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Opisthorchis felineus (Trematoda) is widespread in the Russian Federation, especially in Siberia, and other countries of Europe. Infestation of endemic area population with O. felineus reaches 80 %. On animal models of the infection of closely related Opisthorchis viverrini combined with the nitrosamines' intake it has been shown that the parasite induces cholangiocarcinoma. However carcinogenic potential of O. felineus is still poorly studied. The present study is aimed to investigate the role of O. felineus in cholangiocarcinoma carcinogenesis in hamster treated additionally by dimethylnitrosamine (DMN). Golden hamsters were divided into 4 groups (15 specimens in the control group and 20 for other groups): (I) untreated control, (II) 12.5 ppm DMN solution intake, (III) infected with 50 metacercariae of O. felineus and (IV) infected with 50 metacercariae of O. felineus and 12.5 ppm DMN solution intake. According to the histological data, in the O. felineus-infested group significant hyperplastic and dysplastic biliary changes were found considered as a precancerogenic state. Such pathological changes of bile ducts were more severe in group treated with both factors, with cholangiocarcinoma being found out at 18th week in all the animals of this group. These results demonstrate that O. felineus could play promoting role in two-step model in cholangiocarcinoma carcinogenesis and may be used to define the O. felineus group in the International Agency for Research on Cancer classification of agents, mixtures and exposures (IARC categories).

Deepwater fishes from the Amundsen Sea, d'Urville Sea, Ross Sea, Cooperation Sea, Weddell Sea, and open waters of the South-West part of the Pacific sector of the Antarctic were examined. Fourteen species of trematodes were found. Muraenolepitrema magnatestis Gaevskaya et Rodjuk, 1988 and Helicometrina sp. were found in the Antarctic for the first time. Muraenolepitrema magnatestis was redescribed and basing on this, diagnosis of the genus Muraenolepitrema was amended. Muraenolepitrema magnatestis possesses the uroproct and glandular cells that are loosely arranged around the external seminal vesicle (without a membranous sac).

Based on the analysis of own and literature data, it is concluded that the following main permanent system of ecological-parasitological factors prevents the effective vector functions of the tick Ixodes persulcatus in transmission of Babesia microti: lack of distinct nymphs' anthropophily; small spontaneous invasion of hungry adults; a duration of the parasitic phase in humans is insufficient to complete the sporogonic development, because victims interrupt the phase. Therefore, not excluding the possibility of sporadic babesiosis diseases, it can be stated that within the boundaries of a vast territory, where the taiga tick is the only potential source of infection for humans, the B. microti infection has not, and will not reach significant values in infectious pathology.

The cuticular structure of the horsehair worm Gordionus alpestris (Villot, 1885) was studied under scanning and transmission electron microscopes. Adult worms were collected in the Syuk River near Nickel' Village in the Republic of Adygea (Russia) in June 2013. In the sampling area, the G. alpestris juveniles parasitize diplopods Pachyiulus krivolutskyi Golovatch, 1977. Similarities with other Nematomorpha species with the known cuticle ultrastructure are discussed.

Experimental infestation of the marsh frog tadpoles with the Cosmocerca ornata (Dujardin, 1845) larvae was conducted. In vitro development of the first, second and third larval stages of the helminth were studied. Tadpoles of different age were infested both i) by feeding larvae to them and ii) by placing them in containers simultaneously with the C. ornata larvae. In both cases dissection revealed the presence of living larvae of the nematode in the tadpoles' intestines. Despite their motility and activity, the I and II larval stages were not invasive; they never persisted inside the tadpoles' digestive tract. In 4—5 days after the second molt, a "temporary persistence" was observed. Living larvae were in suppressed condition, motility lowered, further development never occurred. In 3 days occurrence of the III stage nematode larvae lowered drastically. Gradual elimination of the most part of C. ornata larvae was observed. Elimination rate depended on the tadpole developmental stage. Later tadpole stages (those that had already developed arms and legs) were the first to get rid of the nematode larvae — on 7-th day after the infestation. On the 9-th day tadpoles of the early stage (those without limbs) got rid of the larvae. Larvae persisted for the longest time in the intestines of middle-staged tadpoles (with the hind legs) — up to 13 days.

The present study was aimed to examine the specific features of the helminth fauna in insular populations of the bank vole (Myodes glareolus) in the north of the species range. The material was collected in and nearby the Kizhi Archipelago (Lake Onega, 62°1' N 35°12' E) during August 1997, 2005—2007, 2012 and 2013. Small mammals were trapped on 23 islands (varying from 2 to 15,000 ha) and on the mainland. Helminthological methods were applied to examine 301 specimens of M. glareolus. Fourteen helminth species were found: trematodes — Skrjabinoplagiorchis vigisi; cestodes — Paranoplocephala omphalodes, P. gracilis, Catenotaenia henttoneni, Taenia mustelae, Cladotaenia globifera, Spirometra erinacei; nematodes — Trichocephalus muris, Aonchotheca murissylvatici, Hepaticola hepatica, Heligmosomum mixtum, Heligmosomoides glareoli, Longistriata minuta, Syphacia petrusewiczi. The parasites S. vigisi, S. erinaci, H. hepatica and T. muris were identified in the bank vole in Karelia for the first time. Significant differences were detected between the helminth faunas of local insular populations of the bank vole. A distinctive feature of all small islands was that samples from them lacked the widespread parasitic nematode Heligmosomum mixtum. The studies have confirmed the general trends observed in the parasite fauna of most isolated populations of small mammals: a poorer species diversity and high infestation rates with certain species of parasites. The Kizhi Archipelago is characterized by the specific high abundance of regionally rare parasite species (H. hepatica, A. murissylvatici), and by the absence of common parasites (H. mixtum, H. glareoli).

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