306 FISH HATCHERY MANAGEMENT 



reservoirs of infection. Furunculosis may break out in virtually any fresh- 

 water fish population, including warmwater species, if conditions such as 

 high temperature and low dissolved oxygen favor the pathogen. 



Among the eastern salmonids, brook trout are the most susceptible to in- 

 fection, brown trout are intermediate, and rainbow trout are least suscepti- 

 ble. Atlantic salmon also are susceptible. Furunculosis has been reported in 

 most of the western salmonids. In addition to salmonids, the disease has 

 been reported in many other fishes, including sea lamprey, yellow perch, 

 common carp, catfish, northern pike, sculpins, goldfish, whitefish, and vari- 

 ous aquarium fishes. 



Sanitation provides the most important long-range control of furuncu- 

 losis. If a population of trout at a hatchery is free of furunculosis and if the 

 water supply does not contain fish that harbor the pathogen, strict sanita- 

 tion measures should be used to prevent the introduction of the disease via 

 incoming eggs or fish. Eggs received at a hatchery should be disinfected 

 upon arrival. lodophors used as recommended are not toxic to eyed eggs 

 but are highly toxic to fry. 



Maintenance of favorable environmental conditions for the fish is of 

 prime importance in preventing furunculosis outbreaks. Proper water tem- 

 peratures, adequate dissolved oxygen, efficient waste removal, and 

 avoidance of overcrowding must be observed. In areas where the disease is 

 endemic, strains of trout resistant to furunculosis are recommended. How- 

 ever, regardless of the trout strain involved, acute outbreaks of furunculosis 

 have occurred when conditions favored the disease. 



Sulfamerazine (lO grams per 100 pounds of fish per day) in the diet has 

 been the standard treatment of furunculosis for years. In recent years, be- 

 cause of sulfa-resistant strains of A. salmonicida, Terramycin (3.6 grams 

 TM-50 or TM-50D per 100 pounds of fish per day for 10 days) has be- 

 come the drug of choice. Furazolidone (not registered by the Food and 

 Drug Administration) has been used successfully under experimental con- 

 ditions against resistant isolates of the bacterium. Furox 50 (also not reg- 

 istered) at 5 grams active ingredient per 100 pounds fish per day has been 

 used successfully under production conditions with Pacific salmon. Drugs 

 are effective only in the treatment of outbreaks. Recurrences of furuncu- 

 losis are likely as long as A. salmonicida is present in the hatchery system 

 and environmental conditions are suitable. 



ENTERIC REDMOUTH (ERM) 



Enteric Redmouth disease refers to an infection of trout caused by an enteric 

 bacterium, Yersinia ruckeri. Initially, the disease was called Redmouth; later 

 the name Hagerman redmouth disease (HRM) was used to differentiate 

 between infections caused by Yersinia and those caused by the bacterium 



