zoonoses and communicable diseases common to ... - PAHO/WHO

PLAGUE 207PLAGUEICD-10 A20.0 bubonic plague; A20.2 pneumonic plague;A20.7 septicaemic plagueSynonyms: Black death, pestilential fever, pest.Etiology: The etiologic agent of plague is Yersinia pestis, a gram-negative, nonmotilebacterium, coccobacillary to bacillary in form and showing bipolar stainingthat is not very resistant to physical and chemical agents. DNA hybridization studiesdemonstrated the close genetic relationship between Yersinia pestis and Y.pseudotuberculosis (Bercovier et al., 1980). On the basis of this, the authors suggestedcalling the etiologic agent of plague Y. pseudotuberculosis subsp. pestis(International Committee on Systemic Bacteriology, List 7, 1981). However, theCommittee’s Judicial Commission (1985) decided to reject this nomenclature andretain the name Y. pestis in order, among other reasons, to avoid possible confusion.Three biological varieties are distinguished: Orientalis (oceanic), Antiqua (continental),and Mediaevalis. This distinction has a certain epidemiological significance,principally for nosography, but there is no difference in the biotypes’ pathogenicity.Some virulence factors of Y. pestis were defined in the 1980s. Apparently, theprincipal factor is a 45-megadalton plasmid. This plasmid encodes calcium dependencyfor growth at 37°C, but not at lower temperatures, as well as the virulence antigensV and W. The two proteins on the outer membranes that are assumed to beimportant in virulence (E and K) are also plasmid dependent. The precise role ofeach of these factors is not yet well defined (Butler, 1989).Geographic Distribution: Natural foci of infection persist on nearly all continents;they do not exist in Australia, New Zealand, or New Guinea. In the Americas,sylvatic plague is maintained in rodents in the western third of the United States, theborder region of Ecuador and Peru, southeastern Bolivia, and northeastern Brazil.Similarly, there are foci in north-central, eastern, and southern Africa, includingMadagascar; the Near East; the border area between Yemen and Saudi Arabia;Kurdistan province (Iran); and central and Southeast Asia, in Myanmar (Burma) andVietnam. There are also several natural foci in the former Soviet Union and inIndonesia (Benenson, 1990).Occurrence in Man: Since the dawn of the Christian era, there have been threegreat pandemics: the first began in 542 (Justinian plague) and is estimated to havecaused 100 million deaths; the second began in 1346, lasted three centuries, andclaimed 25 million victims; and the last began in 1894 and continued until the1930s. However, the data on incidence in the Middle Ages are very approximate anddifficult to verify. As a result of the last pandemic, natural foci of infection wereestablished in South America, West Africa, South Africa, Madagascar, andIndochina.Urban plague has been brought under control in almost the entire world, and ruralplague of murine origin is also on the decline. Nevertheless, epidemics haveoccurred in Indonesia, Nepal, and southern Vietnam. In this last country, there were5,274 cases in 1967 due to contact with domestic rats and their fleas.