Epizootic Ulcerative Syndrome

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Aphanomyces invadans
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Saprolegniales
Family	Saprolegniaceae
Genus	Aphanomyces
Species	A. invadans

Also Known As: EUS — Mycotic Granulomatoses — MG — Red Spot Disease — RSD

Caused By: Aphanomyces invadans  — also known as: Aphanomyces invaderis — A. piscicida — EUS related Aphanomyces — ERA

Introduction

Epizootic ulcerative syndrome (EUS) is a fungal disease of freshwater and brackish fish affecting more than 100 fish species. It is caused by the fungal species, Aphanomyces invadans. The organism requires a specific combination of factors in order to germinate within the dermis of the fish. The disease causes lesions in both the skin and visceral organs.

Distribution

Disease is present in parts of the Asia-Pacific region and Australia. A similar disease has also been reported in the eastern USA.

Signalment

Over 100 species have been confirmed to be affected by EUS.

EUS occurs commonly during periods of low temperature and heavy rainfall in tropical and sub-tropical waters. These conditions favour sporulation and cold temperatures delay the inflammatory response of the fish to infection.

Some species have been shown to be resistant including Chinese carps, milkfish and tilapia.

Clinical Signs

In early disease, red spots or small haemorrhagic lesions are found on the surface of the fish.

These progress to ulcers and eventually large necrotic erosions. Fungal mycelium is often visible on the surface of ulcers.

Death then follows rapidly due to visceral granulomata, septicaemia and failure of osmoregulatory balance.

Diagnosis

Positive diagnosis is made by analysis of histological sections demonstrating mycotic granulomas and isolation of the causal fungus.

Histology of early lesions reveals acute spongiosis and epithelial cell loss. Degenerative changes progress through the dermis with hyperaemia, haemorrhages and inflammatory infiltration. In advanced stages, sarcolysis is also obvious. Fungal hyphae are enclosed by a well defined epithelioid cell layer and mycotic granulation spreads through the infected muscle and internal organs. Muscle fibres eventually disappear altogether and are replaced by fibrosis, inflammatory cells and new blood vessels. These distinct features of EUS ulcers make histological analysis enough for a definitive diagnosis.

Squash preparations of skeletal muscle from beneath an ulcer will also demonstrate septate fungal hyphae for a rapid provisional diagnosis.

Moderate, pale, raised lesions are best for fungal isolation. Muscle should be exposed in a sterile manner by removing scales and searing the skin with a red hot spatula before dissecting with a sterile scalpel. 2mm samples should be placed in a petri dish containing Czapex Dox agar with penicillin G and oxolinic acid. They should be incubated at room temperature and examined daily so that emerging hyphal tips can be transferred onto fresh plates to produce contaminant free cultures.

The fungus can then be identified by inducing sporogenesis and demonstrating its typical asexual characteristics.

Injection of spore containing innoculum into susceptible fish at 20⁰C and demonstration of histological growth after 7 days and granulomas in muscle after 14 days is also diagnostic.

Treatment

Keeping diseased fish in good quality, clean water may allow recovery, but only if lesions are not too extensive and dark scars are often left behind on healing.

There is no effective treatment for advanced disease.

Control

Quarantine and health certification practices for movement of live fish between countries is the method of preventing spread of EUS to currently free areas.

In endemic areas, eradication, exclusion, management, surveillance and treatment are all required to gain control.

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References

Bruno, D.W., van West, P. and Beakes, G.W. 2011. Saprolegnia and Other Oomycetes. In: Fish Diseases and Disorders Volume 3: Viral, Bacterial and Fungal Infections (eds. P.T.K. Woo and D.W. Bruno), CABI, Walingford, UK, pp. 669-720.

This article was originally sourced from The Animal Health & Production Compendium (AHPC) published online by CABI during the OVAL Project. The datasheet was accessed on 10 July 2011.

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