FISH HEALTH MANAGEMENT 31 1 



boil, red plague, or salt water furunculosis. Vibrio anguillarum is now con- 

 sidered to be the etiologic agent of the disease. Although vibriosis generally 

 is a disease of cultured marine fishes, it also occurs in wild populations. It 

 can occur any time of year, even in water temperatures as low as 39°F. 

 However, it is most prevalent in the temperate zones during the warmer 

 summer months and epizootics can be expected when water temperatures 

 reach 57°F. 



Signs of the disease usually do not become evident until the fish have 

 been in salt water for two weeks or more under crowded conditions. Di- 

 minished feeding activity is one of the first noticeable signs. Lethargic fish 

 gather around the edges of holding units; others swim in erratic, spinning 

 patterns. Diseased fish have hemorrhages around the bases of their pectoral 

 and anal fins or a bloody discharge from the vent. When a fish is opened 

 for necropsy, diffuse pin-point hemorrhages of the intestinal wall and liver 

 may be evident. The spleen frequently is enlarged and may be two to three 

 times its normal size. 



Diagnosis of vibriosis caused by V. anguillarum requires isolation of a 

 gram- negative, motile, rod-shaped bacterium on salt medium. The organ- 

 ism may be slightly curved and produces certain biochemical reactions 

 under artificial culture. There is no reliable presumptive diagnosis of vi- 

 briosis because of its similarity to other septicemic diseases caused by 

 gram- negative bacteria. 



The organism is ubiquitous in marine and brackish waters and infections 

 probably are water-borne and may be spread by contact. Salmonids usually 

 die within 1 week after exposure; fish of all ages are susceptible. 



Vibriosis is worldwide in its distribution, but it usually is most severe in 

 mariculture operations. Virtually all species of marine and estuarine fishes 

 are susceptible. Among salmonids, pink salmon and chum salmon are the 

 most susceptible but serious epizootics have occurred in coho salmon, rain- 

 bow trout, and Atlantic salmon. Stresses associated with handling, low oxy- 

 gen, and elevated temperature predispose fish to vibriosis. 



Prevention of vibriosis depends on good sanitation, no crowding, and 

 minimal handling stress. Immunization is an effective means of combatting 

 the disease. Bacterins now are available from commercial sources and ap- 

 pear to provide long-term protection. Hyperosmotic procedures utilizing 

 bacterins appear most suitable for large numbers of small fingerlings. Injec- 

 tions may be preferable for larger fish. In theory, long-term selection and 

 breeding for resistance to the bacterium may be a means of control. 



Sulfamerazine (registered by the Food and Drug Administration) used at 

 the rate of 17 grams per 100 pounds of fish per day for 10 days has con- 

 trolled vibriosis. Terramycin (also registered) at 5.0 to 7.5 grams per 100 

 pounds of fish per day for 10 days also has been successful. 



