Epizootic Ulcerative Syndrome
| 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.
Signalment
Over 100 species have been confirmed to be affected by EUS.
EUS occurs commonly during periods of low temperature and heavy rainfall.[1] These conditions favour sporulation[2] and cold temperatures delay the inflammatory response of the fish to infection.[3]
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 top of the 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 caudal 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.[4] 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 sterilely 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 wih 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.[5]
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 advanced and dark scars are often left behind.
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
- ↑ Bondad-Reantaso, M. G., Lumanlan, S. C., Natividad, J. M., Phillips, M. J (1992) Environmental monitoring of the epizootic ulcerative syndrome (EUS) in fish from Munoz, Nueva Ecija in the Philippines. In: Shariff, M., Subasinghe, R. P, Arthur, J. R. eds. Diseases in Asian Aquaculture 1. Manila, The Philippines: Fish Health Section, Asian Fisheries Society, 475-490
- ↑ Lumanlan-Mayo, S. C., Callinan, R. B., Paclibare, J. O., Catap, E. S., Fraser, G. C (1997) Epizootic ulcerative syndrome (EUS) in rice-fish culture systems: an overview of field experiments 1993-1995. In: Flegel, T. W., MacRae, I. H. eds. Diseases in Asian Aquaculture III. Manila, The Philippines: Fish Health Section, Asian Fisheries Society, 129–138
- ↑ Catap, E. S., Munday, B. L (1998) Effects of variations of water temperature and dietary lipids on the expression of experimental epizootic ulcerative syndrome (EUS) in sand whiting, Sillago ciliata. Gyobyo Kenkyu. Fish Pathology, 33(4):327-335
- ↑ Chinabut, S., Roberts, R. J (1999) Pathology and Histopathology of Epizootic Ulcerative Syndrome (EUS). Bangkok, Thailand: Aquatic Animal Health Research Institute, Department of Fisheries, Royal Thai Government, pp 33
- ↑ Lilley, J. H., Callinan, R. B., Chinabut, S., Kanchanakhan, S., MacRae, I. H., Phillips, M. J (1998) Epizootic ulcerative syndrome (EUS) technical handbook. Bangkok, Thailand: Aquatic Animal Health Research Institute
Animal Health & Production Compendium, Epizootic Ulcerative Syndrome datasheet, accessed 13/07/2011 @ http://www.cabi.org/ahpc/