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Plague is a zoonotic infection primarily maintained in rodents and fleas. Humans are incidental hosts, and transmission from rodents to humans is most commonly by flea bites. This disease still exists worldwide and likely has been responsible for at least 3 devastating pandemics—the Justinian Plague (Byzantine Empire, 541–542), the Black Death (Europe and Middle East, 1346–1353), and the plague pandemic in the mid-19th century, which started in China.
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PATHOGENESIS AND EPIDEMIOLOGY
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Plague is caused by Yersinia pestis, a pleomorphic, gram-negative, non–spore-forming coccobacillus that exhibits bipolar staining with Wright-Giemsa or Wayson stains. When cultures are obtained from clinical samples (eg, blood, sputum, lymph node aspirates), the organism can be recovered on blood, chocolate, or MacConkey’s agar. Care should be taken if the clinician is suspecting Y pestis since many of the automated identification systems may not accurately identify this pathogens, confusing it with similar organisms (eg, Yersinia enterocolitica, Yersinia pseudotuberculosis).
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The clinical presentation of plague is most often dictated by the portal of entry of the organism into the host. When the portal of entry is through the skin (eg, flea bite or contact with infected animals), the organisms most commonly settle out in the lymph nodes and form buboes, but Y pestis can just as easily disseminate throughout the bloodstream to distant organs. The severity of disease is determined by the extent of endotoxin production. Organisms that can continue to replicate within the host macrophages without being killed have the ability to produce high levels of endotoxin. When the portal of entry is the lungs, severe pneumonia, endotoxemia, and septicemia are more likely to occur, causing a fatal infection if not appropriately recognized and treated.
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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 foci occur in Africa, Asia, North America, and South America, with the majority of human cases in Africa. Most cases occur in rural areas. North American foci occur primarily in the southwestern United States and the Pacific coastal region. The disease exists almost entirely in the sylvatic form, among a number of wild rodent species. Urban plague, the cause of the epidemics of the European Middle Ages, is dependent on the Norwegian rat and a flea (Xenopsylla cheopis) but is now quite rare. The last epidemic of urban US plague occurred in 1924 to 1925 in Los Angeles. Currently, a median of 8 cases of human plague are reported annually in the United States, although preliminary data from 2015 suggested a slight increase.
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In the United States and Canada, the epidemiology is complex and involves a number of different rodent hosts and flea vectors, as well as domestic animals. Most commonly, the bubonic or septicemic form of infection is acquired by human exposure to infected tissues or from the bites of fleas of wild ...