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High values of dominance index, low values of evenness and Shannon index are characteristic of component parasite communities of prespawning (Prosopium cylindraceum) and spawn migratory fishes (Coregonus autumnalis, Oncorhynchus nerka). Autogenic specialists are dominant in the component parasite communities of C. autumnalis. The Component parasite communities of P. cylindraceum and O. nerka are dominant in a content of generalist species. The O. nerka parasite communities are communities "sentenced to death". Low values of dominance index, high values of evenness abd Shannon index are peculiar to component parasite communities of prespawning and spawning cyprinid fishes (Phoxinus phoxinus, Oreoleuciscus humilis). Autogenic specialists are dominant. Increase of dominance index and decrease of two other indexes characterize the postspawning period. The same tendency characterises infracommunities. Thus, the component parasite communities respond differently to the fish spawn of the cyprinid and coregonid / salmonid fishes. Similarity is in the dominance of autogenic specialists in one case only (C. autumnalis). These differences are defined by the mode of fish stock formation for spawn. The cyprinids congregate for spawning and move apart for foraging and vice versa is observed in the coregonids and salmonids. Autogenic specialists dominance seems to be the important adaptation to reduce a negative effect to host during spawning. Bush and Kennedy in 1994 established that "parasites live in patches (host individuals) and fragments (host populations)". They consider the host fragmentation as "hedging your bets against extinction" due to frequency and magnitude anthropogenic factors, which increase fragmenting of host populations. This conclusion was made for a species level. From the other hand the fragmentation is a natural feature for a single host population too. Such fragmentation is a host population structure. The role of fragmentation is obvious from the data on component parasite communities during fish spawn. The fragmentation decreases in the cyprinids and increases in the salmonids that leads to the growing of the cyprinid component parasite communities diversity and to declining the salmonid component parasite communities diversity. Nevertheless the role of host population structure in a component parasite community structure is not obvious, because parasites are able or not able to "recognise" different host subpopulation groups. It is well known from data on parasite species population biology. Such recognizable subpopulation groups or groups can be a "real fragment" for the parasite community. The question is what parameters could be used for this purpose on a component community level. Host population age structure can be used as an example, because the age groups are one of the invariable population characters. Value of Shannon index for component parasite communities of spawn migratory O. nerka (5+) is similar to that of fishes of 1+ age. Difference is statistically insignificant. It is insignificant between the parasite communities of 2+ and 3+ age groups too. Fishes of these two groups could be defined as a real united fragment. It spite of similarity between the fishes of 5+ age group and fish of 1+ age group they are not united fragment. The parasite community of 1+ age fishes is not stabilized yet and one of 5+ age is acommunity "sentenced to death". Thus the structuring of O. nerka freshwater parasite communities are defined by 3 real host age fragments: 1+ age group, 2 and 3+ age group, 4+ age group. It looks as that Shannon index is suitable parameter to study a parasite communities structure.

Epidermiological material reflecting some spatio-temporal aspects of the natural foci of tick-born encephalitis resistance has been analysed. The investigation carried out can used as a base for elaboration and realisation of factor prognoses concerning natural foci of zoonosis infection.

The results of two methods of estimation of abundance of adult Ixodes persulcatus ticks, the absolute (at the sample plots) and the relative (per flag-hour) estimations, were compared. Collecting of ticks and estimation of their abundance was conducted during 9 years at the forests of the Far East and Pre-Ural region. The total of 1995 plots (100 sq. m each) were studied and 865 flag-hours were carried out. A good correlation of the data, obtained by these methods, was revealed. The average number of adult ticks, collected per flag-hour, approximately corresponds to the average number of ticks, activated during the season at 100 sq. m. The possibility of corresponding re-calculation of the results of estimation per flag-hour into the parameters of tick population density was evaluated. It was shown that such re-calculation gives good results. Their inexactitude usually doesn't exceed the statistical error of the parameters, obtained by the estimation of tick density at sample plots.

Based on epizootological and epidemiological investigations in the Vologda province since 1958 till 2000, the combined territorial and temporal manifestations of natural foci of diseases was revealed. A natural focus occupies a complex of biotopes, and in different time the focus is activated in different areas, such as a river plane, marsh, meadow-field, and forest. Most commonly an activation of the natural focis appears in a border zone of three biotopes, namely, a forest, meadow-field, and river plane. A dependence of activation of various natural foci upon local climate conditions and fluctuations of micromammalian host number are considered.

Maps and distribution data are provided for the seven mosquito species of the genus Anopheles, the maculipennis group: Anopheles atroparvus, A. beklemishevi, A. maculipennis, A. messeae, A. malanoon, A. sacharovi and A. subalpinus.

The estimation of parasitic pressure on the host populations is frequently required in parasitological investigations. The empirical values of prevalence of infection are used for this, however the latter one as an estimation of parasitic pressure on the host population is insufficient. For example, the same prevalence of infection can be insignificant for the population with high reproductive potential and excessive for the population with the low reproductive potential. Therefore the development of methods of an estimation of the parasitic pressure on the population, which take into account the features the host population, is necessary. Appropriate parameters are to be independent on view of the researcher, have a clear biological sense and be based on easily available characteristics. The methods of estimation of parasitic pressure on the host at the organism level are based on various individual viability parameters: longevity, resistance to difficult environment etc. The natural development of this approach for population level is the analysis of viability parameters of groups, namely, the changing of extinction probability of host population under the influence of parasites. Obviously, some critical values of prevalence of infection should exist; above theme the host population dies out. Therefore the heaviest prevalence of infection, at which the probability of host population size decreases during the some period is less than probability of that increases or preserves, can serve as an indicator of permissible parasitic pressure on the host population. For its designation the term "parasite capacity of the host population" is proposed. The real parasitic pressure on the host population should be estimated on the comparison with its parasite capacity. Parasite capacity of the host population is the heaviest possible prevalence of infection, at which, with the generation number T approaching infinity, there exists at least one initial population size ni(0) for which the probability of size decrease through T generations is less than the probability of its increase. P[nj(T)< ni(0) | ni(0)] <0.5 (i — from 1 to Nmax; Nmax — maximum population size). The estimation of the probabilities of host population size changes is necessary for the parasite capacity determination. The classical methods fot the estimation of extinction probability of population are unsuitable in this case, as these methods require the knowledge of population growth rates and their variances for all possible population sizes. Thus, the development methods of estimate of extinction probability of population, based on the using of available parameters (sex ratio, fecundity, mortality, prevalence of infection PI) is necessary. The population size change can be considered as the Markov process. The probabilities of all changes of population size for a generation in this case are described by a matrix of transition probabilities of Markov process (π) with dimensions Nmax x Nmax (maximum population size). The probabilities of all possible size changes for T generations can be calculated as πT. Analyzing the behaviour matrix of transition at various prevalence of infection, it is possible to determine the parasite capacity of the host population. In constructing of the matrix of transition probabilities, should to be taken into account the features the host population and the influence of parasities on its reproductive potential. The set of the possible population size at a generation corresponds to each initial population size. The transition probabilities for the possible population sizes at a generation can be approximated to the binomial distribution. The possible population sizes at a generation nj(t + 1) can be calculated as sums of the number of survived parents N1 and posterities N2; their probabilities — as P(N1) х Р(N2). The probabilities of equal sums N1 + N2 and nj(t + 1) ≥ Nmax are added. The number of survived parents N1 may range from 0 to (1 - PI) x ni(t). The survival probabilities can be estimated for each N1 as The number of survived posterities N2 may range from 0 to N2max (the maximum number of posterities). N2max is N2max = K • (1-PI) • Pfemales • ni(t) and the survival probabilities for each N2 is defined as where , ni(t) is the initial population size (including of males and infected specimens of host, PI is the prevalence of infection, Q1 is the survival probabilities of parents, Pfemales is the frequency of females in the host population, К is the number of posterities per a female, and Q2 is the survival probabilities of posterities. When constructing matrix of transition probabilities of Markov process (π), the procedure outlined above should be repeated for all possible initial population size. Matrix of transition probabilities for T generations is defined as πT. This matrix (πT) embodies all possible transition probabilities from the initial population sizes to the final population sizes and contains a wealth of information by itself. From the practical point of view, however, the plots of the probability of population size decrease are more suitable for analysis. They can be received by summing the probabilities within of lines of matrix from 0 to ni - 1 (ni — the population size, which corresponds to the line of the matrix). Offered parameter has the number of advantages. Firstly, it is independent on a view of researcher. Secondly, it has a clear biological sense — this is a limit of prevalence, which is safe for host population. Thirdly, only available parameters are used in the calculation of parasite capacity: population size, sex ratio, fecundity, mortality. Lastly, with the availability of modern computers calculations do not make large labour. Drawbacks of this parameter: 1. The assumption that prevalence of infection, mortality, fecundity and sex ratio are constant in time (the situations are possible when the variability of this parameters can not be neglected); 2. The term "maximum population size" has no clear biological sense; 3. Objective restrictions exist for applications of this mathematical approach for populations with size, which exceeds 1000 specimens (huge quantity of computing operations — order N ^mах3*(T- 1), work with very low probabilities). The further evolution of the proposed approach will allow to transfer from the probabilities of size changes of individual populations to the probabilities of size changes of population systems under the influence of parasites. This approach can be used at the epidemiology and in the conservation biology.

Morphology of schistosomatid cercariae of the genus Trichobilharzia (ocellata group) is described based on original and reference data. The material (Trichobilharzia cercariae) was collected in 1999—2000 from naturally infected snails Lymnaea stagnalis from Moscow and Saint-Petersburg megalopolises. The more accurate flame cell formula for these cercariae, as well as some aspects of chaetotaxy and eye morphology are given. The differences between cercariae from these regions were found. Based on a comparison of nef data with the reference data, it was found that the cercariae examined differ from those from the Central Europe.

Results of comparative morphological and gene analyses of the cestode Proteocephalus thymalli, an intestine parasite of the lakes Hovsogol (Thymallus arcticus nigrescens) and Lake Baikal graylings (Thymallus arcticus baicalensis) are presented for the first time. The results indicate that representatives of the P. thymalli from these fishes are really two different species.

The components of the female reproductive system of Diphyllobothrium latum, including ovary, ovicapt, oviduct, vitelline ducts, vitelline reservoir, vagina, seminal receptacle, ootype with unicellular Mehlis's gland, ootype-uterine duct and uterus were observed with the electron microscope. The epithelium of the female reproductive system ducts consists of a nucleate syncytial layer. Structural differences in apical surface of the ducts, the number of nuclei and organoids in syncytial layer as well as the number of underlaid muscles were revealed. The regional differentiations of the uterus wall were registered. The middle and distal region of uterus was covered with microtriches. The filamentous microtriches were observed in apical surface of vagina. The epithelium of seminal receptacle and distal region of uterus were underlaided by the powerful muscle layers. The fertilization canal was revealed. It was shown that the formation of egg shells implemented by the deposit of vitelline globules in their surface in the ootype-uterine duct. Structural and functional differences of different parts of female apparatus in various groups of cestodes are conditioned by species biology.