The role of non-commercial cyprinids in maintenance and spread of the opisthorchiasis focus in the middle Ob River basin (Tomsk region, Russia).

The study assessed the role of non-commercial cyprinid species in maintaining the opisthorchiasis focus in the middle Ob River basin, Tomsk region, Russia. The source of O. felineus infection for humans and carnivores is fish of the family Cyprinidae. This is the most numerous family, 14 species live in the middle Ob River basin, which includes 6 commercial species and 8 non-commercial species. This study aimed to investigate the current situation on infestation of non-commercial cyprinids with O. felineus metacercariae and their role in maintaining and spreading the natural focus of opisthorchiasis in the middle Ob River basin. We investigated 4 non-commercial species (tench, sunbleak, common bleak, gudgeon), which are highly abundant in water bodies. Tench, common bleak and gudgeon are objects of amateur fishing. These species are traditionally included in the diet of the local population. Opisthorchis felineus metacercariae were recorded in muscles of all the examined fish species. The identification of metacercariae was confirmed by morphological methods and PCR diagnostics. Tench and sunbleak are the main sources of opisthorchiasis infection in the floodplain lakes of the Ob River basin (the prevalence of tench infection is 89.3% and mean intensity of infection is 11.2 metacercariae per fish, the prevalence of sunbleak infection is 50.9% and the intensity of infection is 4.25 metacercariae per fish). The prevalence of infection in the introduced common bleak from the rivers of the middle Ob River basin is rapidly increasing from 2.4 (2016-2018) to 37.5% (2020-2021), and mean intensity of infection increased from 1 to 4.15. The epizootic state of water bodies in the middle Ob River basin remains unfavorable in relation to opisthorchiasis. Tench, common bleak and sunbleak, along with ide and dace, are the main source of infection for humans and animals, which is evidenced by high infection with Opisthorchis felineus metacercariae in these numerous fish species. They pose the greatest danger of infection of people and animals with opisthorchiasis. These species should be included in the campaign to avoid raw and poorly cooked fish in the diet. In addition, such species as roach, bream and sunbleak also pose the danger of infection with opisthorchiasis, but to a lesser extent.

Abundance of Opisthorchis felineus Metacercariae in cyprinid fish in the middle Ob River basin (Tomsk region, Russia).

Infection with the liver fluke, Opisthorchis felineus, caused by the consumption of infected raw or undercooked cyprinid fish is common in humans and carnivores in the middle Ob River basin (Tomsk region, Russia) and can lead to diseases in humans. The goal of this study was the status of fish infection with O. felineus metacercariae in order to assess the role of fish in the infection of the human population in the middle Ob River basin. Of the 14 Cyprinidae species recorded in the middle Ob River basin, we examined 6 cyprinid species for prevalence and intensity of infection with O. felineus metacercariae. Five of these species (Leuciscus idus, Leuciscus leuciscus, Rutilus rutilus, Abramis brama, and Carassius gibelio) are of commercial value, while the common bleak is an object of amateur fishing. In addition, we conducted a survey of the fish consumption habits as part of a community-based cross-sectional study in the rural Shegarsky district, Tomsk region, Russian Federation. O. felineus metacercariae were observed in muscles of all examined species except for the Prussian carp. The ide is the main infection source in the Ob River (prevalence of infection, 100%, and intensity, 50.5 metacercariae per fish) and the common dace in the Tom River (91.1% and 12.7 metacercariae). Although the two alien species (bream and common bleak) are susceptible to infection with O. felineus metacercariae, the prevalence of infection in these fish and in the roach did not exceed 3%. The prevalence and intensity of infection in fish increased with age and size. The infection characteristics of fish in different water flows were different. The cyprinid species account on average for 69.8% of the commercial fish harvested in the Tomsk region. According to epidemiological survey, among 600 participants, 87.5% (n = 525) of respondents consumed river fish, with the most popular fish being cyprinids including Prussian carp, dace, ide and bream, followed by pike and perch. Thus, the epizootological state of the water flows in the middle Ob River basin is adverse with respect to opisthorchiasis, as indicated by high infection rates of liver fluke metacercariae in ide and common dace, which are abundant species of high commercial value. An extremely high prevalence of infection suggests a strong transmission cycle with a high rate of infection from infected humans and/or animal reservoir hosts to snails and fish. In addition to treating humans, there should be a strong focus on identifying these potential reservoir hosts to reduce subsequent infection in humans. Furthermore, since the intensity of infection in humans is determined by the presence of fish species such as ide and dace in the diet, they should be included in a dietary change campaign by eliminating the consumption of raw fish.