CHAPTER 1 History , clinical aspects , epidemiology of plague and characteristics of Y . pestis

DNA/DNA hybridization studies and 16SrDNA sequence analysis indicate that Y. pestis evolved from Yersinia pseudotuberculosis 1500 to 20,000 years ago. There are three primary clinical forms of Y. pestis infections in humans which without treatment exhibit a high incidence of death: bubonic plague, pneumonic plague, and septicemic plague. The designation "Black Plaque" is derived from extensive gangrene of the extremities and other skin surfaces resulting from systemic infection. Present outbreaks are of the bubonic form, characterized by extensive swelling in one or several lymph nodes (buboes). The classical paradigm of infection involves fleas feeding on the blood of infected rodents and then transmitting the organism to humans during feeding. Approximately 3,000 human cases of plaque occur worldwide annually. Without treatment, fatalities can be as high as 57%. In the U.S.A. 12 to 15 cases of plague are reported each year in western U.S., primarily in the states of California, New Mexico, Arizona, and Colorado. Approximately 3,000 human cases of plaque occur worldwide annually. The polymerase chain reaction has allowed the detection of Y. pestis in archaeological skeletons dating to 600AD.