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E. N. Pavlovsky's concept of natural focality of diseases and the development of general knowledge about natural foci and their structural (components), functional (mechanisms of pathogen maintenance), and ecosystem-related organization (assortment and interrelations of ecosystems) are reviewed from principal (in authors' opinion) aspects. The 60-year history of this theory includes three stages at which its scope and contents differed. At the first stage, it concerned transmissible zoonoses. It had been assumed that structurally, natural foci necessarily include the pathogen — vector — host triad, and the functioning of the focus is provided for by only pathogen circulation in terrestrial ecosystems. At the second stage, it became clear that vector is not a necessary structural component of any focus (an example of nontransmissible diseases), although the functioning of foci remained to be unequivocally attributed to the continuous pathogen circulation among animals of terrestrial ecosystems. The third stage is characterized by an understanding that, in general, the presence of a warm-blooded host in the focus is also unnecessary for pathogen survival, and natural foci can be represented by soil and aquatic ecosystems. The only necessary and specific component of any natural focus is the pathogen population. In this context, modern views on natural focality of diseases are reviewed, and the essence of the terms "natural focus" and "epizootic process" is defined. It is proposed to distinguish the phases of pathogen reservation and epizootic spread (circulation) in ecosystems of any type. The current state of this concept provides evidence that, in general biological terms, studies on natural focality of diseases belong to one of the fields of symbiotology.

Conceptions of different authors on the natural focus of zoonosis infections are analysed and an improved definition of this phenomenon is proposed. The natural focus of disease is the antropoecosystem of certain hierarchic level (biosocial system), where the existence and appearance of the infection agent is supported and the disease is realized in people at certain infection rate.

This review concerns the role of the fleas in survival and spread of the plague, their influence on the seasonal dynamics of the epizootics, and infection rates of these insects in different natural foci. The critical evaluation is given to the data which are used to calculate the flea transmission probability for mathematical simulation of plague epizootics.

During 60 years passed after the creation of the conception on natural focus of human infections by the academician E. N. Pavlovsky our knowledge on a role of blood-sucking insects and ticks in this phenomenon have been significantly enlarged. It has been recovered, that these arthropodes serve not only as vectors of infection agents, but also as natural reservoirs and amplificators of these agents. In the process of the infection agent circulation in the natural focus there are several additional pathways of circulations without a participation of vertebrates. These pathways are as follows: the dispersion of infection agent in cases of simultaneous feeding of infected and "clean" ticks, and also the sexual, transovarial and transphase transmissions of pathogens. These mechanism of circulation are most often occur in the ixodid ticks, which can play the main role in supporting the infection agent in the focus. The research was supported by the Russian Fund of Fundamental Investigations (grants 96-15-97 882, 99-04-49568) and in part by FIRCA, USA (Grant R03 TW00279-01A1).

Sympatry of the following pairs (or groups) of closely related species in the territory of the former Soviet Union amd some neighbouring countries has been revealed as a result of a revision of the fam. Ixodidae: I. persulcatus — I. ricinus, I. persulcatus — I. pavlovskyi, I. crenulatus — I. kaiseri, Dermacentor marginatus — D. silvarum — D. ushakovae, D. ushakovae — D. niveus, D. pavlovskyi — D. montanus, Rhipicephalus turanicus — R. sanguineus, R. turanicus — R. rossicus, R. turanicus — R. pumilio, R. rossicus — R. pumilio. Biotope and host relationships of sympatric species, as well as seasons of parasitizing have been described on the basis of the field researches conducted by the author, the collections of the Zoological Institute, Russian Academy of Scienses, and the publications substantiated by collection materials studied by the author. Hypothetic ways of the paleogenesis of each sympatric area and geological age have been considered. The recent sympatric area for I. persulcatus — I. ricinus (fig. 1) is of Holocene age (max. 8.000—10.000 years). During that period approximately 1600—2000 sympatric generations could have been realized. Some recent sympatric areas (I. persulcatus — I. pavlovskyi, and some species of other genera) are of Pleistocene and even Pliocene age (2—10 million years). In these areas the number of realized sympatric generations must have been much higher. Several variants of joint simultaneous and successive feeding on one and the same host individual have been established for stages of life cycles of sympatric species of ticks. Contacts of this kind could have created canals for multiple oscillatory exchange of pathogenic taxa (species, genospecies, strains) in a few or many thousands of sympatric generations of closly related vectors. Thus, the same taxon of a pathogen may be subject to different influence of ixodid vectors in sympatric area and in the area of separate habitat of close species of vectors. The research was supported in part by the Russian Fund of Fundamental Investigations (Grant 97-04-50 094) and in part by FIRCA, USA (Grant R03-TW00279-01A1).

Spatial location of epizootic events in the Tuva plague focus is determined at a considerable degree by the population structure of the flea Citellophilus tesquorum — the main plague microbe vector. Within the enzootic territory occupied by five populations of the long-tailed ground squirrel (Citellus undulatus) there are six populations of C. tesquorum. Each population of fleas has a corresponding autonomic plague focus. Various conditions for the microbe life activity in these populations are recovered. The circulation of the microbe is closely connected with certain intrapopulation groupings of fleas — the nuclei of populations.

The infection with Borrelia burgdorferi s. l. increases the questing activity of adults and nymphs of the taiga tick Ixodes persulcatus. High temperature inhibits the moving and questing activities of the infected ticks in a greater extent than that in the non-infected ones. Than more borreliae per a specimen of tick are present, the more tick's activity is affected.

A time series analysis of 23-years observations of the Ixodes persulcatus imago dynamics was carried out. The tick were counted in forests of the western Sikhote-Alin foothills near Ussuriisk. The fluctuations of the ticks number are quasiperiodical. They are formed by near 3-years oscilations, long-term non linear trend and accidental component. The observed process was described by means of a statistic model. This model combines the harmonic regression analysis with linear regression analysis. The determination coefficients are 90.8% and 92.7% for the discriptive and prognostic models respectively. The difference between two groups of environmental factors, which influence on the tick number, is discussed. These factors are: 1) the directly influencing factors (functional factors), 2) the factors influencing by dint of natural rhythms being general for a biocoenosis (synchronizing factors).

The borreliosis etiology was verified in 60% cases of the morphea, 100% cases of the atrophoderma of Pasini-Pierini and 100% cases of anetoderma by means of IFA with Ip-21 antigen (В. afzelii) and silver staining after Levadity. Borreliae were located near the capilars, muscular and areolar tissue fibers of the derm and near the granulated cells of epidermis.