A Novel Carp Vaccine for Cyprinid Herpesvirus-3

A group of US and Israeli researchers, supported by the BARD Fund, have made great progress in their study of the “cyprinid herpesvirus-3,” a major carp fish pathogen. Their research has led to the development of an effective vaccine for cyprinid herpesvirus-3, currently employed by Israeli fisheries.

The "common carp" (Cyprinus carpio carpio) fish is widely cultivated for human consumption, while the sub-species koi (Cyprinus carpio koi) is ornamental colorful fish, the focus of a hobby worldwide. According to the Food and Agriculture Organization of the United Nations, 3.4 million tons of carp are harvested annually, principally by China, other Asian countries, the US, and Europe. The carp industry suffers enormous economic losses due to a lethal disease caused by the virus "cyprinid herpesvirus-3" (CyHV-3); the disease causes the death of 80-100% of fish within 6 to 22 days of infection.
The CyHV-3 disease afflicts fish in the spring and fall, when the water temperature is between 18-28^oC. With the support of the BARD Fund, a team of scientists has been studying the method CyHV-3 uses to preserve itself between the transient seasons. To resolve this question, the researchers assessed how long CyHV-3 persists in cultured carp cells maintained at 30^oC. The researchers found that this temperature is sufficient for CyHV-3 infection of carp cells; however, the virus does not propagate, remaining dormant in the cell for a period of 30 days. Furthermore, the scientists observed that during the first period after infection, 89 out of the 156 CyHV-3 genes are expressed in the carp cell; this figure gradually declined, with only three genes expressed after 18 days. The scientists are now working to determine whether these three genes are involved in virus preservation during the latent period.
The scientists have also been working on a viable vaccine for CyHV-3. They have isolated a CyHV-3 clone that, while able to propagate in tissue culture and infected fish, is unable to induce the viral disease. Impressively, they found this clone to be an efficient prophylactic vaccine, endowing the carp fish with resistance to CyHV-3 infection for more than a year. This attenuated virus clone is currently used as an efficient, low-cost vaccine in most carp farms in Israel, thereby reducing the economic threat caused by CyHV-3.
The scientists found that the virus clone's genome bears a set of unique mutations, currently used as molecular markers to distinguish between regular and attenuated viruses. Further investigation of the genomic variation between the regular and clone CyHV-3 has led the scientists to identify a battery of non-essential genomic regions, primary among them is a gene that is dispensable for virus propagation in cultured cells, but is essential for virus proliferation in the living fish. The scientists believe that these non-essential genes may be instrumental in using the CyHV-3 clone as a vector for introducing vaccines into the carp fish.

The Core Group of Scientists:

• Moshe Kotler, The Hebrew University, Hadassah Medical School, Israel
• Larry Hanson,Mississippi State University, USA