First Live-Attenuated Vaccine for S. Iniae in Development

January 14, 2008

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

Are cabezon a single, genetically homogenous unit or are they composed of multiple, genetically distinct populations? A mitochondrial DNA study suggests this commercially exploited nearshore species – the largest of the region’s sculpin fishes – may be a single genetic unit.

Cabezon (Scorpaenichthys marmoratus).
Credit: Joanna Grebel (www.mlml.calstate.edu/groups/ich/grebel.htm)

“To date, we have not observed significant genetic differences in cabezon from different locations,” said

biology professor Royden Nakamura of Cal Poly San Luis Obispo, who is collaborating with conservation biology professor Francis Villablanca, also of Cal Poly on the California Sea Grant/California Department of Fish & Game study. “There is gene flow among fish from different locations, more so than might have been expected.”

“If I were a betting man, at the beginning I would have bet that we would have seen differences,” he said, “but we are not done with the research. We have only one piece of information.”

That one piece of information is a mitochondrial DNA analysis of tissue samples from 244 cabezon from eight locations between Puget Sound, Wash. and San Diego, Calif. Mitochondria are “the power houses” of a cell. They produce a form of energy called ATP. Mitochondrial DNA (mtDNA) is inherited exclusively through the maternal line.

“We see different ranges of variations in mtDNA at different locations,” he said. “But not enough to exclude a particular location, or array of locations, from all the others.” In other words, there is no one area in which cabezon are reproductively isolated from the rest.

“Our work supports the idea that currents redistribute larvae and mix populations,” he said. When cabezon are in their planktonic stage, they end up wherever ocean currents take them.

We might have expected to see genetic differences because cabezon live in a very heterogeneous environment compared to open-ocean species,” he said. “Many nearshore species are connected to reefs, kelp beds or embayments. Superimposed on this are larger-scale breaks in topography such as Point Conception and Puget Sound.”

The cabezon’s life history characteristics would also seem to predispose it to mate with the same groups of individuals and hence to form sub-populations. Adult cabezon don’t move around much and have strong fidelity to home sites. “We have caught cabezon, moved them two miles and seen them return to within 60 meters of where they were first caught,” Nakamura said, referring to an ongoing “translocation” study.

It might be that cabezon sub-populations do exist but have yet to be documented. To examine this possibility, researchers are conducting a microsatellite DNA analysis of genetic regions with dramatically faster mutation rates than mtDNA. Sea Grant Trainee Enos Kline said, “The microsatellite regions might have sufficient resolution to detect the formation of distinct populations.”

“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 in fish. Infected fish are anemic-looking, Buchanan, who is now the chief of finfish research at Aqua Bounty Technologies in San Diego, said. They can swim abnormally and have “popeye,” caused by swelling in the brain.

For U.S. farms, there is no satisfactory treatment for S. iniae. Infected fish can be fed antibiotics but sick fish often don’t eat, Carlberg said. To treat outbreaks, which tend to occur at facilities where warm-water fish are kept at high density in recirculating tanks, Kent SeaTech has developed its own killed vaccine for S. iniae. However, it too requires that fish be individually injected.

Sample sites in mitochondrial DNA study of cabezon. Click on image to view larger map.
Credit: Todd Olive.

Carlberg estimates that an orally delivered, live-attenuated S. iniae vaccine would generate revenues of about $10 million to $15 million annually worldwide. The savings to the global aquaculture industry could be worth 10 times this amount, he said.

Nizet, Buchanan and Kent SeaTech have patented the technology that makes their live-attenuated vaccine possible and entered into discussions with pharmaceutical companies about possible licensing agreements.

Meanwhile, researchers with funding from California Sea Grant continue to study genes involved with pathogenesis. Locke, for example, has identified two new S. iniae genes whose mutated forms represent potential new vaccines.

Scientists are hoping to present to a pharmaceutical partner a complete portfolio of potential mutants for development of a commercial product. “There may be one gene that contributes more to virulence than all the others,” Locke explained.

The USDA has already approved two other live-attenuated vaccines for aquaculture for other diseases, so there is optimism that a live-attenuated vaccine for S. iniae would clear regulatory hurdles.

Buchanan said that economics is the reason pharmaceutical companies have not pursued USDA approval for the two killed vaccines now used in Asia.

Royden Nakamura
Credit: Cal Ploy San Louis Obispo

“The cost of getting the vaccine approved in the United States is more significant than the size of the market,” he said. “Farms are allowed to make their own vaccines with bacterial isolates from their own facilities. That is what Kent SeaTech does.”

An effective oral vaccine that could treat a variety of fish species would have greater value to American farms and therefore industry would pursue USDA approval, Buchanan said. Because the USDA is considered relatively strict, its nod would grease the regulatory process in nations that otherwise might be afraid of allowing a vaccine based on an infectious agent. “A killed vaccine is the norm,” he said.

Kent SeaTech is optimistic about the research outcomes. “I think there is an 80 percent chance that a real commercial product will come out of this research,” Carlberg said.