Histopathological changes in the stomach wall of the European grayling Thymallus thymallus associated with severe infection by the nematode Cystidicoloides ephemeridarum

This study is a report of lesions associated with the nematode Cystidicoloides ephemeridarum (Linstow, 1972). It is a common parasite of the stomach mucosa of fish, mainly salmonids. In stomachs of 30 graylings were observed strong infection with C. ephemeridarum caught in July 2013 and February 2014 in the Dobrzyca River were examined. Adult nematodes, including females with mature eggs, and L3 and L4 larvae were recorded. Stomachs of graylings with intensive infection (146-261examples) were examined. The histological examination of a sample taken from the tissue surrounding nematodes revealed the presence of more or less confluent focal areas of destruction of gastric laminae. The numerous invasions of C. ephemeridarum are associated with intensive feeding of fish, but the different developmental stages of the parasite do not induce sufficiently strong histopathological changes in the stomach of graylings to be a cause of their death.