Publications

Unless otherwise noted, all information presented in this article is derived from Inglesby TV, Dennis DT, Henderson DA, et al., for the Working Group on Civilian Biodefense. Plague as a biological weapon: medical and public health management. JAMA. 2000;283(17):2281-2290.

Background

Plague is an infectious disease caused by Yersinia pestis, a naturally occurring bacterium found primarily in wild rodents. Plague has been the cause of 3 of the great pandemics of the modern era-in the mid-6th century, the mid-14th century (known as the Black Death), and the early 20th century.

Three forms of naturally occurring plague infection exist:

• Pneumonic plague is the result of Y. pestis infection of the lungs. Primary pneumonic plague results from inhalation of Y. pestis bacteria and would be the expected form of disease following an aerosol attack with Y. pestis. Secondary pneumonic plague can occur if bubonic or septicemic plague goes untreated and the plague bacteria are allowed to spread to the lungs.
• Bubonic plague is the most common form of naturally occurring plague and is typically acquired through the bite of an infected flea. Bubonic plague is characterized primarily by swollen, tender lymph nodes (called buboes).
• Septicemic plague is the result of plague bacteria multiplying in the blood and disseminating throughout the body. Septicemic plague usually occurs as a result of untreated bubonic or pneumonic plague.

Plague is recognized by public health experts as a re-emerging infectious disease. Worldwide incidence has reached approximately 2,000 to 3,000 reported cases each year. For example, according to the WHO fact sheet on plague, the worldwide reported incidence in 2003 was 2,118 cases with 182 deaths. The disease is endemic in rodent populations in much of Southern Asia, parts of Africa, and the Americas. According to the CDC, approximately 5 to 15 naturally occurring human cases of plague are reported in the United States each year, usually in rural areas of the Southwest (eg, New Mexico, California, Colorado, and Texas). Advances in living conditions, public health, and antibiotic therapy make future natural plague pandemics improbable, but plague outbreaks following an attack with a biological weapon do pose a serious risk.

Plague as a Biological Weapon

Plague is currently considered to be one of the most serious bioterrorism threats. Y. pestis was developed as an aerosol weapon by several countries in the past. Aerosol dissemination of bacteria would cause primary pneumonic plague in the exposed population, an otherwise uncommon, highly lethal, and contagious form of plague.

A number of factors contribute to concern over the use of plague as a biological weapon:

• There is widespread availability of Y. pestis in microbe banks around the world.

• There is widespread natural availability of the Y. pestis bacteria in endemic areas

• Reports indicate that techniques for mass production and aerosol dissemination of Y. pestis have been developed.

• The fatality rate of pneumonic plague in untreated cases is extremely high.

• Plague has the potential for secondary spread from person to person following an attack.

• Y. pestis can be disseminated in aerosol form.

• Plague has a low infectious dose.

• Antibiotic resistant strains of Y. pestis exist in nature.

A 1970 World Health Organization (WHO) assessment asserted that in a worst-case scenario a dissemination of 50 kg of Y. pestis in an aerosol cloud over a city of 5 million might result in 150,000 cases of pneumonic plague. Of those, 80,000 to 100,000 cases would require hospitalization, and 36,000 victims would be expected to die. (See "The History of Bioterrorism: Plague," a short video from the CDC.)

Signs and Symptoms

Diagnosis of plague is based on clinical presentation of symptoms and confirmed by laboratory testing, which usually takes 24 to 48 hours. There are no widely available rapid diagnostic tests for plague. The first sign of a bioterrorist attack with plague would most likely be a sudden surge of patients presenting at hospitals and doctors' offices with symptoms of severe pneumonia and sepsis.

A: Cervical bubo in patient with bubonic plague.
B: Bleeding into the skin in patient with septicemic plague.
C: Gangrene of the digits during the recovery phase of illness of patient shown in B.
Source: CDC, Division of Vector-Borne Infectious Diseases, Fort Collins, Colo.

X-ray of a patient with pneumonic plague infection involving both lungs.
Source: CDC Public Health Image Library (ID #4136).

Transmission

Fleas are natural vectors (carriers) of Y. pestis, and the bacteria are typically transmitted to and among rodents via flea bite. Humans may contract plague through a flea bite or by handling an infected animal or breathing in an aerosolized form of the bacteria.