The current situation of opisthorchiasis in the Tomsk region

Trematodes of the family Opisthorchidae are the cause of liver and biliary tract diseases in humans and carnivores, and a risk factor for malignant tumors of the hepatobiliary system. The world's largest focus of opisthorchiasis is associated with the Ob-Irtysh basin. Almost all territories adjacent to this basin are unfavorable for opisthorchiasis, and Tomsk Oblast occupies one of the leading places in terms of infestation of the population with cat fluke. The aims of the study were to assess the current epizootic and epidemiological situation regarding opisthorchiasis and to identify the key links that determine the maintenance of the focus of opisthorchiasis in Tomsk Oblast. This article is dedicated to the memory of M. D. Ruzsky, one of the founders of parasitological research at Tomsk State University. Snails of the family Bithyniidae are the first intermediate hosts of Opisthorhis fe-lineus Rivolta, 1884. According to previous studies held in the 1970s, in the hyperendemic foci of the Middle Ob region, the extent of snail invasion did not exceed 3%. According to later data by S.A. Beer, the extent of invasion was also low (3.7%), reaching 37% only in some biotopes located near populated areas, while the intensity of invasion was very high (an average of 8130 cercariae per snail). As a result of the studies in the period from 2019 to 2022 in floodplain reservoirs of the Ob and Tom rivers, O. felineus parthenites were detected only in Bithynia troschelii Bielz, 1853. The number of snail varied widely from 3 to 52.5 specimens/m2. In May, infected snails were not recorded, they began to be observed only from mid-June. The extent of invasion (EI) of B. troscheli snails by O. felineus parthenites as a whole was 1.09% (varied from 0.38% to 14.3%.), significantly varied depending on the reservoir, season, and year of observation. Thus, the number of snails infected with O. felineus cercariae in Tomsk Oblast is at a low level. Of the 14 species of carp fish living in the Middle Ob basin, 11 species were examined for infection with metacercariae O. felineus (ide Leuciscus idus, dace Leuciscus baicalensis, roach Rutilus rutilus, bleak Alburnus alburnus, belica Leucaspius deline-atus, bream Abramis brama, golden crucian carp Carassius carassius, silver crucian carp Carassius gibelio, tench Tinca tinca, gudgeon Gobio sibiricus, and minnow Phoxinus ujmonensis). High EI rates were observed in ide (100%), dace (81.6%), tench (89.3%), and belica (41.5%), whereas EI of other fish varies from 2.1 to 17.1%. It has been shown that the infestation of fish increases with age, as metacercariae accumulate in the muscles of the fish, while no dependence of infestation on the sex of the fish has been identified. We recorded bleak infected with O. felineus metacercariae for the first time in the Middle Ob basin, in the period from 2016 to 2021, an increase in the extent of invasion was noted. A sharp increase in EI was detected in 2020, the figure was 52.6%, and in this year the maximum value of the intensity of invasion (II) was noted up to 13.1 spec./ind.; in the remaining years of observation, II averaged 1 specimen/individual. It may be noted that in the period 2020-2021 in tench and belica, the maximum infection was noted in 2020, so the EI of tench was 100% in 2020 and 40% in 2021, in belica it was 73.3% and 32.4%, respectively. Thus, the greatest epizootological burden of opisthorchiasis is carried by four species of fish: ide and dace, mainly in rivers; tench and belica in lakes and oxbow lakes. Wild and domestic carnivores, the same as humans, are the definitive hosts of the cat fluke O. felineus. Of the 27 species of mammals recorded in Tomsk Oblast, trema-todes O. felineus were previously recorded in sable Martes zibellina, muskrat Ondatra zibethicus, weasel Mustela sibirica, and fox Vulpes vulpes. We examined scatological material from 5 species of wild animals (brown bear Ursus arctos, muskrat Ondatra zibethicus, american mink Neovison vison, sable Martes zibellina, and fox Vulpes vul-pes) and from domestic dogs. No parasite eggs were found. Thus, at present, carnivores play a minor role in the circulation of opisthorchiasis in Tomsk Oblast. Another key link that plays an important role in maintaining the focus of opisthorchiasis is humans, as they represent the most infected component among the definitive hosts. On the territory of Tomsk Oblast from 2002 to 2022, 58 253 new cases of this disease were registered. The incidence of opisthorchiasis in 2002 exceeded the average in the Russian Federation by 22 times, in 2022 - by 7 times. Regional morbidity rates ranged from 677 per 100 thousand populations (2002) to 56.44 per 100 thousand populations (2012) with a downward trend. The average annual incidence rate of the entire population of Tomsk Oblast over the past 20 years was 278.59 cases per 100 thousand populations. At the same time, the incidence rate is unevenly distributed throughout the region. The maximum rates are recorded in the north of Tomsk Oblast: Alexandrovsky (420.51 per 100 thousand population), Verkhneketsky (502.76 per 100 thousand population), Kargasoksky (637.68 per 100 thousand population), Parabelsky (472.98 per 100 thousand population), Teguldetsky (902.91 per 100 thousand population), Chainsky (395.16 per 100 thousand population) districts and the city of Kedrovy (475.22 per 100 thousand population). Of the total number of cases of opisthorchiasis, 23.1% were children under 17 years of age. The highest incidence of opisthorchiasis in children under 17 years of age was noted in Verkhneketsky (996.42 per 100 thousand population), Teguldetsky (2667.38 per 100 thousand population), Parabelsky (1234.15 per 100 thousand population), Kargasoksky (1867.53 per 100 thousand population), and Aleksandrovsky (1428.8 per 100 thousand population) districts. Thus, the current epizootic situation regarding opisthorchiasis in Tomsk Oblast is extremely tense. Analysis of the relationships between the components of the life cycle of the trematode O. felineus revealed the key links in maintaining the focus of opisthorchiasis: snails and humans. The development of measures to interrupt connections in the life cycle of opisthorchiasis should be aimed at reducing the number of snails near populated areas, preventing the release of eggs from wastewater into water bodies and therapeutic and preventive work with the population. The article contains 1 Figures, 2 Tables, 28 References. The Authors declare no conflict of interest.

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