A Population-Based Approach to Determining the Borders and Number of Natural Plague Foci

A population-based approach to determining the borders and number of natural plague foci implies that the specialists in the field accept the concept that the existence of a natural plague focus is always provided by a single population of the main carrier. The discussion of this approach was motivated by the data collected in the course of a long-term study of the Hissar natural plague focus in Tajikistan. It was demonstrated that the area of this focus constitutes only 300 km², and its functioning is maintained by a single relatively small population of Microtus carruthersi Thomas, 1909, the main plague carrier. The Hissar mountain range territories neighboring the focus have been surveyed for plague for more than 20 years. However, the plague pathogen (Yersinia pestis Lehmann, Neuman, 1986) could not be detected notwithstanding the absolute identity of the species composition of rodents and fleas. Many known natural plague foci are considerably larger in size than the Hissar focus and are inhabited by several populations of the main carrier. For example, five groups of populations of the common vole (Microtus arvalis Pallas, 1778), the main plague carrier, are described in the Eastern Caucasus high mountain focus occupying an area of 23 500 km². In the Tuva mountain focus, the plague pathogen was found in six populations of the main carrier, the long-tailed ground squirrel (Spermophilus undulatus Pallas, 1778). On the territory of the Muyunkum desert natural plague focus occupying an area of 93 000 km², 17 populations of the great gerbil (Rhombomys opimus Lichtenstein, 1823), the main plague carrier, are distinguished. Similar data are available for a number of other foci. Significantly, in different populations of the main carriers, the differences in the genotypes of both rodents and the plague pathogen were described, and asynchrony in plague epizootics could be traced. In such a way, the plague parasitic cenosis functioning within the borders and the habitation areal of a single population of the main carrier constitutes an autonomous standalone natural focus. Hence, the foci occupied by several populations of the main carrier are, in fact, groups of autonomous foci with a similar biocenotic structure. Therefore, a considerably larger number of natural (autonomous) foci are present within the plague natural focality zones than is commonly assumed.