Читать книгу The Power of Plagues - Irwin W. Sherman - Страница 45

Y. pestis has been subdivided into three varieties—Antiqua, Medievalis, and Orientalis. Epidemiological and historical records support the hypothesis that Antiqua, presently resident in Africa, is descended from bacteria that caused the first pandemic, whereas Medievalis, resident in central Asia, is descended from bacilli that caused the second pandemic; those of the third pandemic, and currently widespread, are all Orientalis. It is believed that Y. pestis probably evolved during the last 1,500 to 20,000 years because of changes in social and economic factors that were themselves the result of a dramatic increase in the size of the human population, which was coincident with the development of agriculture. This triggered the evolution of virulent Y. pestis from the enteric, food-borne, avirulent pathogen Y. pseudotuberculosis. This required several genetic changes: a gene whose product is involved in the storage of hemin resulted in blockage of the flea proventriculus and enhanced flea-mediated transmission, and other gene products (phospholipase D and plasminogen activator) facilitated blood dissemination in the mammalian body and allowed for the infection of a variety of hosts by fleas.