Author:

Keywords:

Science & Technology, Life Sciences & Biomedicine, Public, Environmental & Occupational Health, Infectious Diseases, 1117 Public Health and Health Services, Tropical Medicine, 4202 Epidemiology

Abstract:

Plague is globally distributed but in most cases occurs locally in relatively small areas. These areas are linked to enzootic foci that seem to be stable and from where plague rarely moves away. This is remarkable for a disease which through history has become iconic for a rapidly spreading infection travelling over large distances. It is usually assumed that ecological conditions determine local persistence and that human movements are responsible for long-range spreading. Well established plague foci exist in eastern Africa. Their borders are fairly well documented but the underlying ecology for their persistence remains unsure. Ecological Niche Modelling (ENM) suggests that plague foci are found in highly diverse ecological conditions, and hence predict enormous areas where plague could occur, but nevertheless also equally large areas where plague is unlikely to persist. At a regional scale, predictions show more patchy patterns that are confirmed by recently emerged epidemic areas and observations of plague infected rodents in the absence of human plague. The ecological mechanisms that are responsible for these patterns remain elusive, however, and the niche dimensions selected in the ENMare not easy to interpret.At the local scale, assemblages of rodents and fleas may differ in a complex way between sites with high and low plague incidence but patterns seem to differ between separate foci. As a result, it is extremely hard to convincingly explain the patchy distribution of plague, or to predict how focimay change or where they could emerge. Anthropogenic spreading of the infection may play a role but is certainly not sufficient for the establishment of foci, as illustrated by the numerous outbreaks along Africa’s coast that did not result in the persistent infection of local rodent populations. Climatic factors have been shown in several studies to have an effect on the temporal variation in plague transmission in an area, and therefore it has been suggested that climate change would also affect the spatial characteristics of the foci. Data to support this are mostly lacking, but we make attempts to see whether climate variation can be linked to the growing foci in northeastern R.D.Congo, or the emergent foci of Mbulu and Karatu in Tanzania. Based on the available information, we make a hesitant prediction of areas and situations where plague foci could emerge or grow, with the explicit warning that for now, such predictions must still be used as the testing of hypotheses rather than practical instruments for public health.