Extensively harvested hay contaminated with common windgrass induced an ulcerative stomatitis–gingivitis outbreak in horses

Summary After starting feeding on a new batch of hay, 29 out of 33 horses showed clinical signs of oral ulcerative and bleeding papules and mucous membrane erythema of the gingiva of the mandible within 2–3 days. The results of the complete blood cell count and biochemistry analysis did not yield significant findings in relation to horses affected by ulcerative stomatitis. Swab analyses of the oral mucosa for vesicular stomatitis virus (VS, Type Indiana and Type New Jersey) were negative in all horses ( n = 33). Common windgrass with long awns in the deeper stomal‐gingival ulcerations was confirmed through botanical analysis by botanical identification. The hay batch fed to the affected horses was assessed via botanical analysis, finding 15% common windgrass ( Apera spica‐venti ). After the removal of the contaminated hay batch, full recovery was seen in all horses within 2–6 weeks depending on the severity of the lesions. The grassland from which the hay was harvested belonged to a semi‐natural landscape restored to increase biodiversity and as a protection zone in case of overflow of the river Rhine. In conclusion, feeding forages harvested from these semi‐natural landscapes may be harmful to the health of horses.