Population Approach to Determination of the Borders and Number of Natural Plague Foci

Abstract

Population approach to determination of the borders and number of natural plague foci implies the specialists’ acceptance of the concept that the existence of a natural plague focus is always provided by a single population of the main carrier. Motivation for our discussion of such an approach was the data of a long-term study of the Hissar natural plague focus (Tadzhikistan). The area of the focus was only 300 km2 and its functioning was maintained by a single, relatively small population of Microtus carruthersi (Thomas, 1909) – the main carrier of plague. The Hissar mountain range territories adjacent to the focus have been surveyed for plague for more than 20 years, however, no plague agent (Yersinia pestis Lehmann, Neuman 1986) has been found, despite the complete identity of the species composition of rodents and fleas. Many of the known natural plague foci are significantly larger in size than the Hissar focus and inhabited by several populations of the main carrier. For instance, in the East-Caucasian high-mountain focus, covering the area of 23,500 km2 , five groups of common vole populations (the main carrier of plague) are distinguished (Microtus arvalis Pallas, 1778). In the Tuva mountain focus, the plague agent was detected in six populations of the main carrier – the long-tailed souslik (Spermophilus undulates Pallas, 1778). In the territory of the Mojynkumsky desert natural plague focus that occupies 93,000 km2 , 17 populations of the great gerbil (Rhombomus opimus Lichtenstein, 1823) were singled out. Similar data are available for a number of other foci. Revealingly, in different populations of the main carriers, genotype differences were identified, both in rodents and the plague agent, as well as asynchrony in the plague epizooty development. Thus, the plague parasitic coenosis, operating within the boundaries and in the territory of a single population of the main carrier, constitutes an autonomous, standalone natural focus. Therefore, the foci with several populations of the main carrier are, in fact, groups of autonomous foci with a close, similar biocoenotic structure. There are many more natural (autonomous) foci within the natural plague focality zone than is widely assumed.