Currently, the Irkutsk focus of opisthorchiasis is the easternmost known in Russia. It is located in the Taishetsky district of the Irkutsk region, most of which belongs to the Biryusa River basin. The area of the Irkutsk focus of opisthorchiasis is significantly smaller than the western outbreaks. The climate of the Taishet region is close in its characteristics to the climate of the KhantyMansi Autonomous Okrug, on the territory of which a significant part of the Ob-Irtysh focus of opisthorchiasis is located. In the middle and lower reaches, Biryusa is flat in nature and forms a network of floodplain reservoirs. The water temperature in the summer months exceeds 15°C, which makes the reservoirs suitable for habitation of the mollusks Opisthorchophorus (Bithynia) troschelii (Paasch, 1842) - the first intermediate hosts of Opisthorchis felineus (Rivolta, 1884). The period of activity of bithinids in the water bodies of the Biryusa River basin lasts about 2 months. The second intermediate hosts of opisthorchis in the Taishet region are cyprinids (order Cypriniformes): roach Rutilus rutilus, bream Abramis brama), Siberian dace Leuciscus leuciscus baicalensis). In addition to the listed species, golden crucian carp Carassius carassius and tench Tinca tinca inhabit the reservoirs of the Taishet region, but currently we have no information about the infection of these fish species with opisthorchiasis. The level of fish infestation with O. felineus metacercariae, according to our own and literature data, remains relatively low throughout the entire study period from 1982 to 2023. The maximum peak of invasion (20%) was recorded in 2023. The definitive hosts of opisthorchis in the Taishet region are people, domestic and wild animals. The water bodies of the Taishet region are inhabited by muskrat (Ondatra zibethica), whose infestation with O. felineus was 16.7% in 2022, and 2.9% in 2023. Irkutsk focus of opisthorchiasis is similar to western eurasian foci in a number of factors: the climate of the region; flat terrain; a developed network of floodplain reservoirs in the middle and lower reaches of the river; high trophicity of most reservoirs where intermediate and definitive hosts of opisthorchis live; the presence of shallow coastal parts of reservoirs with a large amount of fine detritus. Distinctive features of the Irkutsk outbreak are: the small area of the Taishet region; limited number of reservoirs suitable for habitation of the first intermediate hosts of opisthorchis; relatively poor species composition of second intermediate hosts.

Veprikov Sergey Pavlovich, Junior Research Scientist, Baikal Museum SB RAS, 1, Akademicheskaya st., Listvyanka Settl., Irkutsk Region, 664520, Russian Federation, e-mail: pochtovik108@yandex.ru

Russinek Ol’ga Timofeevna, Doctor of Sciences (Biology), Principal Research Scientist, Baikal Museum SB RAS, 1, Akademicheskaya st., Listvyanka Settl., Irkutsk region, 664520, Russian Federation, e-mail: rusinek@isc.irk.ru

Veprikov S. P., Rusinek O. T. Focus of Opisthorchiasis in Irkutsk Region, East Siberia: Habitat Features of Intermediate and Definitive Hosts of Opisthorchis felineus. The Bulletin of Irkutsk State University. Series Biology. Ecology, 2024, vol. 49, pp. 33-51. https://doi.org/10.26516/2073-3372.2024.49.33 (in Russian)

Andreeva S.I. Mollyuski semeistva Bithyniidae (Mollusca, Gastropoda) Tyumenskoi oblasti [Molluscs of the family Bithyniidae (Mollusca, Gastropoda) of Tyumen Region]. Ruthenica, 2023, vol. 33, no. 2, pp. 47-57. https://doi.org/10.35885/ruthenica.2023.33(2).1 (in Russian)

Beer S.A. Biologicheskie aspekty problemy opistorkhoza [Biological aspects of the problem of opisthorchiasis]. Parazitologiya, 1977, vol. 11, is. 4, pp. 289-299. (in Russian)

Beer S.A. Biologiya vozbuditelya opistorkhoza [Biology of the causative agent of opisthorchiasis]. Moscow, KMK Publ., 2005, 336 p. (in Russian)

Boyarkin V.M. Geografiya Irkutskoi oblasti [Geography of Irkutsk Region]. Irkutsk, Irkutsk St. Univ. Publ., 1972, 293 p. (in Russian)

Bykhovskaya-Pavlovskaya I.E. Parazity ryb. Rukovodstvo po izucheniyu [Fish parasites. Study Guide]. Nauka Publ., 1985, 121 p. (in Russian)

Veprikov S.P., Rusinek O.T. Mnogoletnyaya dinamika zarazhennosti karpovykh ryb lichinkami Opisthorchis felineus (Trematoda) v Irkutskom ochage opistorkhoza v basseine reki Biryusy [Long-term dynamics of infestation of carp fish with Opisthorchis felineus (Trematoda) larvae in the Irkutsk focus of opisthorchiasis in the Biryusa River basin]. Sovremennye problemy okhotovedeniya [Modern Problems of Hunting: Int. Sci. Conf., Irkutsk, Russia]. Irkutsk, Irkutsk St. Agr. Univ. Publ, 2021, pp. 317-321. (in Russian)

Generalnyi plan g. Taisheta. T. 1 [General Plan of Taishet City. Vol. 1]. Irkutsk, Irkutskgrazhdanproekt Publ., 2011. (in Russian)

Gomelskaya oblast [Gomel’ Region] / Karopa G.N. (Ed.). Gomel, Gomel St. Univ. Publ., 2011, 165 p. (in Russian)

Gosudarstvennyi vodnyi kadastr SSSR. T. 16. Angaro-Eniseiskii raion. Vyp. 2. Angara [State water cadastre of the USSR. T. 16. Angara River. Vol. 2]. St.-Petersb., Gidrometeoizdat Publ., 1978. (in Russian)

Karmaliev R.S., Kereev Ya.M. Opistorkhoz ryb v Zapadno-Kazakhstanskoi oblasti [Opisthorchiasis of fish in the West Kazakhstan region]. Russ J. Parasitol., 2013, no. 3, pp. 11-15. (in Russian)

Kartashev N.N., Sokolov V.E., Shilov I.A. Praktikum po zoologii pozvonochnykh [Workshop on vertebrate zoology]. Moscow, Aspect Press Publ., 2004, 383 p. (in Russian)

Klebanovsky V.A., Zhurina T.A., Zhitnitskaya E.A., Sekulovich A.F., Usoltseva Z.N., Afrakov V.F., Afrakova T.V., Kolokol'tsev M.M., Starikova N.A. Novye dannye ob areale opistorkhoza v Tsentralnoi Sibiri [New data on the range of opisthorchiasis in Central Siberia]. Medical Parasitology and Parasitic Diseases, 1984, no. 3, pp. 7-11. (in Russian)

Kolokoltsev M.M., Kazakova A.A., Zhitnitskaya E.A. Opistorkhoz v Taishetskom raione Irkutskoi oblasti [Opisthorchiasis in Taishet district of Irkutsk region]. Gigiena i zdorovye cheloveka [Hygiene and human health]. Irkutsk, Irkutsk St. Med. Inst. Publ., 1982, pp. 48-49.

Kolokoltsev M.M. Rasprostranenie i ekologiya mollyuskov Bithynia inflata promezhutochnogo khozyaina Opisthorchis felineus v vodoemakh basseina reki Biryusy [Distribution and ecology of the mollusk Bithynia inflata of the intermediate host Opisthorchis felineus in water bodies of the Biryusa River basin1988, no. 3, pp. 58-60. (in Russian)

Kudryavtseva T. M. Rasprostranenie metatserkarii sem. Opisthorchiidae v rybakh vodoemov severo-zapada Rossii (epizootologiya, diagnostika) [Distribution of metacercariae of the family. opisthorchiidae in fishes of water bodies of the north-west of Russia (epizootology, diagnostics): Candidate in Veterinary dissertation]. St.-Petersb., 2020, 143 p. (in Russian)

Kurachenko I.V. Otsenka zarazhennosti lichinkami trematod mollyuskov i ryb vodoemov yugo-vostoka Belarusi [Assessment of infestation of mollusks and fish with trematode larvae in water bodies of the south-east of Belarus]. Nauka i mir [Science and World], no. 9 (37), vol. 1, pp. 67-68. (in Russian)

Lavrov N.P. Rukovodstvo po rasseleniyu pushnykh zverei [Guide to the resettlement of furbearing animals]. Moscow, Tsentrosoyuz Publ., 1958, 91 p. (in Russian)

Maslennikov P.V. Presnovodnye mollyuski vodoemov basseina reki Chulym (srednyaya Ob) [Freshwater mollusks of reservoirs of the Chulym River basin (middle Ob): Candidate in Biology dissertation]. Tomsk, 2015, 167 p. (in Russian)

Mayurova A.S. Geoekologicheskaya otsenka prirodnogo ochaga opistorkhoza na territorii Khanty-Mansiiskogo avtonomnogo okruga – Yugry [Geoecological assessment of the natural focus of opisthorchiasis on the territory of the Khanty-Mansiysk Autonomous Okrug – Ugra]. Cand. sci. diss. St.-Petersburg, 2021, 159 p. (in Russian)

Myasoedov V.S. Epidemiologiya opistorkhoza [Epidemiology of opisthorchiasis]. Tomsk, Tomsk St. Univ. Publ., 1960, 99 p. (in Russian)

Opredelitel zooplanktona i zoobentosa presnykh vod Evropeiskoi Rossii [Key to zooplankton and zoobenthos of fresh waters of European Russia. Vol. 2. Zoobenthos]. Alekseev V.R., Tsalolikhin S.Ya. (Eds.). Moscow, St.-Petersburg, KMK Publ., 2016, 457 p. (in Russian)

Romashov B.V., Romashov V.A., Semenov V.A., Filimonova L.V. Opistorkhoz v basseine Verkhnego Dona (Voronezhskaya oblast): fauna opistorkhid, ekologo-biologicheskie zakonomernosti tsirkulyatsii i ochagovost opistorkhidozov [Opisthorchiasis in the Upper Don basin (Voronezh region): opisthorchid fauna, ecological and biological patterns of circulation and focality of opisthorchidosis]. Voronezh, Voronezh St. Univ. Publ., 2005. 201 p. (in Russian)

Pavlovskii E. N. Prirodnaya ochagovost transmissivnykh boleznei v svyazi s landshaftnoi epidemiologiei zooantropozoonozov [Natural focality of vector-borne diseases in connection with the landscape epidemiology of zooanthropozoonoses]. Moscow, St.-Petersburg, Nauka Publ., 1964, 211 p. (in Russian)

Gerasimov I.P. (ed.) Predbaikal'e i Zabaikal'e [Cisbaikalia and Transbaikalia]. Moscow, Nauka Publ., 1965, 492 p. (in Russian)

Kereev Ya.M., Shalmenov M.Sh., Nurzhanova F.Kh., Lukmanova Zh.G., Sariev B.T., Sidikhov B.M. Raspredelenie lichinok Opisthorchis felineus u ryb v vodoemakh ZapadnoKazakhstanskoi oblasti [Distribution of Opisthorchis felineus larvae in fish in water bodies of the West Kazakhstan region]. Russ. J. Parasitol., 2011, no. 4, pp. 70-73. (in Russian)

Rusinek O.T. Irkutskii ochag opistorkhoza: itogi i voprosy dalneishego izucheniya [Irkutsk focus of opisthorchiasis: results and issues for further study]. Perspektivy razvitiya biomeditsinskikh tekhnologii v Baikalskom regione [Prospects for the development of biomedical technologies in the Baikal region: Int. Sci. Conf. Irkutsk, Russia]. Irkutsk, Inst. Geogr. SB RAS Publ., 2019, pp. 84-85. (in Russian)

Rusinek O.T., Veprikov S.P. Irkutskii ochag opistorkhoza (k 40-letiyu otkrytiya) [Irkutsk focus of opisthorchiasis (to the 40th anniversary of its discovery)]. Russ. J. Parasitol., 2023, vol. 17, no. 4, pp. 488-500. (in Russian)

Rusinek O.T., Kondratistov Yu.L. Izuchenie zarazhennosti karpovykh ryb metatserkariyami trematod v ochage opitorkhoza (Taishetskii raion, Irkutskaya oblast, Rossiya) [Study of infestation of cyprinids with trematode metacercariae in the focus of opitorchiasis (Taishetsky district, Irkutsk region, Russia)]. The Bull. Irkutsk St. Univ. Ser. Earth Sci., 2010, vol. 3, no. 1, pp. 132-142. (in Russian)

Serbina E.A. Pervoe obnaruzhenie Opisthorchis felineus i Metorchis bilis v pervykh promezhutochnykh khozyaevakh bitiniidakh iz basseina ozera Chany (Novosibirskaya oblast') [The first discovery of Opisthorchis felineus and Metorchis bilis in the first intermediate hosts, bithinids from the Lake Chany basin (Novosibirsk region)]. Russ. J. Parasitol., vol. 37, is. 3, pp. 421-429. (in Russian)

Chereneva V.A. Strukturno-gidrogeologicheskie usloviya formirovaniya resursov i zapasov Taishetskogo mestorozhdeniya pit'evykh podzemnykh vod [Structural hydrogeological conditions for the formation of resources and reserves of the Taishet deposit of drinking groundwater]. Izvestija Sibirskogo otdelenija Sekcii nauk o Zemle RAEN [Proc. Siberian branch of the section of Earth Sciences RAS]. 2014, no. 1 (44). pp. 44-49. (in Russian)

Chugunova N.I. Rukovodstvo po izucheniyu vozrasta i rosta ryb [Guide to the study of age and growth of fish]. Moscow, AS USSR Publ., 1959, 164 p. (in Russian)

Lepekhin A.V., Mefodiev V.V., Filatov V.G., Buzhak N.S. Epidemiologiya, klinika i profilaktika opistorkhoza [Epidemiology, clinical picture and prevention of opisthorchiasis]. Tomsk, Tomsk St. Univ. Publ., 1992, 232 p. (in Russian)

Falniowski A., Glöer P., Szarowska M. Bithynia troschelii (Paasch, 1842), a giant of unknown origin? Folia Malacologica, 2004, vol. 12, no. 3, pp. 137-139. https://doi.org/10.12657/folmal.012.009

Fromming E. Biologie der mitteleuropaischen Susswasserschnecken. Berlin, Duncker and Humbolt Verl., 1956, 313 p.

De Liberato C., Scaramozzino P., Brozzi A. , Lorenzetti R., Di Cave D., Martini E., Lucangeli C., Pozio E., Berrilli F., Bossù T. Investigation on Opisthorchis felineus occurrence and life cycle in Italy. Vet. Parasitol., 2010, vol. 177, pp. 67-71. https://doi.org/10.1016/j.vetpar.2010.11.042

Serbina E.A. Bithyniid Abundance in the South of Western Siberia Water-Courses and Water Reservoirs (Russia). Diversity, 2022, vol. 14, no. 10, p. 791. https://doi.org/10.3390/d14100791