This experiment, infecting fish with the VHSV, is the first to be performed using Atlantic herring. There are various types of VHS, called genotypes, and there are considerable differences between them. Herring at approximately 3 grams were bathed in two different types of infectious VHS: one was genotype III that had been isolated from herring in Denmark; the other was an isolate of genotype Ib from herring found in Norwegian waters. Genotype 1b proved fatal for almost half (47%) of the herring tested. Infection experiments with the genotype III virus, however, proved fatal for only 6% of fish tested.
- Tissue samples confirm that the herring died of VHSV, and this raises many new questions about how this virus may affect Norwegian herring stocks, says researcher Renate Johansen at the National Veterinary Institute in Oslo.
VHSV is known to cause mortality in many different fish species in many parts of the world. The virus may cause high mortality to some species, especially rainbow trout. In some cases, significant mortality has also been observed in wild fish. The virus is not dangerous to humans, however.
Can High Mortality be followed by Immunity?
The Pacific herring, a different species than Atlantic herring, has undergone more extensive research on the effects of VHSV. Infection experiments on Pacific herring have shown that VHSV can cause very high rates of mortality in populations that have been exposed to the virus earlier on; while low rates of mortality have been observed when the same populations have been newly-infected. This suggests that immunity may develop in such herring populations.
- In earlier studies we found only small amounts of VHSV in North Sea herring. Therefore, it was surprising when we found large amounts of the virus in Norwegian spring-spawning herring during the 2010 spawning season, says researcher Øivind Bergh at IMR. Almost half of the herring examined in 2010 had genotype Ib VHSV.
Virus - a Part of Nature
- Herring is very difficult species to maintain in laboratory facilities, and we are delighted to have successfully completed infection experiments using them, says Bergh. He believes the results provide new insights into the importance of this virus in marine ecosystems.
- We know that different types of viruses play an enormously important role in regulating populations of bacteria, algae, and other micro-organisms in the ocean. Although we are aware of many viruses and bacteria that can kill fish, we have remarkably little knowledge about epidemics in the ocean. Here we have a virus that attacks fish, and data suggest that it occurs naturally in large and important fish stocks, says Bergh.
- Although the virus can cause mortality, it is necessary to ask how other factors such as climate variability, food availability, and population size can also affect both herring stocks and spread of the virus. It is also important to remember that both the disease and the virus are a part of nature, he emphasized.
Possible Infection from Wild to Farmed Fish
Previous infection experiments using both Ib and III genotypes have shown little receptivity in both salmon and rainbow trout, but in nature mortality has been observed. The same type of VHSV demonstrated to affect Norwegian herring (genotype Ib) also caused mortality in farmed-raised rainbow trout in Sweden. Another example is the outbreak of VHSV in rainbow trout in Storfjorden during 2007, which demonstrated VHSV (genotype III) to be not completely harmless for farm-raised Norwegian fish.
- Since genotype alone does not provide a clear answer to how deadly VHSV is for various fish species, it is necessary to conduct control experiments, says Johansen. She emphasizes that thus far surveillance and disease control programs have not shown VHSV (genotype Ib) to occur in Norwegian farmed-raised fish, however we still cannot rule out the possibility that salmon and rainbow trout are susceptible. VHSV has not been detected in Norwegian farm-raised fish since 2008, and Norway has again been certified as a VHSV-free zone even though VHSV occurs in wild fish along the Norwegian coast.
Much Left to Learn
- Through the Norwegian Research Council-funded project "VHSV-Wild-Farmed" efforts are underway to obtain biological samples from approximately 3000 fish from over forty different fish species caught along the Norwegian coast from 2009 through 2011. Cultivations in cell culture have detected VHSV only in a few herring, when testing with more sensitive molecular methods such as the polymerase chain reaction (PCR) process a lot more pisitive fish were detected. However, we expect to find a lot more of both VHSV and other viral pathogen through examining this very valuable material, says Øivind Bergh. IMR will continue its cooperation with the National Veterinary Institute to clarify the importance of the VHSV in both wild and farmed-raised fish.
Testing for VHSV infection was funded by the EU project NADIR (Network of Animal Disease Infectiology Research), and conducted at the Network of Animal Disease in Aarhus, in cooperation with the National Veterinary Institute in Oslo, and the Institute of Marine Research in Bergen.