What is Epizootic Ulcerative Syndrome (EUS) caused by?

Epizootic Ulcerative Syndrome (EUS) is primarily caused by an infection with the oomycete fungus Aphanomyces invadans. This organism is not a true fungus; rather, it belongs to a group of water molds that are related to certain pathogenic fungi. The disease occurs in a wide range of freshwater and estuarine fish species and is characterized by the development of ulcers on the skin and internal tissues, which can lead to significant morbidity and mortality rates among affected populations.

The pathology associated with EUS is a result of the sharp increase in virulence of the oomycete under specific environmental conditions, such as high temperature and low oxygen levels. Aquatic ecosystems that experience these stresses can trigger outbreaks of the disease, resulting in substantial fish kill events.

Understanding the specific causative agent of EUS is crucial for effective management and control strategies, particularly in aquaculture and wild fish populations. Hence, identifying the correct etiological agent as an oomycete rather than a virus, fungus, or protozoan is essential for appropriate diagnosis and treatment in veterinary medicine focused on aquatic species.