Israeli virologists have conducted a study to investigate the antibody response and disease resistance of carp exposed to Koi Herpes Virus and KHV vaccines.
The scientists experimentally infected fish with either the wild-type virus or the attenuated-virus in the KHV vaccines and measured their antibody response and their resistance to the disease.
Both groups of fish developed an immune response which made them resistant to further exposure to the virus, which is now officially known as Cyprinid Herpes Virus 3 (CyHV-3).
Those fish inoculated with the virus and vaccine developed anti-CyHV-3 antibodies, but the number of these gradually decreased, and at 280 days after exposure they contained levels only slightly higher than those seen in naiive fish - the term used to describe fish which haven't been exposed.
The protection against the virus is proportional to the amount of anti-virus antibodies in the fish, with those fish that have experienced recent exposure to the virus or vaccine being better equipped to fight off further infections than those who were exposed long before.
However, the study claims that even immunised fish, with antibodies that are no longer detectable, remain resistant to the virus, an effect the scientists believe to be caused by the rapid response of high affinity anti-virus antibodies.
"The fact that anti-virus antibodies neutralize in vitro the pathogenic effects of the virus emphasizes the central role probably played by the antibodies in anti-CyHV-3 protection in vivo," the authors wrote.
The study, which was undertaken by Ayana Perelberga, Maya Ilouzeb, Moshe Kotlerb and Michael Steinitz from Israel's Ministry of Agriculture and Rural Development and The Hebrew University, has just been reported in the journal Vaccine.
Is the vaccine safe?The news will add to the confusion and debate regarding the safety of fish vaccinated with the attenuated virus.
Suppliers of Israeli Koi have been vaccinating their fish with attenuated vaccines, which are based on live but less virulent viruses, rather than inactivated or 'dead' viruses, and shipping them around the world with claims of resistance to KHV.
However, it is legal to import vaccinated stock, it is illegal to administer the vaccine in this country, and concerns have been raised over the safety of the fish.
A previous paper by Perelberg, Kotler and co-authors in 2005 claimed that the attenuated virus was safe, however, others have been skeptical. (See KHV vaccine is safe says study)
In 2006, the Institute of Fisheries Management (IFM) released a position statement on KHV advising fisheries against fish that had been vaccinated against the disease. (See IFM warns against KHV vaccine)
It said: "The technology used to develop this vaccine is unproven, and there is a risk that the live viruses in the vaccine will revert back to a full pathogenic Koi Herpesvirus.
"The vaccine is also unlikely to confer protection on the fish longer than six months, making it inadequate long term protection."
Koi Herpes Virus affects Koi and Common carp and has been responsible for a number of mass mortalities around the world.
For more information see the paper: Perelberg A, Ilouze M, Kotler M, Steinitz M (2008) - Antibody response and resistance of Cyprinus carpio immunized with cyprinid herpes virus 3 (CyHV-3). Vaccine. 2008 May 12.