USMLE – Plague
Yersinia pestis is a gram-negative facultative intracellular bacterium that is transmitted by flea bites or by aerosols and causes a highly invasive, frequently fatal systemic infection called plague. Plague, also named Black Death, caused three great pandemics that killed an estimated 100 million persons in Egypt and Byzantium in the 6th century; one quarter of Europe’s population in the 14th and 15th centuries; and tens of millions in India, Burma, and China at the beginning of the 20th century. Currently, wild rodents in the western United States are infected with Y. pestis, which is rarely transmitted to humans. Most cases of plague occur in urban foci in Southeast Asia, where rats are the reservoir of infection. Y. enterocolitica and Y. pseudotuberculosis are genetically similar to Y. pestis; these bacteria cause fecal-orally transmitted ileitis and mesenteric lymphadenitis.
Y. pestis makes a plasmid-encoded secreted protease that activates plasminogen and cleaves complement C3 at a specific site. This secreted protease is essential for spread of the bacteria from the local site of inoculation and inflammation into the bloodstream, so mutant bacteria lacking this protease are 1 million times less virulent to mice when inoculated into the skin.
Plague causes lymph node enlargement (bubo), pneumonia, or sepsis, all with a striking neutrophilia. The distinctive histologic features include (1) massive proliferation of the organisms, (2) early appearance of protein-rich and polysaccharide-rich effusions with few inf1ammatory cells but with marked tissue swelling, (3) necrosis of tissues and blood vessels with hemorrhage and thrombosis, and (4) neutrophilic infiltrates that accumulate adjacent to necrotic areas as healing begins.
In bubonic plague, the site of entry is usually on the legs and is marked by a small pustule or ulceration. The nodes of drainage enlarge dramatically within a few days and become soft, pulpy, and plum colored and may infarct or rupture through the skin. In pneumonic plague, there is a severe, confluent, hemorrhagic, and necrotizing bronchopneumonia, often with fibrinous pleuritis. In septicemic plague, lymph nodes throughout the body as well as reticuloendothelial organs develop foci of necrosis. Fulminating bacteremias also induce DIC with widespread hemorrhages and thrombi.