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The causative agent of plague, Yersinia pestis, is unique within the family of intestinal pathogens Enterobacteriaceae because it is transmitted between hosts by flea bites. This method of transmission reflects the unique natural circumstances of Y. pestis speciation. It is shown that the conversion of the population (clonal complex) of Y. pseudotuberculosis O:1b into the population of the causative agent of plague was accomplished in the host-parasite system of Mongolian marmot — tarbagan (Marmota sibirica) — marmot flea Oropsylla silantiewi. The inducer of speciation was the last maximal (Sartan) cooling in Northern and Central Asia and deep soil freezing. Freezing caused behaviour modification in larval fleas — conversion during the winter months from saprophagy in nests of hosts to hematophagy in the mouth cavity of sleeping marmots. Scarification, caused by flea larvae in the oral cavity of sleeping marmots, became the «gateway» for mass infection of marmots by pseudotuberculosis infection through trauma. Such trauma resulted in intense bacteremia, which is the compulsory starting requirement for formation of vector-borne transmission by adult fleas.

In the present work, we conducted a comparative study on inbred C57BL/6 and outbred CD-1 mice after injection of O. felineus larvae. Body weight and weights of the liver and spleen as well as biochemical parameters of blood serum were evaluated at 4 and 6 weeks postinfection. An increased relative weight of the liver was observed after 4 weeks in outbred CD-1 mice and after 6 weeks in inbred C57BL/6 mice. In response to the infection, in CD-1 mice after 4 weeks, the relative weight of the spleen increased and returned to baseline 6 weeks postinfection. Alanine aminotransferase activity in C57BL/6 mice, compared to CD-1 mice, was substantially higher than the control value 6 weeks after the injection of O. felineus larvae. In CD-1 mice, the O. felineus infection contributed to a significant increase in the activity of aspartate aminotransferase and to a modest upregulation of y-glutamyl transpeptidase after 4 weeks. Both of these parameters normalized 6 weeks postinfection. Furthermore, inbred C57BL/6 mice had significantly more maritae of O. felineus in hepatic biliary ducts than outbred CD-1 mice did. Meanwhile, in CD-1 mice, the number of maritae decreased by more than twofold at 6 weeks postinfection as compared to 4 weeks postinfection. Our results revealed differences in susceptibility to O. felineus infection between the mouse strains (depending on the genotype) and indicate that inbred C57BL/6 mice are preferable as a model for studies on experimental O. felineus--induced opisthorchiasis.

The host range of the Bursaphelenchus mucronatus (subspecies kolymensis) to seven tree species (conifers and deciduous) was tested. 20 cuttings of every plant species (not ro-otstocks) were used for infected and control plants; 200 nematode individuals per plant as an inoculum; 45 days at 20—22 °C. Parameters estimated after 45 days were: the final population density, percentage of wilted leaves or needles, size and color of the necrotic spot in the point of inoculation. The population density exceeded inoculum in Pinus sylvestris that was the native host of the nematode isolate. In other plant species the nematode abundance decreased. The immune response was expressed as the necrotic reaction of hypersen-sitivity: in P. sylvestris it was no necrosis reaction, weak necrosis in Picea abies, in deciduous plant species the immune reaction had the maximum expression. It was concluded that the plant host range of B. mucronatus was conditioned not only by its vector preferences, but own nematode adaptations to the natural plant host to overcome the immune response of the plant.

The structure and composition of the helminth fauna of Sorex araneus (Soricomorpha: Soricidae) in the European part of the species range are characterized. Ninety nine helminth species have been recorded in total, including 21 trematodes, 39 cestodes, 34 nematodes, 5 acanthocephalans. The high taxonomic diversity of helminths is due to the host's wide range and habitat versatility. Representatives of the Palaearctic faunal complex prevail among the parasites (70%). A majority of Holarctic and cosmopolitan species were polyxenous parasites found in shrew at larval stages of their development. The helminth fauna mainly comprises species with a complex life cycle, where S. araneus serves as a definitive or intermediate host. Comparative analysis of the helminth fauna of Sorex araneus in the European part of the species range has demonstrated a high level of difference in local component communities of parasites, mainly owing to rare (or accidental) species. The widespread species generating similarity in the helminth fauna of different parts of the S. araneus European range are specific parasites of Soricidae: trematodes Brachylaima fulvum, Rubenstrema exasperatum, cestodes Monocercus arionis, Staphylocystis furcata, Neoskrjabinolepis schaldybini, Lineolepis scutigera, Ditestolepis diaphana, Vigisolepis spinulosa and nematodes Aonchotheca kutori, Eucoleus oesophagicola, Longistriata didas and L. codrus.

The dynamics of development of the population of taiga ticks Ixodes persulcatus Schulze, 1930 inhabiting Eastern Siberia is shown in the experimental conditions. Ticks were reared at 23—25 °С, relative humidity of 80—90 % and light/darkness ratio of 16/8 hours. The average time of egg incubation comprised 35.2 days, the feeding of females, larvae and nymphs took 6.4, 3.5 and 4.5 days respectively. It was shown experimentally that there are two subpopulations of taiga ticks — with long and short time of development of fed larvae into the nymphs and nymphs into the imago. The majority of ticks exhibited short period of development of larvae and nymphs that lasted for 25 ± 2 and 40 ± 4 days respectively. However, part of ticks exhibited prolonged development time of 75.5 ± 2 and 95 ± 9 days, respectively. The total duration of life cycle comprised 121 days in the short development time group and 226 days in the group with prolonged development time.

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