By Charmain Tan Courcelle

MISSISSIPPI STATE -- Vaccine and pharmaceutical combinations are being used in aquaculture production as the first line of defense against disease-causing organisms.

Sometimes catfish producers lose some of their stock to disease because an effective treatment does not exist or resistance has developed to available drugs. Scientists at Mississippi State University's College of Veterinary Medicine, in association with the Mississippi Agricultural and Forestry Experiment Station, are using biotechnology to develop and test vaccines against some important catfish diseases.

The new vaccines may provide animal health benefits and increase catfish production, which are two of MAFES' research goals.

One control agent being developed is a live-attenuated vaccine for the bacterium Edwardsiella ictaluri, the cause of enteric septicemia of catfish. This type of vaccine is a live bacteria modified in the laboratory so it is unable to cause disease.

ESC is a major economic problem for the catfish industry, causing annual losses on nearly all catfish farms. Fingerlings infected with the bacteria have a characteristic "hole-in-the-head" lesion that appears as a red ulcer between the eyes. Other symptoms include large red or white skin lesions, a distended abdomen, bulging eyes and pale-colored gills.

ESC reduces catfish feeding, and once fish are sick, there is no way to treat them with oral antibiotics. Because ESC-infected catfish are more vulnerable to other diseases, the preferred treatment is prevention through vaccination and improved management.

"The first vaccine that was developed against ESC was a killed vaccine," Mark Lawrence, CVM researcher, said. "This treatment gave inconsistent results, so it never gained acceptance among producers."

Catfish treated with killed ESC vaccine developed antibodies to a membrane sugar found on the bacteria. But because the bacterium lives part of its life cycle inside a type of host catfish cell where it is hidden from these antibodies, even when catfish produce high antibody numbers, they are not always protected from ESC, Lawrence said.

Defense against pathogens that live within cells relies on the activity of T-cells, a type of white blood cell, Lawrence explained. Antigens &endash; proteins from invading bacteria or viruses &endash; are present on the surfaces of infected cells. T-cells recognize these antigens and cause a chain of events clearing the host of infection. Once an animal has been exposed to a pathogen, its body develops "immunological memory" that protects against later infections.

Unlike killed vaccines that rely on dead microorganisms, live-attenuated vaccines can reproduce within a vaccinated animal's cells and stimulate T-cell immunity without actually causing disease, Lawrence said. This type of vaccine strategy may help to control ESC.

To develop a live-attenuated vaccine, Lawrence is first identifying and characterizing some of the genes responsible for E. ictaluri virulence. He has developed a modified form of the bacteria that is less able to cause ESC disease in catfish. This less-virulent mutant could serve as a potential live vaccine.

"We've tested one mutant of E. ictaluri in catfish and observed that it takes 100,000 times more bacteria to cause ESC death using this mutant E. ictaluri compared to wild-type (normal) E. ictaluri," Lawrence said.

He is now working to further define how certain genes contribute to the bacteria's ability to cause disease. This will help researchers make better selections in vaccine design.

CVM scientist Larry Hanson is taking a different approach in the fight against ESC. He is using channel catfish viruses to deliver pieces of E. ictaluri that may induce immunity in catfish.

"Our big focus has been to identify protective antigens (proteins that cause a good immune response) of E. ictaluri and move these into channel catfish virus," Hanson said.

In 1996, Hanson and a graduate student built a less virulent channel catfish virus and he is trying to add to this vaccine an antigen that causes immunity to ESC as well.

"By carrying a small fragment of E. ictaluri DNA in with channel catfish virus, we may be able to express bacterial proteins that stimulate T-cell immunity," Hanson said. "This form of immune response is important for clearing this bacterium."

Hanson has developed a genetic tool to screen for potential protective antigens that could induce an effective immune response against ESC. He has some encouraging results, and it appears he will one day be able to transmit ESC immunity to catfish using channel catfish virus as a vaccine carrier.

For more information, contact: Dr. Mark Lawrence, (662) 325-1195 or Dr. Larry Hanson, (662) 325-1202