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Fieldfare and Song Thrush Confirmed as New Intermediate Hosts of Sarcocystis turdusi

Fieldfare perched on a tree branch in Kraków, Poland
Research · Disease, parasites & microbiology

A study bringing together more than two decades of material has confirmed fieldfare (Turdus pilaris) and song thrush (Turdus philomelos) as new intermediate hosts of Sarcocystis turdusi. Researchers examined dead thrushes collected in Lithuania and at Rybachy in Russia’s Kaliningrad region between 2003 and 2024.

The dataset comprised 72 birds: 30 song thrushes, 24 fieldfares, 14 Eurasian blackbirds (Turdus merula) and four redwings (Turdus iliacus). Screening of leg muscle found sarcocysts — tissue cysts formed by parasites in the genus Sarcocystis — in 28 birds, or 38.9% of the sample.

Sarcocysts occurred in all four thrush species

Cysts were found in 14 of 30 song thrushes, seven of 24 fieldfares, six of 14 Eurasian blackbirds and one of four redwings. The analysis did not support clear differences among the four species in either infection prevalence or parasite load. The redwing result is especially uncertain because only four individuals of that species were available.

Most cysts had the same form under the microscope: ribbon-shaped structures with a wall bearing finger-like projections. This morphology matched S. turdusi, a species previously known to form muscle cysts in Eurasian blackbirds.

DNA confirmed two new intermediate hosts

The researchers isolated individual cysts and analysed DNA from the ITS1 region, a spacer between ribosomal genes that often varies enough to distinguish closely related parasite species. This confirmed S. turdusi in Eurasian blackbirds, song thrushes and fieldfares, establishing song thrush and fieldfare as new intermediate hosts. A morphologically similar cyst was also found in the single infected redwing, but DNA could not be recovered from that sample, so the parasite could not be confirmed to species level.

Sarcocystis parasites have an obligatory two-host predator–prey life cycle. Asexual stages and tissue cysts develop in an intermediate host, while sexual stages develop in the intestine of a definitive host after infected tissue is eaten. Earlier molecular studies have detected S. turdusi in raptors and corvids, although the authors note that detecting parasite DNA alone does not always establish definitive-host status.

The study also compared two genetic markers. ITS1 separated S. turdusi clearly from related species, whereas sequences from the mitochondrial cox1 gene were too similar among several avian Sarcocystis species to provide reliable species-level identification.

Sarcocystis halieti found once in a Eurasian blackbird

One Eurasian blackbird carried both S. turdusi and a second, smooth-walled cyst. ITS1 analysis identified the second parasite as Sarcocystis halieti, a potentially pathogenic species previously recorded from several other birds. This was the only S. halieti detection in the two-decade dataset, and the authors describe it as an atypical host record whose epidemiological significance remains uncertain.

The authors also caution that the material was opportunistically collected from birds found dead, sample sizes differed substantially among species, and only part of the material received molecular confirmation. The prevalence and host-association patterns therefore need to be interpreted within those limits.

Overall, the study expands the known intermediate-host range of S. turdusi and shows that ITS1 is more useful than cox1 for distinguishing these closely related parasites. Fieldfare and song thrush now join Eurasian blackbird as molecularly confirmed intermediate hosts in the material examined.

Sources and image credit

  1. Rudaitytė-Lukošienė, E., Kutkienė, L., Švažas, S., Butkauskas, D., & Prakas, P. (2026). Two Decades (2003–2024) of Investigating Sarcocystis in Thrushes (Turdus spp.). Pathogens, 15(7), 709.

Image: Jakub Hałun / Wikimedia Commons — fieldfare in Kraków, Poland; Wikimedia Commons Quality Image; representative species image, not a study bird · CC BY 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

Disease, parasites & microbiology

SPECIES IN THIS STORY

Species in this story

Fieldfare Turdus pilaris Explore species Song Thrush Turdus philomelos Explore species Eurasian Blackbird Turdus merula Explore species

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