The major route of transmission appears to be via infected fish and contaminated water (Hastings, 1988). Although the disease causes mortality of all ages, the most serious losses occurs during spring-autumn in the sea water farms. An important aspect of furunculosis is the carrier state, which is often established after the fish have been exposed to A. salm. Clinical outbreaks and mortality appear to be triggered by stress factors such as crowding, poor water quality, fright, high temperature and physical trauma (Ellis, 1997).

While Aeromonas salmonicida was traditionally thought of as a pathogen of Salmonids, global reports now confirm that this pathogen has been associated with clinical or covert disease in a variety of Salmonid and non-salmonid species in freshwater, brackish water and sea water.

The geographical distribution of A. salmonicida subsp. Salmonicida is almost worldwide, including Japan and the mainland of Asia. The possible exceptions to this distribution are South America and New Zealand, from which reports of the isolation of A. salmonicida have yet to be made. To date, there have been no reports of 'typical' Furunculosis in Salmonids in Australia, despite many attempts to isolate the organism (Bernoth, 1997a). Atypical A. salmonicida was, however, identified from diseased goldfish in South Africa.

Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.