Epizootic Ulcerative Syndrome (EUS): A Disease Guide

Epizootic ulcerative syndrome, caused by the oomycete Aphanomyces invadans, produces deep necrotic ulcers in many freshwater fish. Control relies on water quality and biosecurity.

Overview

Epizootic ulcerative syndrome (EUS), also known as mycotic granulomatosis or red spot disease, is a severe, seasonal disease of freshwater and estuarine fish. The causative agent is the oomycete (water mould) Aphanomyces invadans, also referred to as Aphanomyces piscicida; secondary bacteria often complicate the lesions. EUS is a notifiable disease of the World Organisation for Animal Health (WOAH/OIE). It was first recorded in Japan in 1971 and has since been reported in more than 26 countries across Asia, Africa, Australia, and the Americas, affecting a very wide host range.

Affected species

EUS affects a remarkably broad host range, with more than 160 freshwater and estuarine fish species reported as susceptible, both wild and farmed. Commonly affected groups include snakeheads, catfish, and mullet. Because so many species are vulnerable, outbreaks can cause high morbidity in natural waters as well as in aquaculture.

Clinical signs

Red spots on the body surface, head, operculum, or caudal peduncle in early stages
Large red or grey shallow ulcers progressing to deep necrotic lesions
Extensive erosions filled with necrotic tissue and fungal mycelium
Invasive hyphae penetrating from the skin into muscle and internal organs
Mycotic granulomas on internal organs in later stages
Inappetence, lethargy, and erratic swimming

Seasonality and transmission

Outbreaks are strongly associated with periods of low water temperature and heavy rainfall or flooding in tropical and subtropical waters. The pathogen spreads through water by motile zoospores, and flood water moving between water bodies and the movement of infected fish are important routes of introduction.

Diagnosis

Provisional diagnosis can be made from squash preparations of skeletal muscle beneath an ulcer to identify the broad, non-septate (aseptate) invasive hyphae. Definitive diagnosis relies on histopathology showing penetrating hyphae surrounded by granulomatous inflammation, together with isolation of the pathogen in culture and confirmatory molecular tests such as real-time PCR.

Treatment, control and prevention

There is no specific vaccine and no simple cure. In open natural waters, control is generally impractical, so management focuses on small, closed ponds and on preventing introduction.

Liming ponds with agricultural lime and improving water quality, together with removal of infected fish, can reduce mortality
Salt (sodium chloride) and lime are used to treat or limit spread in managed systems
Sun-drying and liming ponds between cycles disinfect against the pathogen
Raise and repair pond embankments above flood level and screen inlets to keep out flood water
Reduce stress, restrict movement of infected fish, and apply general biosecurity and surveillance