Aqua: US researchers in vaccine breakthrough

IN preliminary trials, NOAA Sea Grant researchers have for the first time demonstrated the feasibility of using a live-attenuated vaccine to prevent the deadly Streptococcus iniae infection in fish.

The success raises the possibility of being able to inoculate hybrid striped bass, tilapia, rainbow trout and other cultured species orally through feed, instead of having to inject individual fish – a prohibitively labour-intensive process for American farms.

Besides the economic benefits, live-attenuated vaccines also stimulate a more robust immune response than vaccines from killed pathogens, thus offering better protection from infection, said John Buchanan, a former researcher at the University of California at San Diego.

There are currently two vaccines on the market for preventing S. iniae infections – AquaVac Garvetil and Norvax Strep Si. Both are classical vaccines based on exposing fish to killed versions of bacterial pathogens. However, neither is approved for use in the United States, Buchanan said. In addition, AquaVac is for use in tilapia only, and Norvax is most effective when fish are immersed in a 60-second dip initially; subsequent booster doses can be delivered orally.

The vaccine that Buchanan and UC San Diego pediatrics professor Victor Nizet are testing, in collaboration with Kent SeaTech, is based on mutating genes of the bacterial pathogen – not on killing the pathogen outright. These mutants have weakened virulence, but they can still infect fish, eliciting a strong adaptive immune response, in which antibodies to the real pathogen are created.

In the trials so far, their vaccine has been administered through injection, which means that each fish has to be given a shot. However, as Jim Carlberg, president of Kent SeaTech, emphasised: “The beauty of live-attenuated vaccines is that you have the potential to put the vaccine in feed.” The key is to be able to mutate a gene that does not wipe out the weakened pathogen’s ability to orally infect the animal.

“A vaccine that can be put in feed would have a huge potential advantage in cost,” Carlberg said.
“Oral delivery is the gold standard for aquaculture,” agreed Jeff Locke, a doctoral student with Nizet, who used to work at Kent SeaTech, a large hybrid striped bass farm in Southern California.

S. iniae is a ubiquitous disease and a fairly chronic problem,” Carlberg said. “It has a huge economic impact on worldwide aquaculture.”

About 26 species of fish are susceptible to S. iniae, which causes meningitis.

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Aqua: US researchers in vaccine breakthrough