Epizootic ulcerative syndrome

Overview

Epizootic Ulcerative Syndrome (EUS) is a serious fish disease that affects both wild and farmed freshwater and estuarine fish species. It is characterized by the development of red lesions, ulcers, and necrosis (tissue death) on the skin and underlying muscle tissues. EUS has caused large-scale fish kills and economic losses in aquaculture, particularly in Southeast Asia, South Asia, and parts of Australia and Africa. The disease is caused by an invasive water mold and is listed as a notifiable disease in many countries due to its devastating effects on aquatic ecosystems and fisheries.

Causes

The primary causative agent of EUS is a pathogenic water mold known as Aphanomyces invadans, a type of oomycete. The infection often occurs following environmental stressors that compromise the fish’s immune system. Common contributing factors include:

• Temperature fluctuations

• Poor water quality: Including low dissolved oxygen or high organic load

• Heavy rainfall or flooding events

• Mechanical injury: Skin wounds that allow the pathogen to enter

• Co-infections with bacteria or parasites

Symptoms

Clinical signs of EUS typically begin with small red spots or hemorrhagic lesions on the skin and can progress rapidly. Common symptoms include:

• Redness and inflammation: Especially on the head, sides, and base of fins

• Open ulcers and lesions: Which may become deep and necrotic

• Frayed or eroded fins

• Lethargy: Reduced movement and activity

• Loss of appetite

• Secondary infections: Due to exposure of internal tissues

• High mortality: Especially in susceptible or stressed fish populations

In advanced cases, muscle tissues and internal organs may be affected, often leading to death.

Diagnosis

Diagnosis of EUS involves clinical observation, histopathological analysis, and laboratory testing. Key diagnostic steps include:

• Visual inspection: Presence of characteristic ulcers and lesions

• Histopathology: Tissue samples show granulomatous inflammation and presence of invasive fungal hyphae

• Fungal culture: Isolation of Aphanomyces invadans on specific media

• PCR testing: Molecular methods to confirm the identity of the pathogen

Differential diagnosis may be necessary to distinguish EUS from other ulcerative conditions such as bacterial infections or parasitic diseases.

Treatment

There is currently no universally effective treatment for EUS in wild fish populations, but management in aquaculture systems focuses on prevention and control:

• Water quality improvement: Maintaining optimal dissolved oxygen and reducing organic waste

• Environmental management: Avoiding overstocking and minimizing stress during seasonal changes

• Chemical treatments:

  • Use of antifungal agents (e.g., potassium permanganate, formalin) may reduce fungal load but are not always effective once lesions are established

• Quarantine and biosecurity: Preventing the spread to new areas or unaffected populations

• Selective breeding: Development of resistant strains in aquaculture is being explored

Prognosis

The prognosis for fish affected by EUS is generally poor, particularly in severe outbreaks or where environmental conditions are unfavorable. In aquaculture settings, early detection and improved management practices can limit losses. However, in natural water bodies, EUS can cause high mortality rates and long-term ecological disruption. Recovery of fish populations may take months or years, depending on species affected and environmental restoration efforts. Global surveillance and better understanding of the pathogen’s behavior remain essential for mitigating future outbreaks.