New Methods Could Control Fish Disease

According to Business World Online, these methods could boost production and reduce economic losses in the marine fish industry, scientists Leobert D. de la Peña and Rolando Pakingking, Jr. said in abstracts of their studies.

Viral nervous necrosis (VNN), otherwise known as viral encephalopathy and retinopathy (VER), has over the last two decades continued to inflict serious economic losses among high value marine fish species not only at the larval and juvenile but importantly at the grow-out stages, Mr Pakingking wrote.

At the hatchery, Mr de la Peña cited an urgent need for preventive measures that include the selection of disease-free broodstocks, disinfection of eggs to control the vertical transmission of the virus from broodstocks and vaccination to enhance fish immunity.

The eggs were artificially infected with the virus and were either washed with ozone-treated seawater or immersed in iodine and Virkon. The eggs were checked for the virus before and after infection as well as after disinfection using the polymerase chain reaction (PCR) test and cell culture.

Mr de la Peña said his experiments showed that the effective time exposure and concentrations using ozone-treated seawater, iodine and Virkon were 2.5 minutes, 25 parts per million (ppm) and 1.5 ppm, respectively.

Mr de la Peña’s study also determined that the groupers (E. fuscoguttatus and E. coioides) were susceptible to the virus. The milkfish (Chanos chanos) and siganid (Siganus guttatus) were not.

Grouper larvae weighing eight grams and below were highly susceptible with a mortality rate of 80-100 per cent in four to eight days. Fish weighing 90 grams and above are no longer susceptible to the virus.

The most susceptible size of the grouper was then vaccinated. The vaccine, which was prepared from DNA plasmid encoding the capsid protein of the virus, was intramuscularly injected to the fish at 5, 10 and 100 nanograms (ng). Challenge tests using intramuscular injection of the virus were conducted after 15, 30 and 45 days from the vaccination.

Mr de la Peña reported that the results showed that fish vaccinated with 100 ng had the highest survival rate.

Meanwhile, Mr Pakingking and colleagues developed a formalin-inactivated vaccine to control the disease at the grow-out stage. Three studies pertaining to the efficacy of the vaccine were conducted from June 2007 to November 2009 under the project "Fish Disease Surveillance System" funded by the Government of Japan Trust Fund 4.

The vaccines were tested on sea bass (Lates calcarifer), grouper (Epinephelus fuscoguttatus), and pompano (Trachinotus blochii).

The first study, which was published in the Journal of Fish Diseases 2009, focused on the immune responses of the Asian sea bass to a formalin-inactivated betanodavirus vaccine.

The fish produced neutralizing antibodies at high titer levels from Day 10 to Day 116, with the highest titer at Day 60 post-vaccination. No mortalities were reported after the vaccinated fish were challenged with the red-spotted grouper nervous necrosis virus (RGNNV, a betanodavirus).

"The present results indicate the potential of the formalin-inactivated RGNNV vaccine against viral nervous necrosis of Asian sea bass," the abstract stated.

The second study, which is still under review, dealt with the vaccination of the brown-marbled grouper (Epinephelus fuscogutattus) while the third study, the manuscript of which is still being prepared, was conducted to determine the susceptibility of hatchery-reared pompano (Trachinotus blochii) to betanodavirus and its immune responses to a formalin-inactivated betanodavirus vaccine.

AQD chief Joebert D. Toledo said the Japanese evaluators were pleased with the development of a vaccine against VNN.

"They [Japanese evaluators] also said that research is a difficult process but that the AQD has done it in the ’correct, step-by-step process,’" Mr Toledo said.