Biological Warfare Agent Delivery
Biological agents have some unique characteristics that make weaponizing them attractive. Most biological weapons consist of living organisms (toxins are the exception) and, thus, can replicate once disseminated. A relatively small group of persons, using single individuals deployed in a military staging area, could bring about the infection of a large percentage of targeted persons. The clinical illness could develop within a day of dispersal and last for as long as 2-3 weeks. The mission and political impact of such a strike on a combat or constabulary force of 10,000 soldiers may compromise operations. In a civil situation, major subway systems in a densely populated urban area could be targeted for biological agent strike, resulting in massive political and social disorganization. Approximately 10 grams of anthrax spores can kill as many persons as a ton of sarin.
Any country having pharmaceutical, cosmetic, or advanced food storage industries will have stabilization facilities similar to those that could be used for biological weapons. The ability to disseminate the biological agent over a wide area would be limited to those countries having cruise missiles or advanced aircraft. Even the smallest country or a terrorist group, however, has the capability to deliver small quantities of BW agent to a specific target.
Stabilization and dissemination/dispersion are important issues because of the susceptibility of the biological agents to environmental degradation, not only in storage but also in application. This is a problem whether the end use is for biological weapons, pharmaceutics, cosmetics, pesticides, or food-related purposes and is related to the susceptibility of the organisms to inactivation of the biochemical compound by the environment. This loss of bioactivity can result from exposure to high physical and chemical stress environments, such as high surface area at air-water interfaces (frothing), extreme temperatures or pressures, high salt concentrations, dilution, or exposure to specific inactivating agents.
The primary means of stabilization for storage or packaging are initial concentration; direct freeze drying (lyophilization); direct spray drying; formulation into a special stabilizing solid, liquid, or sometimes gaseous solution; and deep freezing. Methods of concentration include vacuum filtration, ultrafiltration, precipitation, and centrifugation. Freeze drying is the preferred method for long-term storage of bacterial cultures because freeze-dried cultures can be easily rehydrated and cultured via conventional means. Many freeze-dried cultures have remained viable for 30 years or more. Deep or ultra freezing of biological products is another long-term storage technique for species and materials not amenable to freeze drying. The method involves storage of the contained products in liquid nitrogen refrigerators (-196° Celsius) or ultra-low temperature mechanical freezers (-70° Celsius). Mechanical freezing systems should include precautionary back-up freezers and electrical generators. Cryo-protective agents, such as dimethyl sulfoxide (DMSO), glycerol, sucrose, lactose, glucose, mannitol, sorbitol, dextran, polyvinylpyrollidone, and polyglycol, are required to ensure cell viability during storage. A toxin agent is most effective when prepared as a freeze-dried powder and encapsulated. Such encapsulation, however, is not necessary for weaponization. Infectious biological agents are generally stabilized and then spray dried.
Under appropriate meteorological conditions and with an aerosol generator delivering 1-10 micron particle-size droplets, a single aircraft can disperse 100 kg of anthrax over a 300 km 2 area and theoretically cause 3 million deaths in a population density of 10,000 people per km2. The mean lethal inhalator dosage is 10 nanograms. On the other hand, some biological agent characteristics can severely limit the effectiveness of BW, which consist of living organisms. A technique to stabilize (protect) the organisms from adverse environments is essential if the weapons are to maintain their effectiveness over some period of time. This requirement of stabilization also extends to the methods of delivery since the organisms are very susceptible to degradation in the environments associated with delivery systems.
Proper dissemination is a non-trivial problem because the agent must be dispersed in 1 to 10 micron particles and be inhaled by the target population. of Biological organisms are subject to being negated by sunlight and the environment, but most can be effectively stabilized against adverse environmental effects. Stress from explosive dissemination and/or missile firing can reduce efficiency to about the 5-percent level, which is why aerosol dissemination by pressurized gases was adopted by munition designers in the old U.S. program. Dissemination efficiencies of up to 70 percent were achieved, with 30 to 50 percent being produced routinely.
Aerosolization of biological agents using spray devices is the method of choice since the extreme physical conditions associated with explosive dissemination can completely inactivate the biological agent. (Aerosol dispersal allows for control of particle size and density to maximize protection from environmental degradation and uptake of the enclosed biological agents in the lungs of targeted populations.)
Aerosol delivery systems aim to generate invisible clouds with particles or droplets between 0.5 and 10 microns in diameter which can remain suspended for long periods. Inhalation of agent aerosols, with resultant deposition of infectious or toxic particles within alveoli, provides a direct pathway to the systemic circulation. Infection by the respiratory route may induce disease at doses lower than those generally associated with naturally acquired infections by the oral route. The subsequent illness may differ from the natural pattern, and the incubation period may be much shorter. The natural process of breathing causes a continuing influx of biological agent to exposed individuals. The major risk is pulmonary retention of inhaled particles. Droplets as large as 20 microns can infect the upper respiratory tract; however, these relatively large particles generally are filtered by natural anatomic and physiological processes, and only much smaller particles (ranging from 0.5-5 microns) reach the alveoli efficiently. Still smaller droplets are inhaled, but they are not efficiently by the human respiratory tract, and are relatively unstable under ambient environmental conditions.
Aerosol particles with a diameter of 1-15 mm mass median diameter (MMD) are readily absorbed by lung cells following inhalation, the primary mode of infection by most biological agents. Some agents can also act following ingestion of contaminated food or water. However, infectious agents generally do not penetrate intact skin. Equipment used with aerosol dispersal of biological agents includes spray nozzles or aerosol delivery systems capable of dispersing particles or droplets and compressors for initial weaponization by agent integration with compressed gas (air). For subnational or terrorist groups, the biological agents can be dispersed by manual aerosol generators. The availability of vaccines against selected biological agents may render the user immune to the effects of the agent although a suffcient dose of agent may overwhelm the vaccine's protective effect.
Dissemination efficiency rates of aerosol delivery systems are in the range of 40-60 percent. Cruise missiles, aircraft carrying gravity bombs or spray attachments, and fixed-wing or rotor craft with attached sprayers are all vehicles for delivery of biological agents. The delivery of biological agents by explosive devices is much less efficient (~1-5 percent).
Effective delivery of these agents must also consider the environmental effects on the agent (inactivation). Dissemination (delivery) of biological agents in biological warfare has been traditionally accomplished by aerosol dispersal using either spray devices or through incorporation of the agents with explosive devices (cluster bombs, missile warheads with submunitions designed for extended biological agent dispersal). The latter, however, must be approached with caution since explosive, heat-generating entities can inactivate the organisms/toxins. The preferred approach is dispersion via the use of a pressurized gas in a submunition. Other preferred platforms from an efficiency standpoint include small rotary-wing vehicles, fixed-wing aircraft fitted with spray tanks, drones, bomblets, cruise missiles, and high-speed missiles with bomblet warheads. Fixed-wing aircraft and ground vehicles with aerosol generators also make excellent delivery systems.
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