Epizootic ulcerative syndrome

Epizootic ulcerative syndrome is category 1 restricted matter.

Under Queensland legislation, if you suspect the presence of this disease in any species of fish, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Disease Watch Hotline on 1800 675 888.

Epizootic ulcerative syndrome (EUS) is an ulcerative disease of fish. It is not known to cause disease in humans. However, it is recommended that people should not eat EUS-affected fish as contaminant bacteria may be present in the flesh. In Queensland this disease occurs in estuarine and freshwater fish anywhere along the eastern coast or inland rivers systems or catchments and in the Gulf of Carpentaria. This disease has also been reported in New South Wales, the Northern Territory, Western Australia, South Australia and in Victoria. EUS has been reported from over 20 countries in 4 continents including North America, Southern Africa, Asia and Australia (OIE 2016).

It has been reported in over 94 species of estuarine and freshwater fish worldwide. Native fish species affected include mullet, barramundi, golden perch, silver perch, sleepy cod, Australian bass, rainbow fish, black bream, whiting, pikey bream, gobies, gudgeons, eels, rockcod, catfish and many other species.

Cause

Aphanomyces invadans (a water mould)

Other names

Red spot disease; mycotic granulomatosis

Distribution in Queensland

EUS affects a wide range of native fish species from both central and coastal areas of Queensland. The disease has been reported from central and inland freshwater rivers or catchments, coastal and estuarine creeks, rivers, and catchment areas and rarely in farmed freshwater fishes (e.g. silver perch).

Affected animals

fish

Symptoms

Initially, the gross signs of early disease are characterised by

  • multiple small red or grey spots or erosions on the skin (can be difficult to see)
  • areas of scale loss.

As the lesions progress the disease is characterised by

  • deep-red to brown ulcers, extending into the deeper skin layers, muscle, or even into the bone or exposing the abdominal cavity
  • weak swimming, or swimming near the surface
  • reduced feeding
  • mass mortalities.

Impacts

Fish that are badly affected usually die during EUS outbreaks. Fish die because the deep ulcers, with underlying muscle necrosis and tissue damage, result in osmoregulatory failure and the ulcers often become secondarily infected with bacterial, viral, parasitic or other fungal pathogens. Rarely, some fish may survive, the unsightly ulcers eventually healing, leaving noticeable scars. Recovered fish are rarely lifelong carriers.

How it is spread

The fungal zoospores are transmitted from 1 fish to another (horizontally) through the water. The fish must first have damaged skin for infection to occur. Once the motile spore attaches to the skin of the fish, the spore will germinate under suitable conditions and its hyphae will invade the fish skin and muscle tissue and reach the internal organs. Fish muscle is the target organ.

If the zoospores cannot find a susceptible fish host, or if water conditions are unfavourable, zoospores can encyst in the pond and wait until for more favourable conditions occur, whereby the cysts will then transform into free-swimming zoospores, and the infection cycle can begin again.

Factors associated with outbreaks

Research has shown that EUS outbreaks occur mostly at water temperatures of 18–22°C and after periods of heavy rainfall. Other factors associated with EUS outbreaks include:

  • water run-off over acid-sulphate soils
  • low water pH
  • rainfall after long dry periods
  • the presence of fungal zoospores in the water
  • high levels of dissolved aluminium in the water; prolonged low salinity
  • a reduction in water temperature to 18–22°C.

These conditions favour the proliferation of the fungus. Scientific studies have also shown that water temperatures of 17–19°C can delay the fishes’ inflammatory response to fungal infection. In some countries, EUS outbreaks occur in wild fish first and then spread to fish ponds (OIE 2016).

Risk period

In Queensland, EUS outbreaks tend to occur in the cooler periods of winter leading into spring, but this is not always consistent. EUS outbreaks often occur after the first heavy rainfall in spring or after the first rains following a prolonged dry period. In northern Queensland, EUS outbreaks may occur following the first heavy rains of the wet season.

References

OIE, 2016, 'Manual of diagnostic tests for aquatic animals', Section 2.3, Diseases of fish, Chapter 2.3.2 'Infection with Aphanomyces invadans (epizootic ulcerative syndrome)', http://www.oie.int/index.php?id=2439&L=0&htmfile=chapitre_aphanomyces_invadans.htm.

Fraser, GC, Callinan, RB, & Calder, LM, 1992, 'Aphanomyces species associated with red spot disease: an ulcerative disease of estuarine fish from eastern Australia', Journal of Fish Diseases, 15, pp. 173-81.

Diéguez-Uribeondo, J, Garcia, MA, Cerenius, L, Kozubíková, E, Ballesteros, I, Windels, C, Weiland, J, Kator, H, Söderhäll, K, & Martín, MP, 2009, 'Phylogenetic relationships among plant and animal parasites, and saprotrophs in Aphanomyces (Oomycetes)', Fungal Genetics and Biology, 46, pp. 365-76.