungi19 such as soybean rust (Phakospora pachyrhizi)20,21 and wheat stem rust (Puc-cinia graminis).22 Wheat stem rust can be an especially effective bioweapon because it can remain viable for more than two years with proper storage conditions and spreads rapidly.23 Wheat smuts are endemic around the world, and large infestations can greatly reduce crop yield. In addition, wheat smuts may have another weaponizable feature not typically considered; they produce trimethylamine gas that is highly flammable. Harvesters containing infected wheat could perhaps become bombs.23 Although chemicals such as Agent Orange for defoliation or chlordane to poison food17 have been used, there are no documented cases of intentional use of pathogens to attack crops or other plants. Natural outbreaks, however, can provide insight into the possible effect of a successful attack. The potato blight in Ireland during 1845-1846 resulted in a famine that caused one million deaths and the emigration of another million Irish.23,24 Also in the 19th century, a coffee leaf rust destroyed plantations throughout southeast Asia.23,24 Brown spot rice contributed to the Bengal famine of 1943.24 In 1970 in the United States, a leaf blight destroyed one billion dollars worth of corn.23 Some of the most devastating crop pathogens pass from plant to plant in an airborne manner or by splashing rain; thus, once an infestation has begun, dispersal can occur easily. There are more than 50,000 known diseases that affect crops. Many crops have low genetic diversity, which render some crop varieties widely susceptible to the terrorist or criminal use of plant pathogens. There are few restrictions on trade and transportation of plant products, especially seeds; thus, an attack on agriculture could be easy to carry out and difficult to prevent. Because fungi are far more complex than viruses, bacteria, or toxins (the major human and animal threats), a greater resource investment will be required to develop the investigative tools and technologies required for forensic detection and analysis.
Toxins, produced by some plants, fungi, and bacteria, also may be used as weapons. Botulinum toxin is one of the most lethal toxins known.5 A single gram of the toxin is sufficient to kill more than 1 million people.25 Moreover, as a potential weapon, botulinum toxin is easy to produce and transport. At least three times in the early to mid-1990s, the Aum Shinrikyo attempted unsuccessfully to disperse botulinum toxin in Tokyo and at U.S. military installations in Japan.25 The castor bean (Ricinus communis) contains the toxin ricin, which is highly lethal can be made easily (recipes can be readily found on the internet) has been involved in several U.S. criminal cases, and has been used as a tool for assassination.7 In London in 1978, Georgi Markov, a Bulgarian dissident, was assassinated and ricin was the weapon. He was stabbed in the back of his right leg with the tip of an umbrella, resulting in the deposition of a ricin-laden platinum/iridium spherical pellet only 1.52 mm in diameter. Interestingly, a similar attempt at assassination was made in Paris in 1978 on Vladimir Kostov, a Bulgarian defector. The pellet was lodged in his back, but the delivery mode was unknown. Because Kostov was aware of the death of Markov, the pellet was identified and surgically removed before any lasting effects developed. Ricin was confirmed as the toxin by the presence of ricin antibodies in Kostov.10 Mycotoxins (toxins from fungi) can cause severe illness and death.19,26 Even as far back as the 6th century B.C., the Assyrians recognized the potential of mycotoxins as a weapon; they poisoned their enemies' water supply with rye ergot.26 Shortly after World War II the potential of mycotoxins as a bioweapon became apparent via the Russian military, when Fusarium-contaminated flour was used to make bread that was then fed to unaware civilians, who suffered severe illness.26 Mycotoxins, in the form of yellow rain, allegedly have been used in Laos (1975-1981), Kampuchea (1979-1981), and Afghanistan (1979-1981). As many as 10,000 people, civilians and guerilla fighters, may have died from the mycotoxin poisoning.26 The Iraqis had an active program developing aflatoxin (although it is difficult to see how this mycotoxin would be used as a significant weapon).27
One can appreciate that there are many potential targets, and the number and variety of potential pathogenic weapons are large. Some pathogens are readily available and may be easy to weaponize and disperse. It is now time to build an effective forensic science program to analyze evidence found at crime scenes where bioweapons have been used.
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