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Plague exists worldwide as continuing enzootic sylvatic disease in rodent-flea cycles that occasionally spreads into the human population. Yersinia pestis, the causative organism, is a nonmotile, gram-negative, non–spore-forming, coccobacillus.

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Epidemiology

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The geographic distribution of plague is largely confined to the semiarid areas of most continents, with the exception of Australia. Enzootic North America foci are the largest in the world, occurring primarily in the southwestern United States (extending east as far as Dallas, Texas) and the Pacific coastal region, extending from Coahuila, Mexico, to Alberta and British Columbia, Canada. The disease exists almost entirely in the sylvatic form, with rodent-flea cycles among a number of wild rodent species.1 Urban plague, the cause of the epidemics of the European Middle Ages, is dependent upon the Norwegian rat and flea (Xenopsylla cheopis), but is now quite rare. The last epidemic of urban US plague occurred in 1925 in Los Angeles. Rarely, pneumonic plague is transmissible from person to person, bypassing both the rat reservoir and the flea vector.1,2

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In the United States and Canada, the epidemiology is more complex and involves a number of different rodent hosts and flea vectors, as well as domestic animals. Most commonly, the infection is acquired by human exposure to infected tissues or from the bites of fleas of wild rodents such as prairie dogs, ground squirrels, chipmunks, rabbits, and other wild rodent species. Contact with squirrels accounts for nearly half of the exposures. Domestic animals, especially cats, may become infected after contact with wildlife and may transmit the infection to humans.2 Prior to 1977, no cat-related cases had occurred, but since that time 18 cases have been reported, with 28% of the cases causing direct pneumonic plague. Therefore, direct contact with wild animals or their fleas is not required for plague transmission. With the exception of cats, most other carnivores, including dogs, are relatively resistant to plague and therefore are rarely involved in plague transmission.

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Plague is considered a select agent and a potential bioterrorism agent. Therefore, its isolation in most clinical laboratories in the United States is restricted to genus and presumptive species based on phenotypic criteria, but final specific identification is performed in appropriate public health laboratories.

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Clinical Manifestations

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In the United States, most human plague cases occur from May to September and usually present as 1 of 3 primary forms: bubonic, septicemic, or pneumonic. Bubonic plague accounts for 78% of US cases, the remainder are septicemic (13.2%), pneumonic (4.4%), and meningitic or unknown (3.4%).3,4 In recent series, the incidence of septicemic plague has been as high as 25%.

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The incubation period of bubonic and septicemic plague is often difficult to determine but is usually within 2 to 6 days (range 1–10 days) of contact. Skin lesions are infrequently present at the site of initial infection (eg, flea ...

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