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epizootic ulcerative syndrome

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epizootic ulcerative syndrome

Summary

  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • epizootic ulcerative syndrome
  • Overview
  • Epizootic ulcerative syndrome (EUS) is a disease of freshwater and brackishwater fish that has other synonyms such as mycotic granulomatosis (MG), red spot disease (RSD) and ulcerative mycosis (UM). More than 100 specie...

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Pictures

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PictureTitleCaptionCopyright
Map showing the spread of EUS across the Asia-Pacific region. Dates indicate the time of the first serious outbreak. There is some doubt about outbreaks marked with asterisks.
TitleDistribution map
CaptionMap showing the spread of EUS across the Asia-Pacific region. Dates indicate the time of the first serious outbreak. There is some doubt about outbreaks marked with asterisks.
CopyrightJ.H. Lilley
Map showing the spread of EUS across the Asia-Pacific region. Dates indicate the time of the first serious outbreak. There is some doubt about outbreaks marked with asterisks.
Distribution mapMap showing the spread of EUS across the Asia-Pacific region. Dates indicate the time of the first serious outbreak. There is some doubt about outbreaks marked with asterisks.J.H. Lilley
Sand whiting, Sillago ciliata, caught in Richmond River, Eastern Australia, infected with EUS.
TitleSymptoms
CaptionSand whiting, Sillago ciliata, caught in Richmond River, Eastern Australia, infected with EUS.
CopyrightR. V. L. Wollongbar
Sand whiting, Sillago ciliata, caught in Richmond River, Eastern Australia, infected with EUS.
SymptomsSand whiting, Sillago ciliata, caught in Richmond River, Eastern Australia, infected with EUS.R. V. L. Wollongbar
Striped snakehead, Channa striata, bath challenge, Thailand, infected with EUS.
TitleSymptoms
CaptionStriped snakehead, Channa striata, bath challenge, Thailand, infected with EUS.
CopyrightSupranee Chinabut
Striped snakehead, Channa striata, bath challenge, Thailand, infected with EUS.
SymptomsStriped snakehead, Channa striata, bath challenge, Thailand, infected with EUS.Supranee Chinabut
Silver perch, Bidyanus bidyanus, from Eastern Australia, infected with EUS.
TitleSymptoms
CaptionSilver perch, Bidyanus bidyanus, from Eastern Australia, infected with EUS.
CopyrightR. V. L. Wollongbar
Silver perch, Bidyanus bidyanus, from Eastern Australia, infected with EUS.
SymptomsSilver perch, Bidyanus bidyanus, from Eastern Australia, infected with EUS.R. V. L. Wollongbar
Grey mullet, Mugil cephalus, caught in the Richmond River, Eastern Australia, infected with EUS.
TitleSymptoms
CaptionGrey mullet, Mugil cephalus, caught in the Richmond River, Eastern Australia, infected with EUS.
CopyrightR. V. L. Wollongbar
Grey mullet, Mugil cephalus, caught in the Richmond River, Eastern Australia, infected with EUS.
SymptomsGrey mullet, Mugil cephalus, caught in the Richmond River, Eastern Australia, infected with EUS.R. V. L. Wollongbar
Histological pictures of EUS muscle samples with various stains: (a) Haematoxylin and Eosin; (b) Uvitex; (c) Grocott's; (d) Perodic Acid Schiff (PAS).
TitleHistology
CaptionHistological pictures of EUS muscle samples with various stains: (a) Haematoxylin and Eosin; (b) Uvitex; (c) Grocott's; (d) Perodic Acid Schiff (PAS).
CopyrightSupranee Chinabut & Varinee Panyawachira
Histological pictures of EUS muscle samples with various stains: (a) Haematoxylin and Eosin; (b) Uvitex; (c) Grocott's; (d) Perodic Acid Schiff (PAS).
HistologyHistological pictures of EUS muscle samples with various stains: (a) Haematoxylin and Eosin; (b) Uvitex; (c) Grocott's; (d) Perodic Acid Schiff (PAS).Supranee Chinabut & Varinee Panyawachira
Hyphae and sporangium of Aphanomyces invadans from EUS-infected fish.
TitleHyphae and sporangium
CaptionHyphae and sporangium of Aphanomyces invadans from EUS-infected fish.
CopyrightJ.H. Lilley
Hyphae and sporangium of Aphanomyces invadans from EUS-infected fish.
Hyphae and sporangiumHyphae and sporangium of Aphanomyces invadans from EUS-infected fish.J.H. Lilley

Identity

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Preferred Scientific Name

  • epizootic ulcerative syndrome

Other Scientific Names

  • mycotic granulamotoses
  • red spot disease

International Common Names

  • English: Aphanomyces invadans infection

English acronym

  • EUS
  • MG
  • RSD

Overview

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Epizootic ulcerative syndrome (EUS) is a disease of freshwater and brackishwater fish that has other synonyms such as mycotic granulomatosis (MG), red spot disease (RSD) and ulcerative mycosis (UM). More than 100 species of fish species with a wide geographic distribution have been reported to be affected by EUS. EUS outbreaks occur only when a number of causal factors combine.

The causative agent of this disease is the fungus, Aphanomyces invadans. There are a number of sufficient causes that lead to exposure of the dermis to fungal spore germination. In the early stages, red spots or small haemorrhagic lesions are found on the body surface. Intermediate stage lesions become small ulcers. Advanced stage lesions expand into large necrotic open ulcers resulting in death.

Positive diagnosis of EUS is made by the presence of mycotic granulomas in histological section and isolation of Aphanomyces invadans from infected fish. Quarantine and health certification practice for the movement of live fish between countries or regions are effective means of preventing the spread of EUS to new areas. For areas where EUS is endemic, prevention programmes should include eradication, exclusion, management, surveillance and treatment.

Outbreaks of EUS have continued to occur periodically in all target countries. The immunity of fish surviving EUS infections should be the subject of interest because records from many countries showed that the severity of EUS outbreak after the first few years tends to decrease (Mohan and Shankar, 1994; Bondad-Reantaso et al., 1992). The serious implications for the development of vaccines against EUS should be deliberated because there was a report on the strong reaction between the sera from immunized fish with the spores and mycelium of pathogenic strains of Aphanomyces and the extracts of pathogenic strains of Aphanomyces (Thompson et al., 1997).

Hosts/Species Affected

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The disease called mycotic granulomatosis was first found in ponds raising goldfish (Carassius auratus) (Miyazaki and Egusa, 1972), and ayu (Plecoglossus altivelis) in various regions in Japan in 1971 (Egusa and Masuda, 1971). Wild species such as the formosan snakehead (Channa maculata), Japanese trident goby (Tridentiger obscurus) and black mullet (Mugil cephalus) were also affected by the disease at that time (Miyazaki and Egusa, 1973a,b,c). Later, the causative agent of mycotic granulomtosis was identified as Aphanomyces piscicida by Hatai (1980). In the following year, 1972, red spot disease (RSD) was first reported in estuarine fish from the Burnett River, Queensland, Australia (McKenzie and Hall, 1976; Rodgers and Burke, 1981). Lately, outbreaks of RSD have been reported in many species of estuarine and freshwater fish in New South Wales (Callinan et al., 1989), North Territory (Humphrey and Langdon, 1986; Pearce, 1990) and Western Australia (D. Pass, personal communication). Sea mullet (Mugil cephalus), sand whiting (Sillago ciliata), yellow fin bream (Acanthopagrus australis) and barramundi (Lates calcarifer) were some of the economically important estuarine species affected. Outbreaks of RSD have occurred in farmed silver perch (Bidyanus bidyanus) which is a freshwater species of Southern Australia (Rodgers and Burke, 1981).

Distribution

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Since 1971-1972 the disease gradually spread to many countries in the Asia-Pacific region. Catfish cultured in the Mekong delta, Vietnam were reported to be infected by EUS in 1973 (Roberts et al., 1994) but this information was not supported by histopathological confirmation. In 1975, there was a report on an EUS outbreak in some freshwater fish in Papua New Guinea. By the year 1980 it had spread to Malaysia and Thailand. In 1982-1983, a severe EUS outbreak in Thailand affecting snakehead (Channa striata), Puntius sp. and many species of rice field fish was recorded (Tonguthai, 1985). The outbreak was also reported in East Kalimantan, Indonesia, in 1982 and 1984, causing serious mortality on wild species such as snakehead, catfish, sand goby, Puntius sp. and kissing gouramy in natural water resources (Roberts et al., 1994). Cambodia, Laos and Myanmar also suffered from EUS in 1984. The first confirmed EUS outbreak of some freshwater fish in Laguna de Bay, Philippines, was recorded in 1985-1986. As early as 1987, Puntius sp. and snakehead in a western province of Sri Lanka were infected by EUS. After the severe flooding in 1988, the first EUS outbreak was reported in Bangladesh. Indian major carps seem to be the main species affected. In the same year, EUS occurred in northeast India and spread throughout the whole country, then to Nepal in the following year (Roberts et al., 1994). By 1996 it was confirmed that EUS had spread to the upper part of the Indus River in Pakistan (Kanchanakhan, 1996).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

AfghanistanNo information availableOIE, 2009
ArmeniaDisease never reportedOIE, 2009
AzerbaijanDisease never reportedOIE, 2009
BahrainDisease never reportedOIE, 2009
BangladeshPresentNULLOIE, 2009
BhutanNo information availableOIE, 2009
CambodiaNo information availableNULLOIE, 2009
ChinaDisease never reportedOIE, 2009
-Hong KongDisease never reportedOIE, 2009
IndiaNo information availableOIE, 2009
IndonesiaDisease never reportedNULLRoberts et al., 1994; OIE, 2009
IranNo information availableOIE, 2009
IraqAbsent, reported but not confirmedOIE, 2009
IsraelDisease never reportedOIE, 2009
JapanDisease not reported20020904Egusa and Masuda , 1971; OIE, 2009
JordanNo information availableOIE, 2009
KazakhstanDisease not reportedOIE, 2009
Korea, Republic ofNo information availableOIE, 2009
KuwaitDisease not reportedOIE, 2009
KyrgyzstanDisease not reportedOIE, 2009
LaosNo information availableNULLOIE, 2009
LebanonNo information availableOIE, 2009
MalaysiaDisease not reported1986OIE, 2009
MongoliaNo information availableOIE, 2009
MyanmarNo information availableNULLOIE, 2009
NepalNo information availableNULLRoberts et al., 1994; OIE, 2009
OmanNo information availableOIE, 2009
PakistanNo information availableNULLKanchanakhan, 1996; OIE, 2009
PhilippinesNo information availableNULLOIE, 2009
QatarNo information availableOIE, 2009
Saudi ArabiaNo information availableOIE, 2009
SingaporeDisease never reportedOIE, 2009
Sri LankaNo information availableNULLOIE, 2009
SyriaNo information availableOIE, 2009
TajikistanNo information availableOIE, 2009
ThailandNo information availableNULLTonguthai , 1985; OIE, 2009
TurkeyNo information availableOIE, 2009
United Arab EmiratesNo information availableOIE, 2009
VietnamNo information availableNULLRoberts et al., 1994; OIE, 2009
YemenNo information availableOIE, 2009

Africa

AlgeriaNo information availableOIE, 2009
AngolaNo information availableOIE, 2009
BeninNo information availableOIE, 2009
BotswanaDisease not reportedOIE, 2009
Burkina FasoNo information availableOIE, 2009
ChadNo information availableOIE, 2009
CongoNo information availableOIE, 2009
DjiboutiNo information availableOIE, 2009
EgyptNo information availableOIE, 2009
EritreaNo information availableOIE, 2009
EthiopiaNo information availableOIE, 2009
GambiaNo information availableOIE, 2009
GhanaNo information availableOIE, 2009
GuineaNo information availableOIE, 2009
Guinea-BissauNo information availableOIE, 2009
KenyaNo information availableOIE, 2009
LesothoDisease never reportedOIE, 2009
MadagascarNo information availableOIE, 2009
MalawiNo information availableOIE, 2009
MaliNo information availableOIE, 2009
MauritiusNo information availableOIE, 2009
MoroccoNo information availableOIE, 2009
MozambiqueDisease not reportedOIE, 2009
NamibiaPresentOIE, 2009
NigeriaNo information availableOIE, 2009
SenegalNo information availableOIE, 2009
South AfricaNo information availableOIE, 2009
SudanDisease never reportedOIE, 2009
SwazilandNo information availableOIE, 2009
TanzaniaNo information availableOIE, 2009
TogoNo information availableOIE, 2009
TunisiaDisease not reportedOIE, 2009
UgandaNo information availableOIE, 2009
ZambiaNo information availableOIE, 2009
ZimbabweNo information availableOIE, 2009

North America

CanadaDisease never reportedOIE, 2009
GreenlandDisease never reportedOIE, 2009
MexicoDisease not reportedOIE, 2009
USADisease not reported2004OIE, 2009

Central America and Caribbean

BelizeDisease never reportedOIE, 2009
Costa RicaDisease never reportedOIE, 2009
CubaDisease never reportedOIE, 2009
El SalvadorNo information availableOIE, 2009
GuadeloupeNo information availableOIE, 2009
GuatemalaDisease never reportedOIE, 2009
HaitiNo information availableOIE, 2009
HondurasNo information availableOIE, 2009
JamaicaNo information availableOIE, 2009
MartiniqueNo information availableOIE, 2009
NicaraguaDisease never reportedOIE, 2009
PanamaNo information availableOIE, 2009

South America

ArgentinaDisease never reportedOIE, 2009
BoliviaNo information availableOIE, 2009
BrazilDisease never reportedOIE, 2009
ChileDisease never reportedOIE, 2009
ColombiaDisease never reportedOIE, 2009
EcuadorNo information availableOIE, 2009
French GuianaDisease not reportedOIE, 2009
PeruNo information availableOIE, 2009
UruguayNo information availableOIE, 2009
VenezuelaNo information availableOIE, 2009

Europe

AlbaniaNo information availableOIE, 2009
AustriaNo information availableOIE, 2009
BelarusDisease not reportedOIE, 2009
BelgiumNo information availableOIE, 2009
BulgariaNo information availableOIE, 2009
CroatiaDisease never reportedOIE, 2009
CyprusDisease never reportedOIE, 2009
Czech RepublicDisease never reportedOIE, 2009
DenmarkDisease never reportedOIE, 2009
EstoniaNo information availableOIE, 2009
FinlandDisease never reportedOIE, 2009
FranceDisease not reportedOIE, 2009
GermanyDisease never reportedOIE, 2009
GreeceNo information availableOIE, 2009
HungaryDisease never reportedOIE, 2009
IcelandDisease never reportedOIE, 2009
IrelandDisease not reportedOIE, 2009
ItalyNo information availableOIE, 2009
LatviaDisease never reportedOIE, 2009
LiechtensteinNo information availableOIE, 2009
LithuaniaDisease never reportedOIE, 2009
LuxembourgNo information availableOIE, 2009
MacedoniaNo information availableOIE, 2009
MaltaNo information availableOIE, 2009
MontenegroNo information availableOIE, 2009
NetherlandsDisease never reportedOIE, 2009
NorwayDisease never reportedOIE, 2009
PolandNo information availableOIE, 2009
PortugalDisease not reportedOIE, 2009
RomaniaNo information availableOIE, 2009
Russian FederationNo information availableOIE, 2009
SerbiaNo information availableOIE, 2009
SlovakiaDisease not reportedOIE, 2009
SloveniaDisease never reportedOIE, 2009
SpainDisease never reportedOIE, 2009
SwedenDisease never reportedOIE, 2009
SwitzerlandDisease never reportedOIE, 2009
UKDisease never reportedOIE, 2009
UkraineDisease not reportedOIE, 2009

Oceania

AustraliaPresentNULLOIE, 2009
-Australian Northern TerritoryPresentHumphrey and Langdon, 1986; Pearce, 1990
-New South WalesPresentCallinan et al., 1989
-QueenslandPresentMcKenzie and Hall , 1976; Rodgers and Burke, 1981
-South AustraliaPresentRodgers and Burke, 1981
French PolynesiaDisease never reportedOIE, 2009
New CaledoniaDisease not reportedOIE, 2009
New ZealandDisease never reportedOIE, 2009
Papua New GuineaPresent

Pathology

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Pathogenic Agents

Based on the investigation of the pathogenic organisms found from EUS-infected samples, a wide range of aetiologies have been attributed to EUS, such as bacteria, fungi, parasites and viruses.

Several species of parasitic protozoans (Chilodonella sp., Costia sp., Epistylis sp., Glossatella sp., Ichthyophthirius sp., Scyphidia sp., Trichodina spp.), myxosporeans (Henneguya sp. and Thelohania sp.), monogeneans and crustaceans (Lernaea sp.) have been recorded from diseased fish (Callinan et al., 1997; Reungprach et al., 1983). Nevertheless, there is no evidence to confirm that these parasites are the causative agent of the disease. Parasitic infections possibly induce stress in fish, thus predisposing them to infection (Subasinghe, 1993).

Aeromonas hydrophila, A. sobria, Pseudomonas spp. and Vibrio spp. are the species of bacteria isolated from the internal organs of EUS-infected fish (Burke and Rodgers, 1981; Llobrera and Gacutan, 1987; Tonguthai, 1985). Among these bacteria, A. hydrophila is the most commonly isolated species from the advanced stages of infected fish. It is usually not isolated from the early stage of the disease. On the other hand, A. hydrophila is known to be part of the normal microflora of fish and aquatic environments and is recognised as an opportunistic pathogen.

A number of birnaviruses, reoviruses and rhabdoviruses have been isolated from diseased snakehead and other susceptible species. These viruses most probably represent adventitious infections unrelated to EUS because of their heterogeneous nature, together with a low and non-consistent level of recovery from epizootics of disease. Therefore, the role of viruses in the aetiology of this disease remains unproven (Kanchanakarn, 1996; Roberts et al., 1994).

At least two species of fungi have been isolated from EUS-infected fish, namely, Achlya sp. which was isolated from the superficial area of the lesions (Pittchayangkula and Bodhalamik, 1983; Subasinghe et al., 1990) and Aphanomyces sp. which is normally isolated from the muscle area near the lesions (Egusa, 1992, Fraser et al., 1992; Hatai et al., 1994; Roberts et al., 1993). Recently, it has been shown that all Aphanomyces identified from EUS samples not only are the same species but also a single clonal lineage (Lilley et al., 1997). Therefore, it is confirmed that the fundamental aetiological agent of EUS is an oomycete fungus, Aphanomyces invaderis (Willoughby et al., 1995). However, the valid taxonomic name according to the International Code of Botanical Nomenclature (ICBN) of this species is A. invadans (Lilley et al., 1998). This specific pathogenic fungus showed the ability to invade the skin of EUS-infected fish (Kiryu et al., 2003). Isolation of this particular fungus is difficult as it is slow growing and most of the hyphae in the muscle are dead except for the tips that are penetrating deeper into the muscle tissue (Roberts et al., 1993)

Pathogenicity of the Fungus

After success in isolating the slow growing fungus, Aphanomyces invadans, from EUS-infected fish in various countries (Egusa, 1992; Fraser et al., 1992; Willoughby et al., 1995), pathogenicity studies were conducted. Intramuscular injection of spore suspensions into some susceptible fish species (barbs and snakehead) demonstrated a severe necrotising myogranulomatous condition at the injected site similar to the pathological feature of fish naturally infected with EUS. At the later stages, the fungus rapidly invaded the other internal organs, causing death to the host. The diameter of the hyphae of the fungus in lesions was always larger and the walls thicker than in cultured specimens (Roberts et al., 1993). Co-habitation transmission experiments with EUS-infected fish were successful (Balasuriya et al., 1990; Cruz-Lacierda and Shariff, 1995).The exposure of healthy Atlantic menhaden to suspension of zoospores of A. invadans demonstrated the germination and penetration of the germ tube through the epithelium layer of the experimental fish (Kiryu et al., 2003).

Histopathology

Lesions in all EUS-infected fish are similar, except that in snakehead chronic lesions can develop. This is because snakehead survive longer and the lesions are able to develop to a very advanced stage; other species die before this stage is reached.

Grossly the lesions are generally small red erosions on the body, head and fins. In the advanced stage, deep haemorrhaged ulcers spread through the whole body. Fungal mycelium is commonly found on top of the ulcerative lesions. The internal organs in the early stage of infection show only minimal inflammatory response. It is important to note that diseased fish with not too advanced lesions, kept in better water quality conditions, often recover and undergo a healing process leaving dark scars at the sites of infection.

Histopathological observation of early stage lesions reveal acute spongiosis with loss of epithelial cells. Degenerative changes are observed in the dermis, with hyperaemia, haemorrhages and inflammatory cells infiltrating between the fibres of the dermis. Inflammatory exudate and haemorrhages are also found in the hypodermis. Sarcolysis with haemorrhages and inflammatory exudate are obvious in the more advanced stage. The fungal hyphae are enclosed by a well developed epithelioid cell layer. Mycotic granulation of these non-septate fungal hyphae spreads through the infected muscle and other internal organs in the very late stages. In the advanced stage, there are no muscle fibres in the granuloma tissues. They are replaced by fibrosis, inflammatory cells and newly developing capillaries (Chinabut et al., 1995; Chinabut and Roberts, 1999). This distinct feature of the typical mycotic granulomas in the lesions of EUS-infected fish provides important histopathological information which can be used to distinguish EUS from other fish ulcerative diseases.

Diagnosis

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Clinical signs

EUS is associated with mass mortality of susceptible fish in areas that are initially affected. However, evidence from EUS-endemic areas shows that it can also occur at low intensity, with fish often recovering towards the end of the cool season. Gross pathology of EUS infected fish is similar to other cutaneous ulcerative syndromes of freshwater and estuarine fishes and it is therefore important not to rely on clinical signs as a means of diagnosis. Outbreaks in estuarine fish have been recorded only in water with salinity less than 2 ppt. The gross appearance of lesions varies between species, habitat and stage of lesion development (Callinan et al., 1989, Viswanath et al., 1997, Chinabut and Roberts, 1999). The most distinctive EUS lesion is the open dermal ulcer. This is often most conspicuous in snakehead which is often used as an "indicator species" of EUS in an area.

Rapid muscle squash

A provisional diagnosis of EUS can be made by demonstrating aseptate fungal hyphae (12-30 µm in diameter) in a squash preparation of skeletal muscle taken from beneath the surface of an ulcer.

Histology

Confirmatory diagnosis requires histopathological demonstration of typical for necrotizing mycotic granulomas using haematoxylin and eosin stain (H&E) and a general fungus stain such as Periodic Acid Schiff (PAS), Grocott's or Uvitex.

Fungus culture and characterization

Isolation and identification of Aphanomyces invadans from infected fish are reported by Fraser et al. (1992) and Willoughby and Roberts (1994). Moderate, pale, raised, dermal lesions are most suitable for fungal isolation. Remove the scales around the lesion and sear the underlying skin with a red-hot spatula to sterilize the surface. Using a sterile scalpel blade and sterile, fine pointed forceps, cut through skin underlying the seared area and then by cutting horizontally and reflecting superficial tissues, expose underlying muscle. Ensure the instruments do not contact the contaminated external surface and otherwise contaminate the underlying muscle. Using aseptic techniques, carefully excise pieces of affected muscle, approximately 2 mm3, and place them in a Petri dish containing Czapex Dox agar with penicillin G (100 units mL-1) and oxolinic acid (100 mg mL-1). Seal plates and incubate at room temperature, examining daily. Repeatedly transfer emerging hyphal tips on to fresh plates of Czapex Dox agar until cultures are free of contamination.

Lesions located on the body or tail of fish less than 20 cm in length can be sampled by cutting the fish in two using a sterile scalpel, slicing a cross-section through the fish at the edge of the lesion. Flame the scalpel until red-hot and use this to sterilize the exposed surface of the muscle. Use a small-bladed sterile scalpel to cut out a circular block of muscle (2-4 mm3) from beneath the lesion and place it in a Petri dish of GP medium (3 g L-1 glucose, 1g L-1 peptone, 0.128 g L-1 MgSO4.7H2O, 0.014 g L-1 KH2PO4, 0.029 g L-1 CaCl2.2H2O, 2.4 mg L-1 FeCl3.6H2O, 1.8 mg L-1 MnCl2.4H2O, 3.9 mg L-1 CuSO4.5H2O, 0.4 mg L-1 ZnSO4.7H2O) with 100 units mL-1 penicillin-K and 10 mg mL-1 oxolinic acid. Instruments should not contact the contaminated external surface of the fish. Incubate inoculated media at approximately 25°C and examine under a microscope (preferably an inverted microscope) within 12 hours. Repeatedly transfer emerging hyphal tips to plates of GP medium with 12 g L-1 technical agar, 100 units mL-1 penicillin-K and 10 mg mL-1 streptomycin sulfate until pure cultures are obtained. They may be maintained at 10°C on GP agar and subcultured at intervals of no greater than 7 days.

The fungus can be identified to genus by inducing sporogenesis and demonstrating typical asexual characteristics of Aphanomyces as described in Lilley et al. (1998). A. invadans is characteristically slow growing in culture and fails to grow at 37°C on GPY agar (GP broth with 0.5 g L-1 yeast extract and 12 g L-1 technical agar). Confirmation that the isolate is A. invadans can be made by injecting a 0.1 ml suspension of 100+ motile spores intramuscularly in EUS susceptible fish at 20°C, and demonstrating histologically growth of aseptate hyphae 12-30 µm in diameter in muscle of fish sampled after 7 days, and typical mycotic granulomas in muscle of fish sample after 14 days.

Epidemiology

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Cutaneous ulcerative disease, named epizootic ulcerative syndrome (EUS), has been a seasonal disease of freshwater and brackishwater fish in Asia, Australia and the United States of America since 1971. Earlier outbreaks of mycotic granulomatosis (MG) in Japan, red spot disease (RSD) in Australia and ulcerative mycosis (UM) in the United States of America are now known to be the same as EUS (Kiryu et al., 2003). EUS commonly occurs during periods of low temperature and after periods of heavy rainfall (Bondad-Reantaso et al., 1992). These conditions favour sporulation of the Aphanomyces pathogen (Lumanlan-Mayo et al., 1997) and low temperature has been shown to delay the inflammatory response of fish to fungal infection (Chinabut et al., 1995; Catap and Munday, 1998). It has been defined as "a seasonal epizootic condition of freshwater and estuarine warm water fish of complex infectious aetiology characterized by the presence of invasive Aphanomyces infection and necrotising ulcerative lesions typically leading to a granulomatous response" (Roberts et al., 1994). Recent studies suggest that a complex infectious aetiology is not necessarily involved in all cases of EUS (Callinan, 1997, Kiryu et al., 2003); and one species of invasive fungus, Aphanomyces invadans, is involved in the outbreaks. Region wide, over 100 species have been confirmed affected by histological diagnosis in Asia (Lilley et al., 1998), but some important cultured species, including Chinese carps, milkfish and tilapia, have been shown to be resistant.

References

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Balasuriya KSW; Kulathilake M; Subasinghe RP, 1990. Preliminary investigations into the experimental transmission of ulcerative diseases syndrome in fish in Sri lanka. In: Hirano R, Hanyo I, eds. The 2nd Fisheries Forum. Manila, Philippines: Asian Fisheries Society, 659-662.

Blazer VS; Vogelbein WK; Densmore CL; May EB; Lilley JH; Zwerner DE, 1999. Aphanomyces as a cause of ulcerative skin lesions of menhaden from Chesapeake Bay tributaries. Journal of Aquatic Animal Health, 11(4):340-349.

Bondad-Reantaso MG; Lumanlan SC; Natividad JM; Phillips MJ, 1992. Environmental monitoring of the epizootic ulcerative syndrome (EUS) in fish from Munoz, Nueva Ecija in the Philippines. In: Shariff M, Subasinghe RP, Arthur JR, eds. Diseases in Asian Aquaculture 1. Manila, The Philippines: Fish Health Section, Asian Fisheries Society, 475-490.

Bruno DW; Wood BP, 1999. Saprolegnia and other Oomycetes. In: Fish diseases and disorders. Volume 3: viral, bacterial and fungal infections [ed. by Woo, P. T. K.\Bruno, D. W.]. Wallingford, UK: CAB International, 599-659.

Burke JB; Rodgers LJ, 1981. Identification on pathogenic bacteria associated with the occurrence of “red spot” in sea mullet, Mugil cephalus L., in southeastern Queensland. J. Fish Dis., 4:153-159.

Callinan RB, 1986. Diseases of native Australian fishes. Diseases of Australian fish and shellfish. Proceedings of the first Australian workshop on diseases of fish and shellfish, Benalla, Victoria, Australia, 27-30 May 1985., 102-117.

Callinan RB, 1997. Pathogenesis of red spot disease (epizootic ulcerative syndrome) in estuarine fish in eastern Australia and the Philippines. PhD Thesis. Queensland, Australia: University of Queensland, 232pp.

Callinan RB; Chinabut S; Kanchanakhan S; Lilley JH; Phillips MJ, 1997. Epizootic ulcerative syndrome (EUS) of fishes in Pakistan. A report of the finding of a mission to Pakistan on 9-19 March 1997.

Callinan RB; Fraser GC; Virgona JL, 1989. Pathology of red spot disease in sea mullet, Mugil cephalus L., from eastern Australia. Journal of Fish Diseases, 12(5):467-479.

Callinan RB; Paclibare JO; Bondad-Reantaso MG; Chin JC; Gogolewski RP, 1995. Aphanomyces species associated with epizootic ulcerative syndrome (EUS) in the Philippines and red spot disease (RSD) in Australia: preliminary comparative studies. Diseases of Aquatic Organisms, 21(3):233-238.

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