Red spot disease

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Beginning of infection. Red spot lesions: Small area of reddening over single scale a ‘red-spot' (circled in red).
Moderately ulcerated fish: ‘Red-spot’ expands and deepens
Severely ulcerated fish: ulceration with loss of scales and skin, exposing underlying muscle.
Epizootic ulcerative syndrome in silver perch. Note classic red sores on body.

Epizootic ulcerative syndrome (EUS) or 'red spot disease' is a disease that can affect many species of fish. Red spot disease is known to be endemic in a number of waterways in NSW. In 2008 reports of red spot disease were confirmed in the Darling River. In 2009 it was confirmed in Port Stephens and the Clarence River and reported from the Richmond, Manning, Macleay and Tweed Rivers.

Many fish species are known to be susceptible to the disease including bony bream, silver scat, sole, bream, mullet, whiting, dusky flathead, silver trevally, eels and catfish.

Cause

A pathogenic fungus, Aphanomyces invadans causes EUS. Infection occurs when motile spores in the water are attracted to the skin of fish. The spores penetrate the skin and germinate, forming fungal filaments or hyphae. The hyphae invade widely into the surrounding skin and deeply into underlying muscle tissues, resulting in extensive ulceration and destruction of tissues. Low water temperatures depress the immune system of fish, which may partially explain the apparent dry season occurrence of EUS. There appears to be no definite relationship between dissolved oxygen levels and EUS; affected fish have been seen in water with dissolved oxygen levels of between 2 to 8 ppm[1].

Previous outbreaks of red spot disease have been associated with acid water run-off, particularly after heavy rain following a prolonged a dry spell, as well as other factors such as prolonged cold temperatures, crowding, and conditions associated with drought.

Clinical signs

The initial sign is usually mass mortality associated with distinct dermal lesions including ulcers. Surviving fish typically have lesions of varying degrees of severity. These may appear as red-spots, blackish burn-like marks, or deeper ulcers with red centres and white rims. Some fish, especially snakeheads survive a long time with such ulcers, which may erode so deeply as to expose the vertebrae, brain and viscera.

Histological occurrences include necrotising, granulomatous dermatitis and myoositis associated with invasive, non-septate fungal hyphae, 10-20 um in diameter. The fungus may penetrate visceral organs, such as the kidney and liver, after it has spanned the musculature.

Diagnosis

Diagnosis is based on clinical signs and histological evidence of the typical aggressive invasiveness of the non-septate fungal hyphae, within the context of high mortalitiy. Isolation of the fungus allows its characteristic growth profile to be used as an aid to identification.

The fungus can be isolated and cultured without difficulty, provided measures are taken to exclude bacterial and other fungal contaminants. Besides their typical vulnerability to temperature above 30°C, A invaderis and A. piscicida are very similar to non-pathogenic opportunistic Aphanomyces spp. that readily contaminate the surface of affected fish and often interfere with isolation attempts. In the laboratory, the fungus has also been shown to be pathogenic to a wide range of fish, inducing similar pathology and mortality under various predisposing experimental conditions.

Treatment

There are no specific control measures in fish for EUS in natural environments. Fish from infected waterways, especially those with lesions of EUS, should not be relocated to other waterways. In captive fish, early 'red-spot' lesions may respond to topical treatment with an antiseptic iodophore solution. Increasing salinity of holding waters may prevent outbreaks of EUS in aquaculture ponds. Severely ulcerated fish should not be eaten, and not be returned to the river or waterway.

Healthy fish with no sign of red spot disease can still be caught and consumed by recreational fishers and caught and sold by licensed commercial fishers.

References

  1. Humphrey, JD & Pearce, M (2010) Epizootic Ulcerative Syndrome (Red-spot Disease)
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