FISH HEALTH MANAGEMENT 305 



contaminated with or exposed to the bacterium at one time or another. 

 The causative agent of the disease is Aeromonas salmonicida. Today, furuncu- 

 losis is enzootic in many hatcheries but severe outbreaks are rare due to 

 advances in fish culture, sanitation, and drug therapy. Outbreaks have 

 been reported among marine fishes. 



The disease is characterized by a generalized bacteremia with focal 

 necrotic lesions in the muscle, often seen as swellings under the skin and 

 not true furuncles (Figure 80). The swollen skin lesions are filled with pink 

 fluid containing blood, and necrotic tissue may have a purple or irridescent 

 blue color. These lesions are especially apparent in chronic infections but 

 similar lesions may occur from other diseases caused by gram- negative bac- 

 teria. Hemorrhaged fin sockets and frayed dorsal fins also are common. 



The disease frequently occurs as an acute form in which death results 

 from massive bacteremia before gross lesions can develop. Only a few clini- 

 cally sick fish may be seen at any one time in spite of the high death rate. 



Internally, diseased fish may exhibit small inflamed red lesions called 

 petechiae in the lining of the body cavity and especially on the visceral fat. 

 The pericardium usually is filled with bloody fluid and is inflamed. The 

 spleen, normally dark red in color, often will be a bright cherry- red and 

 swollen. The lower intestine often is highly inflamed and a bloody 

 discharge can be manually pressed from the vent. 



A diagnosis of furunculosis can be either presumptive or confirmed. 

 Presumptive diagnosis takes into consideration the frequency of outbreaks 

 in a certain area, presence of typical lesions, and the occurrence of short 

 gram-negative rods in the lesions, kidneys, spleen, and blood. Confirmation 

 of a presumptive diagnosis can be made only after Aeromonas salmonicida 

 has been identified as the predominant organism isolated. 



Furunculosis is endemic in many hatcheries and is so widespread that no 

 natural waters with resident fish populations should be considered free of 

 this disease. The incidence pattern of furunculosis generally follows the 

 seasonal temperature pattern. Almost twice as many cases are reported in 

 July as in any other month. The number of cases drops sharply in August, 

 possibly indicating increased resistance in the remaining fish population or 

 death of most of the susceptible fish. 



Acute cases of furunculosis have incubation periods of 2-4 days with few 

 apparent signs. Chronic cases usually occur at temperatures below 55°F 

 and may have an incubation period of one to several weeks, depending 

 upon the water temperature. Latent cases may develop during low- 

 temperature periods, and flare up with greater severity, displaying many 

 typical signs, when water temperatures rise. 



Fish exposed to furunculosis form protective antibodies. Some fish be- 

 come immune carriers of the disease. Suckers and other nongame fish in 

 the water supply may become infected and should be considered likely 



