Plague, the disease caused by the Gram-negative pathogen Yersinia pestis, presents in a variety of clinical forms in naturally acquired disease. Pandemic plague has significantly impacted world history; its impact may have been so great in the Middle Ages as to have led to genetic selection within Europeans, thus possibly affecting the course of future epidemic diseases such as HIV through changes in one of the viral co-receptors (Galvani and Slatkin, 2003). Plague is endemic in parts of Southeast Asia, Africa, and the western United States, with nearly all of the 13 annual US cases occurring in four states of the desert southwest (CDC, 1996).

The allure of plague as an agent of bioterrorism is related to a number of factors: it can be mass produced and disseminated as an aerosol, as successfully accomplished experimentally by both the US (Christopher et al., 1997) and the Soviet (Alibek, 1999) bioweapons programs in the past; the pneumonic form of the disease is communicable from person-to-person and associated with a high mortality rate if untreated; drug-resistant mutants occur in nature (Galimand et al., 1997); and an effective vaccine is not widely available. Perhaps the greatest appeal to terrorists is the stigma attached to plague, largely based on its historical track record of social and economic devastation. While the outbreak in Surat, India, in 1994 resulted in only 52 deaths, hundreds of thousands fled the city and mass chaos followed in its wake (Ramalingaswami, 2001).

Aerosolized preparations of the agent, the expected vehicle in bioterrorism, would be predicted to result in cases of primary pneumonic plague outside of usual endemic areas. As was the case with the anthrax attacks in 2001, however, additional forms of the disease, such as bubonic and septicemic plague, might also be expected to occur. Primary pneumonic plague classically presents as an acute, febrile, pneumonic illness with prominent respiratory and systemic symptoms; gastrointestinal symptoms, purulent sputum production or hemoptysis occur variably (Artenstein and Lucey, 2000). Chest roentgenogram typically shows patchy, bilateral, multilobar infiltrates or consolidations. In the absence of appropriate treatment, there may be rapid progression to respiratory failure, vascular collapse, purpuric skin lesions, necrotic digits, and death. The differential diagnosis, as noted in Table 12.5, is largely that of rapidly progressive pneumonia. The diagnosis may be suggested by the characteristic small Gram-negative coccobacillary forms in stained sputum specimens with bipolar uptake - the "safety pin" appearance - of Giemsa or Wright stain (Inglesby et al., 2000). Culture confirmation is necessary to confirm the diagnosis; the microbiology laboratory should be notified in advance if plague is suspected, because special techniques and precautions must be employed to avoid inadvertent exposures.

Treatment recommendations for plague have been reviewed elsewhere (Inglesby et al., 2000). Pneumonic plague can be transmitted from person-to-person by respiratory droplet nuclei, thus placing close contacts, other patients, and health-care workers at risk. Prompt recognition and treatment, appropriate deployment of postexposure prophylaxis, and early institution of droplet precautions will interrupt secondary transmission of plague.

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