Protection of Rainbow Trout from Experimentally Induced Bacterial Gill Disease Caused by<i>Flavobacterium branchiophilum</i>

Abstract

Rainbow trout Oncorhynchus mykiss from the same genetic pool and varying in size from 10 to 400 g (a) were challenged with broth cultures of Flavobacterium branchiophilum and the survivors were held for approximately 2 months, (b) were injected intravenously with rainbow trout anti-F. branchiophilum or rainbow trout anti-sheep red blood cell hyperimmune serum, (c) were vaccinated by bath or the intraperitoneal route two or three times with acetone-killed F. branchiophilum, or (d) were used as unprotected challenge controls. These groups of fish were then challenged with various doses of broth cultures of F. branchiophilum. Those challenged with large doses (8 × 1010 colony-forming units, cfu) of bacteria displayed mortality rates similar to that of unprotected controls. As the challenge dose decreased (from 2 × 1010 to 2 × 109 cfu), the cumulative percent mortality of controls decreased. The cumulative percent mortality of groups that had been previously exposed to live F. branchiophilum, had received F. branchiophilum-specific serum intravenously, or had been bath-vaccinated declined to a proportionately greater extent than unprotected controls. Fish that were bath-vaccinated three times were almost completely protected from experimental challenge. A quantitative enzyme immunoassay for F. branchiophilum antigen revealed significantly (P < 0.05) decreased antigen loads on the gills of fish that had been vaccinated-compared with loads on untreated fish-only during the first 3 d following challenge. No significant (P > 0.05) differences in antigen load were seen in fish that had been previously exposed nor in those that had received intravenous rainbow trout anti-F. branchiophilum antiserum.