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Novel virus detected in ballan wrasse may explain fry mortality

Friday, December 18, 2020

PatoGen, in collaboration with a farmer, discovered a novel virus in ballan wrasse fry which is associated with high mortality rates shortly after hatching. The virus was named ballan wrasse birnavirus (BWDV).

 

PatoGen has been working closely with a producer of ballan wrasse in Norway that has experienced increased mortality rates shortly after hatching, in an attempt to clarify possible causes. 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 and were examined with PCR for relevant pathogens and histology. PCRs did not detect any virus and histology gave no explanatory findings. Furthermore, production-related, environmental and feed-related mortality causes were ruled out. 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 of the Birnaviridae family. The gene sequences of the novel ballan wrasse virus were genetically different from other known birnaviruses and aquabirnaviruses.

 

The most known birnavirus in Norwegian aquaculture is the aquabirnavirus, infectious pancreatic necrosis virus (IPNV), known to cause fry mortality in Atlantic salmon. Furthermore, other birnaviruses are associated with early fry mortality on other marine fish species.

 

A Real-Time qPCR assay was developed based on the gene sequences produced by the full genome sequencing and it was used to investigate the presence of the virus. 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 believes that biosecurity measures against BWDV will reduce mortality in this critical phase of the ballan wrasse production. PatoGen is now offering a PCR analysis for the ballan wrasse birnavirus.

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