A novel virus, which is associated with high mortality shortly after hatching, has been discovered in farmed juvenile ballan wrasse. Discovered by PatoGen, in collaboration with a Norwegian wrasse farmer, the condition has been named ballan wrasse birnavirus (BWDV). The diseased fish were investigated with PCR for relevant pathogens and histology, without detecting any obvious causes. Furthermore, production, environmental and food related causes for the mortality was ruled out. As a result, PatoGen performed a full genome sequencing on diseased fish in which they detected the genome of a novel birnavirus. Downstream PCR studies detected large amounts of virus in fish larvae from groups with increased mortality. PatoGen believes that biosecurity measures against BWDV will reduce the mortality in this critical phase of wrasse production.
The mortality occurs after a short period of loss of appetite, and peak mortality typically occurs between 17 and 25 days post-hatching (DPH). The gene sequences of the novel ballan wrasse virus were genetically different from other known birnaviruses and aquabirnaviruses.The most common birnavirus in Norwegian aquaculture is an aquabirnavirus; infectious pancreatic necrosis virus (IPNV), which is known to cause fry mortality in Atlantic salmon. Other birnaviruses are associated with early fry mortality in other marine fish species.
The PCR analyses confirmed the presence of the virus in large amounts from day 21 after hatching and at later timepoints. The investigation showed that the virus infection is maintained in the population for a long time after the peak mortality and at high levels.