Biological Terrorist Agents Part I – Bacterial Agents

In January 2002, anthrax was covered in detail in the Hazmat Studies column. This column will cover other potential biological agents that could be used by terrorists. Biological terrorist agents are microorganisms or toxins derived from living...


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Plague (Yersinia pestis) is a zoonotic bacterium that is normally spread among rodents by infected fleas. Zoonotic bacterium are capable of being transmitted from lower animals to humans under natural conditions. Three forms of the disease can affect humans: bubonic, pneumonic and primary septicemic. (Another type of plague, pharyngeal, resembles acute tonsillitis.)

Periodic outbreaks of the plague occur naturally in rodent populations, which may result in a high death rate. Fleas that have lost their usual hosts pursue alternative sources of blood. When this happens, the risk to humans and other animals is increased.

Epidemics of plague in humans commonly involve house rats and their fleas. Animals prone to be carriers in the United States include rock squirrels, prairie dogs and other burrowing rodents. The last plague epidemic in the United States occurred in 1924 and 1925. Since then, only isolated cases have been reported, usually in rural areas from wild rodents. Plague cases in the United States during the 1980s averaged 18 per year, mostly in Arizona, California, Colorado and New Mexico.

Death rates from bubonic plague can reach as high as 60% if victims are not treated. When victims are treated, the death rate is reduced to about 15%. If treatment is not begun within 24 hours after symptoms develop, pneumonic plague has a near 100% death rate. (Plague can also be transmitted when the organism enters the body through a break in the skin. This type of exposure occurs from direct contact with tissue or body fluids of a plague-infected animal, such as skinning a rabbit or other animal. This, however, is a rare occurrence.)

Plague can be transmitted through inhalation by contacting infected droplets from a person or domestic animal coughing. Plague that develops from this type of exposure is called pneumonic. This infection involves the lungs as a result of inhalation of organisms, which results in primary pneumonic plague. Secondary pneumonic plague results from septicemia (a blood infection) when the organisms spread to the lungs.

Symptoms from plague exposure usually develop within two to six days after exposure. Pneumonic plague occurs a little faster, from one to three days, which is also dependent on the amount of organisms inhaled. Symptoms of bubonic plague include enlarged lymph nodes, fever, chills and prostration. Pneumonic plague symptoms are similar to those of bubonic plague and include high fever and chills that are accompanied by coughing and difficulty breathing, toxemia and the production of a bloody sputum. Symptoms may be followed by rapid shock and death if treatment is not begun early. Death results from respiratory failure, circulatory collapse and a predisposition toward bleeding.

Bubonic plague can progress spontaneously to septicemic plague accompanied by fever, chills, prostration, abdominal pain, shock and bleeding into the skin and other organs. It can also affect the central nervous system, lungs and other parts of the body.

As many as 80% of bubonic plague victims have positive blood cultures for septicemia. Approximately 25% of the patients also have various types of skin lesions. Pustules, vesicles, eschars (dead tissue separating from living tissue) or papules (small elevations of the skin) containing leukocytes and bacteria may also be present near the site of the fleabite.

Antibiotic treatment should begin as soon as possible. Streptomycin is the drug of choice, but others such as tetracyclines, chloramphenicol, gentamicin or one of the sulfonamides may also be effective. A vaccine for the plague is available, but it is effective only as a preventative measure; once someone is exposed, the vaccine will not help. The initial dose of vaccine is followed by a second dose one to three months later and a third dose three to six months after that. Booster shots are administered at six, 12 and 18 months and then every one to two years. As with all vaccines, the level of protection is related to the size of the dose; vaccination defense could be overwhelmed by extremely high doses of the bacteria.

Once infected with plague through aerosol dissemination, people and animals could pass the disease to each other. Aerosol delivery would result in pneumonic plague developing in those exposed. If infected fleas are disseminated into a geographic area, bubonic plague would follow in those bitten by the fleas. The bacteria can survive in blood for 100 days and in human bodies for up to 270 days. They remain viable in water and moist meals and grains for several weeks.