Epizootic ulcerative syndrome or EUS is an infection caused by an oomycete fungi known as Aphanomyces invadans or A. piscicida. Aphanomyces is a member of a group of organisms formerly commonly known as water moulds. EUS is an epizootic condition affecting wild and farmed freshwater and estuarine finfish. Also EUS is known as mycotic granulomatosis (MG) or red spot disease (RSD). It has a complex infectious aetiology and is clinically characterised by the presence of invasive Aphanomyces infection and necrotising ulcerative lesions, typically leading to a granulomatous response.