Epidemiology of infectious hematopoietic necrosis disease in net-pen reared Atlantic salmon in British Columbia, Canada

The epidemiology of infectious hematopoietic necrosis viral disease (IHN) in net-pen reared Atlantic salmon was investigated using a combination of study designs including outbreak investigations, laboratory studies, and surveillance. The knowledge gained from this study was useful in designing prevention strategies to minimize the occurrence of the disease.; A ribonuclease protection assay analysis suggested that IHNV isolates collected from farmed Atlantic salmon over a six year period were genetically similar. Investigation of the spatial and temporal pattern of IHN outbreaks between 1992 and 1996 suggested that the virus spread from one farm to another.; A surveillance program designed to detect exposure to IHNV and new outbreaks of disease found a reversal in a four-year rising incidence rate of IHN once an area management plan, aimed at reducing the probability of farm to farm spread of IHNV, was implemented. Furthermore, there was no evidence of exposure to IHNV on farms for at least 10 months after sites were fallowed. These findings further supported the hypothesis that the majority of farm outbreaks were related.; Laboratory exposure studies and outbreak investigations suggested that once the virus was present on a net-pen site the fish were infected for several months. This long infective period, combined with the high frequency of fish and equipment transfers between sites in a small area, made farm to farm spread of the virus highly probable.; It is possible that IHN will re-occur in the Atlantic salmon industry in B.C. as the virus is endemic in the region; however, it may be possible to minimize the number of affected sites by preventing the farm to farm spread of the pathogen. In this particular study preventing the secondary spread of the pathogen could have prevented the majority of cases. In order to minimize the magnitude of the outbreaks in the future it is important that if IHNV is detected on a salmon farm, the site is isolated and the fish are culled or harvested as soon as possible to reduce the likelihood of spreading the virus to other sites.