New aquatic pathogen detected

A novel virus, which is associated with high mortality shortly after hatching, has been discovered in farmed juvenile ballan wrasse.

The new virus causes peak mortalities in farmed wrasse 17-25 days after hatching. Photo: PatoGen

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).

Samples were collected weekly after hatching and were examined with PCR for relevant pathogens and histology. PCR produced no detections and histology gave no explanatory findings. This was consistent with previous investigations from similar cases at the site. It was therefore decided to perform a full genome sequencing on samples from moribund fish. The analysis revealed high amounts of gene sequences of a novel virus that grouped within the virus family Birnaviridae. 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.

A real time qPCR assay was developed, based on the gene sequences produced by the full genome sequencing and this was used to investigate the presence of the virus. 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.

PatoGen is now offering a PCR analysis for the ballan wrasse birnavirus and asks wrasse breeders to contact its key account managers should they require more information about the virus.