The primary precursor to disease in wild populations is usually stress from physical or chemical factors such as lowered water temperature, rapid changes in salinity, chronic pollution or crowded conditions. In many cases the disease-causing parasites, viruses, microbes or fungi are present in low levels even in healthy populations, with environmental conditions or other stressors reducing the resistance to disease and making the fish susceptible to infection.

Australia is relatively free of many of the major fish diseases present overseas. Introduction of new diseases, for example via imported products, can lead to increased deaths in native species which have no exposure and hence little resistance to the new disease. For example, the much publicised death of millions of pilchards in coastal waters of southern Australia in May 1995 is thought to have been due to a newly introduced viral disease (see Whittington 1996 (www.int-res.com)).

Diseases in Abalone

Perkinsus

The protozoal parasite Perkinsus has had a devastating effect on abalone stocks around Port Stephens, Terrigal, Sydney and Kiama. In some areas, it has caused abalone numbers to drop to just 5% of their former populations. The major risk to abalone occurs when the infected animals are stressed or if the parasite is introduced into a new population. The Perkinsus parasite is not harmful to people.

To help abalone stocks recover, a total ban has been placed on the taking of abalone from these areas until November 2007.

• Summary of Proceedings from the Perkinsus Workshop held at the Cronulla Fisheries Centre on 3 September 2003
• See Legislation – declared diseases and quarantine orders for details of the abalone fishing closure.

Ganglioneuritis

Another abalone disease, a herpes-like virus that causes ganglioneuritis (inflammation of the nervous tissue, resulting in curling and paralysis of the foot and swelling of the mouth), has caused devastating losses to the Victorian abalone industry. It has not currently been recorded from any other State, though there is potential for it to spread.

• New closure to help keep abalone disease out of NSW
• Information on ganglioneuritis – Victorian Department of Primary Industries (www.dpi.vic.gov.au)

Diseases in Wild Fish

Redspot Disease

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 coastal waterways in NSW. In 2009 reports of red spot disease have been confirmed in Port Stephens and the Clarence River and reported from the Richmond, Manning, Macleay and Tweed Rivers. 

2008 was a particularly bad year for red spot disease, with reports of ulcerated fish and confirmation of this disease in a number of estuaries including the Manning River in February, the Wisemans Ferry area of Hawkesbury River in March and in Myall Lakes in September. Red spot disease was also reported from a number of mid-north coast NSW estuaries in 2008, including Macleay, Richmond, Clarence, Hastings and Wallamba Rivers. In addition to these coastal reports of red spot disease, fish sampled between Bourke and Brewarrina in the Darling River during May 2008 were also diagnosed with EUS.

EUS is caused by a fungus (Aphanomyces invadans) and shows as red lesions (sores) or deep ulcers Secondary bacterial infections are often also associated with red spot disease.

Red spot disease is reasonably common in NSW coastal catchments and has been previously reported in many freshwater catchments and estuaries throughout Australia, including NSW, Queensland, Western Australia and the Northern Territory. 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.

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.

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.

SEVERELY ulcerated fish should:

• Not be eaten (on advice from the NSW Food Authority)
• Not be returned to the river or waterway
• Be euthanased by quickly removing the head (following immersion in ice slurry if possible)
• Be disposed of in landfill away from any waterway.

EUS Stages

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.

NSW DPI monitors the occurrence red spot disease, as well as other aquatic diseases . Please report any suspected occurrences of red spot disease to NSW DPI by phone on 02 4916 3877 (recorded 24 hour service) or by e-mail to aquatic.pests@dpi.nsw.gov.au

Bony Bream from the Darling River at Bourke showing red spot disease lesion (picture R. Reece).