Black-tailed Gull (Larus crassirostris): A Novel Host for Trichomonas gypaetinii with High Prevalence of Avian Trichomonads in South Korea.

This study detected Trichomonas gypaetinii in Black-tailed Gulls (Larus crassirostris) in South Korea. We collected 83 samples (80 oropharyngeal swabs from live birds and three carcass-derived specimens) from breeding colonies on Nando and Hongdo islands and coastal habitats in Taean, South Korea, in 2023-24. Molecular analysis using nested PCR targeting the ITS region revealed an exceptionally high prevalence (98.8%) of Trichomonas spp., with both Trichomonas gallinae and Trichomonas gypaetinii identified. We observed significant seasonal variation in Trichomonas spp. distribution, with T. gallinae predominating in winter (85%), while T. gypaetinii became more prevalent during breeding and migration periods (67%). Co-infections were documented at both breeding colonies, suggesting potential interspecies interactions. Sex-based differences in infection patterns were statistically significant (P<0.05); T. gypaetinii showed higher prevalence in adult males at Nando Island (88%) and in adult females at Hongdo Island (88%). Despite the high infection rate, no distinctive lesions were observed in examined carcasses, raising questions about pathogenicity and host adaptation. These findings expand the known host range of this protozoan parasite beyond raptors. This, together with previous detection of T. gallinae in other seabird species (Streaked Shearwater [Calonectris leucomelas] and Swinhoe's Petrel [Hydrobates monorhis]), highlights the need for expanded surveillance of Trichomonas spp. in nonraptor species and further investigation into transmission dynamics, pathogenicity, and potential impacts on reproductive success and population health in colonial nesting birds.