Ichthyocotylurus erraticus Rudolphi 1809 infecting the heart of the whitefish Coregonus lavaretus Linnaeus 1758 in Lake Sevan, Armenia

Metacercariae of Ichthyocotylurus erraticus Rudolphi 1809 have historically shown high prevalence and infection intensities in Sevan whitefish Coregonus lavaretus Linnaeus 1758. However, recent data on parasite infection levels in this introduced fish species in Lake Sevan are lacking, as are molecular identification of the parasite and its potential pathological effects on the host. To evaluate the status of the trematode, sampling was conducted at multiple locations across Lake Sevan, Armenia, in spring, summer and autumn of 2024, collecting a total of 300 whitefish. A total of 80.3% of whitefish were infected with metacercariae of I. erraticus , encysted on the host’s pericardium and the bulbus arteriosus. Тhe histopathological features are consistent with a mild chronic epi- and pericarditis and a strong fibrotic response to the presence of metacercariae, but no significant damage to the myocardial tissue. The phylogenetic analysis based on the mitochondrial DNA locus ( cox 1) inferred that the Lake Sevan specimens clustered with the I. erraticus reference sequence with high support. The prevalence of I. erraticus in whitefish showed an increasing trend from spring to summer and autumn, and mean abundance increased significantly with age. Both prevalence and mean abundance differed significantly between infected adults and juveniles of Sevan whitefish. This study reveals a high prevalence of I. erraticus infection in Lake Sevan whitefish, confirms its taxonomic identity using molecular tools and demonstrates its limited pathological impact on host tissues. Age and season were the primary determinants of infection abundance, while sex and gonadal development stages played minor roles.