Two Decades (2003-2024) of Investigating Sarcocystis in Thrushes (Turdus spp.)

Sarcocystis spp. are apicomplexan parasites that form sarcocysts mainly in the muscles or central nervous system of intermediate hosts and sporocysts in the intestines of definitive hosts. Three species, Sarcocystis falcatula, Sarcocystis calchasi and Sarcocystis halieti, are potentially pathogenic to their intermediate hosts. Over the past two decades, we have examined 72 thrushes across four species (redwing (Turdus iliacus), common blackbird (Turdus merula), song thrush (Turdus philomelos), and fieldfare (Turdus pilaris)) for sarcocysts to better understand their role as intermediate hosts. Sarcocysts were detected in 28 individuals (38.9%). Most sarcocysts observed by light microscopy were of a single morphological type consistent with Sarcocystis turdusi. A molecular analysis of ITS1 sequences confirmed the presence of S. turdusi in the common blackbird, song thrush, and fieldfare, establishing the latter two bird species as new intermediate hosts. In contrast, cox1 was not a sufficiently variable locus for species differentiation. Additionally, a single sarcocyst with a smooth cyst wall, distinct from S. turdusi, was detected in a common blackbird and identified as S. halieti based on ITS1 sequence analysis. This atypical host record represents an isolated finding in a long-term dataset. Further sampling is required to confirm its epidemiological significance.