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Trematode Was Found in 65% of Grass Snakes From Northern Ukraine

Grass snake moving through a wetland in Ukraine with its head unobstructed
Research · Disease, parasites & microbiology

The trematode Telorchis assula was found in 64.9% of grass snakes (Natrix natrix) examined from northern Ukraine in a study published online on 3 February 2026. Across all 238 grass snakes in the Ukrainian dataset, 49.8% were infected.

The researchers combined parasitological material collected across Ukraine between 1951 and 2025. In total, the study included 461 snakes: 238 grass snakes and 223 dice snakes (Natrix tessellata). T. assula lives in the stomach and intestine of snakes, which are its definitive hosts.

Infection was more common in northern grass snakes

Among 148 grass snakes from northern regions, 64.9% were infected, compared with 41.5% of 65 snakes in the southern sample. The north–south difference in prevalence was supported statistically. Parasite abundance was also higher on average in the north, but that difference was less clear.

Across the full species samples, 49.8% of grass snakes and 33.2% of dice snakes were infected. In southern areas where the two species occurred together, however, their infection prevalence was nearly identical. This comparison is important because the species differ in diet: grass snakes commonly use amphibian-rich habitats, while dice snakes feed primarily on fish.

The parasite depends on an aquatic transmission cycle

Telorchis assula uses aquatic planorbid snails as its first intermediate hosts and tadpoles as second intermediate hosts. Snakes become infected when they eat infected tadpoles. The authors suggest that conditions in northern and central Ukraine may support this transmission cycle better than those in the south, including through greater availability of aquatic snails and amphibians.

In the western Ukrainian sample of 25 grass snakes, T. assula was not detected. The authors point to regional ecosystem differences and the comparatively low abundance of the snail Planorbis planorbis as a possible factor.

Sources and image credit

  1. Yermolenko, S., Nechai, A., Kuzmin, Y., Gasso, V., Karmyzova, L., & Holub, I. (2026). Analysis of the infection of the dice snake Natrix tessellata and the grass snake Natrix natrix with Telorchis assula in Ukraine. Acta Zoologica Bulgarica, 78(1), 127–132. https://doi.org/10.71424/azb78.1.003017Primary peer-reviewed study published online 3 February 2026. Among 238 grass snakes, T. assula prevalence was 49.8% overall, 64.9% in the northern sample and 41.5% in the southern sample; the north–south prevalence difference was statistically significant.

Image: George Chernilevsky / Wikimedia Commons — grass snake (Natrix natrix) moving through a bog in Ukraine, photographed 5 July 2009; representative Ukrainian habitat image, not a snake from the parasite dataset. Cropped to news-card format. · Public domain · Source

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