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Opisthorchis felineus or Siberian liver fluke is a trematode parasite (Opisthorchiidae) that infects the hepato-biliary system of humans and other mammals. Despite its public health significance, this wide-spread Eurasian species is one of the most poorly studied human liver flukes and nothing is known about its population genetic structure and demographic history. In this paper, we attempt to fill this gap for the first time and to explore the genetic diversity in O. felineus populations from Eastern Europe (Ukraine, European part of Russia), Northern Asia (Siberia) and Central Asia (Northern Kazakhstan). Analysis of marker DNA fragments from O. felineus mitochondrial cytochrome c oxidase subunit 1 and 3 (cox1, cox3) and nuclear rDNA internal transcribed spacer 1 (ITS1) sequences revealed that genetic diversity is very low across the large geographic range of this species. Microevolutionary processes in populations of trematodes may well be influenced by their peculiar biology. Nevertheless, we suggest that lack of population genetics structure observed in O. felineus can be primarily explained by the Pleistocene glacial events and subsequent sudden population growth from a very limited group of founders. Rapid range expansion of O. felineus through Asian and European territories after severe bottleneck points to a high dispersal potential of this trematode species., Competing Interests: The authors have declared that no competing interests exist.

The paper gives data on an opisthorchiasis focus in the Taishetsky district, Irkutsk Region. The species affiliation of Bithynia from the Biryusa River basin was established. These mollusks are morphologically close to Opisthorchophorus hispanica (= Bithynia inflata). No O. felineus trematodes were found in the snails examined. Roach, dace, and bream were the second intermediate hosts. Analysis of long-term data revealed that fish and human infection rates have remained comparatively high for almost 20 years. The peak for Opisthorchis infestation in carps were in 1982 (9.3%), 1983 (2.9%), 1998 (5.5%), 2000 (2.5%), and 2007 (5.9%). In other years, fish infection ranged from 0.4 to 1.6%. The performed investigations showed that dace and bream were most infected. Data on the focus area that was 1500 km2 are first presented. The Irkutsk focus of opisthorchiasis is considered to be little studied since there is no evidence (infection rate, prevalence) for mollusks, wild (muskrat, otter, bank vole, fox, wolf) and domestic (dog, pig) animals. The considerable pathogenicity of the parasite and the natural focal pattern of the disease call for regular investigations at all stages of its life cycle: mollusks, fishes, fish-eating mammals, local residents, as well as for comprehensive analysis and elaboration of preventive and controlling measures.

Experimental data on the development of the Tetraonchus borealis larvae and species of the genus Salmonchus are given for the first time. The experiments was carried out in field conditions, in the estuary of the Shegnanda river (Baikal Lake basin), during 25 days. It was established that temperatures from 15 to 22 degrees C are favorable for the culture of the Salmonchus eggs followed by the emergence of larvae at eighth or ninth day. At the same time, T. borealis larvae need in lower temperatures, from 15 to 17 degrees C for their development. Detailed descriptions of eggs and larvae of the Tetraonchidae species under are given.

Hitherto data on the morphology of Tetraonchidae and their eggs obtained with scanning electron microscopy (SEM) were absent in the literature. In the present paper results of SEM study of two genera of Tetraonchidae, Tetraonchus and Salmonchus, are given. Fine morphological traits of eggs, larvae, and definitive individuals of Tetraonchidae, as well as patterns of their attachment to host gill, were established for the first time. A deep penetration of the worms into the branchial epithelium of the host was shown.

The fauna of fish parasites in Lake Baikal is represented by 5 faunistic complexes, namely the boreal plain, boreal submountain, arctic freshwater, Baikal, and Sino-Indian ones. The parasites of the boreal plain complex are dominant by the number of species (43 %). Hypotheses on the origin of the recent fish and parasite faunas of Lake Baikal were advanced on the base of the data on the parasite species composition and their distribution among hosts, as well as on the base of paleontological data. It is shown that invasion of new fish species and their parasites to Baikal led to the change of the composition of natural faunistic fish complexes and parasite systems. Invading fishes play the roles of intermediate and definitive hosts in parasite systems of Baikal, that led to the change of the initial structure of these systems.

Analysis of infracommunities and component communities of fish parasites in Lake Baikal has been conducted for the first time. It has been revealed that parasite infracommunities for the majority of Baikal fishes are weakly balanced and impoverished (the Berger-Parker Index is > 0.5; Evension is < 0.5; the Brillouin Index is < 1). The highest diversity and balance of the communities are characteristic for carnivorous fishes (Brachymystax lenok, Hucho taimen, Thymallus arcticus, Esox lucius, and Percafluviatilis). The component parasitic communities of Leuciscus leuciscus baicalensis, Rutilus rutilus, and Leocottus kesslerii are the most diverse in Lake Baikal since the Shennon index for L. leuciscus baicalensis, R. rutilus, and L. kesslerii is 2.4, for Paracotlus knerii--2.2, Limnocoitus godlewskii--2.3, Phoxinus phoxinus--2.1, Lota lota and Limnocuttus pallidus--1.9, P. fluviatilis--1.8, Leuciscus idus--1.8. The component parasitic communities of other fishes in Lake Baikal have low indices of biological diversity (H = 0.5-1.05, Smp is close to 1). A classification of mature and immature components of parasitic communities based on the ratio of specialist species and generalist species has been proposed. It is established that the component parasitic communities in sublitoral, profundal, and pseudoabyssal zones are mature, while in the littoral zone they are immature (impoverished and weakly balanced). The component parasitic communities in benthophagous fishes and predators are mature, in planktivorous fishes they are immature. The component parasitic communities are mature in the family Cyprinidae and immature in the families Coregonidae and Cottidae. The component parasitic communities of the Boreal Plain and Boreal Submountain faunal complexes are mature, but they are immature in Lake Baikal and Arctic freshwater complexes.

The parasite fauna of two endemic fish species of the genus Comephorus (Cottoidei: Comephoridae) includes 11 species. Six species were found in Comephorus baicalensis and 9--in C. dybowskii. Two species only are endemic for Baikal: Myxidium perniciosum and Gyrodactylus comephori. Other 9 species are distributed throughout Palaearctic region. Ecology of parasites and their hosts is briefly discussed.

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.

Data on a hook morphology (in plerocercoid and mature helminths) and gene systematics of Triaenophorus nodulosus from the Baikal Lake are presented for the first time. Sequence analysis of rDNA fragments (5'-end portion of 18S rDNA) yelded 562 nucleotid positions.

A new cestode species, Proteocephalus pronini sp. n., is described from the Lake Hovsgol Grayling Thymallus arcticus nigrescens (Mongolia, Hovsgol Lake). The new species differs from P. thymalli from the Baikal Lake by the length of strobile, number of proglottids, form of mature proglottids, and number of diverticles of uterus.

Morphological data on Schistocephalus nemachili plerocercoid (a parasite of Nemachilis babatulus) and a nucleotide sequence of its 18S rRNA are presented. Samples were collected from the groundling inhabiting the Isumrudnoe lake (karst lake, belonging to the Lena River basin, 1200 m altitude). Length of plerocercoids 37-70 mm (mean 52.9 mm), width 1.6-7.3 (3.8), number of segments in srobile--229-348 (290). The sequence data on the 5'-terminal major domain of small rRNA of S. nemachili were obtained by mean of polymerase chain reaction and direct sequencing, they displayed 574 base positions. The nucleotide sequence was compared with known Plathelminthes sequences from the EMBL data base to compute data matrix under Juke and Cantor's model (1969). Distance matrix was used for obtaining a similarity dendrogram by Saitou and Nei's method (1987). Comparative analysis showed S. nemachili nucleotide sequence clustering with the cestode sequence (E. granulosus) within the dendrogram containing four trematodes, one cestode and one turbellarian sequences.

The life cycle of Proteocephalus thymalli (Cestoda, Proteocephalidae), a parasite of Siberian glame (Thymallus arcticus), was repeated under experimental conditions. The first intermediate hosts, the copepods Epischura baicalensis, Cyclops kolensis and C. vicinus, were determined. The developmental time of P. thymalli in the first intermediate host was determined and the morphology of the larval and adult phases was described.