Molecular biological tools to detect fish pathogens *

Multiple approaches to control viral infections in fish are being employed on an experimental basis in many fish disease laboratories. They include techniques to monitor fish populations for viruses by tissue culture infectivity, tagged antibody reagents to detect viral proteins, and nucleic acid probes for in situ hybridization or polymerase chain reaction amplification. The specificity and resolution of these detection methods are being constantly improved to increase their ease‐of‐use and sensitivity. In addition, scientists are developing prophylactic treatments in the form of traditional vaccines and subunit, peptide and genetic vaccines using molecular biological techniques. The success of all these approaches is obviously dependent on an understanding not only of the molecular structure of the virus and its genome but on the pathogenic mechanisms that lead to disease in the host animal as well. It is at this level, where there is so little known, that the formulation of appropriate control strategies has been difficult. For example, molecular techniques have provided evidence that the survivors of infection with infectious haematopoietic necrosis virus are long‐term carriers of the virus. This finding raises questions regarding the policy of releasing anadromous fish that have survived the disease. Viral vaccines have been shown to work in preventing virus‐induced mortalities in rainbow trout fry in laboratory trials, but no determination has been made on whether vaccination also prevents the formation of a virus‐carrier state in the survivors. More importantly, is there a vaccine formulation that will prevent carrier formation?