Declining fish stocks and losses caused by diseases are a major constraint to improving the nutrition and income of rural people. EUS, a disease characterised by ulceration and mortalities in wild and cultured freshwater fish affects more than 30 species of commercially important freshwater and estuarine cultured and wild fish.

Rational control and prevention methods remain a problem, as the pathogenesis of EUS is not understood. Patterns of occurrence in the Asia-Pacific suggest an infectious cause linked with the fungus Aphanomyces sp., which invades the skin of fishes exposed to rapid water changes such as decreases in alkalinity, hardness, chloride concentrations and fluctuations in pH.

In Australia and the Philippines, EUS is most often seen in estuaries with acid sulfate soils in their lower catchments, and in Indonesia a link between the occurrence of EUS in rivers and the presence of peat (acid) soils in their catchments has been observed.