|[Image: Photo of striped bass]|
Two serious problems affecting Louisiana aquaculture are the bacterial pathogens that cause enterics epticemia of catfish (ESC) and photobacteriosis in hybrid striped bass. ESC causes the loss of millions of dollars annually in the catfish industry. Photobacteriosis restricts the development of hybrid striped bass culture in coastal Louisiana and causes financial losses in the Mediterranean region and Japan.
The only tools for controlling these diseases are antibiotics used after clinical signs appear. Because the antibiotics are delivered in feed, and because sick fish often cease feeding, the “treatment” actually prevents disease in the fish not yet infected. Effective vaccines, on the other hand, act to prevent disease by stimulating the natural immune system of the fish. The immune system becomes“primed” in vaccinated fish, increasing the ability of the immune system to recognize and kill the bacterium before establishment of disease.
Researchers previously vaccinated against ESC and photobacteriosis by immersing fish in a bacterial cell suspension that was killed by the addition of formalin. Vaccination by injection, the method of choice for human and animal medicine, is not economically feasible for catfish or hybrid striped bass because of the small size at vaccination, the large numbers of fish and the relatively low value per individual fish. Protection from disease using these killed vaccines, however, was very weak, partly because of the poor uptake of killed vaccines by immersion. Other studies indicated that both bacteria can survive and grow within living cells in the fish. By “hiding” in cells, the bacteria are able to avoid the parts of the immune system that are most strongly activated by killed vaccines, resulting in a weak protective response.
To induce a protective immune response, it was apparent that a larger dose of vaccine needed to be delivered, and that it needed to be delivered in a way that more closely mimicked the natural, intracellular location of the infection. Because both of the bacteria that cause ESC and photobacteriosis are capable of rapid invasion of the fish from the water, live vaccine strains that retain this invasive capability will “inject” large doses of vaccine. If the live vaccine also retains its intracellular residence, it will stimulate the parts of the immune system that are important in controlling bacteria that can hide in host cells. The combination should result in a safe, effective vaccine that will protect fish from disease.
With support from the Louisiana Catfish Promotion and Research Board, Alpharma Inc. inSeattle, the Louisiana Education Quality Support Fund, the National Sea Grant Program and the U.S. Department of Agriculture (USDA), we have successfully developed and are evaluating live vaccine strains that prevent ESC and photobacteriosis. Using molecular genetic techniques, we deleted from each bacterium a gene required for growth. Its deletion does not affect the bacteria’s ability to invade fish and establish an initial infection within host cells. The bacteria can infect the fish and distribute themselves in the cells, but their growth is limited because of the deleted gene. As a result, the fish clear the bacteria from their bodies within 48 hours to 72 hours. alarge portion of the gene, the vaccine strain can not revert to the wild, disease-causing strain.
During the abortive “infection” with the live vaccine strain, the fish immune system interacts with the bacteria as it would in the early stages of a natural infection, stimulating the part of the immune system required to protect against intracellular bacteria. Essentially, we have create daversion of the bacteria that “injects” itself, but does not cause disease. In our experiments, we have demonstrated that vaccination with the vaccine strains result sin significant protection from disease after exposure to the wild, virulent bacteria. The ESC vaccine is approved for field testing by the USDA, and experiments are under way to generate data to support a full vaccine license application to the USDA. Preliminary data for the photobacteriosis vaccine are almost completed, and an application for field testing will be submitted to the USDA in 1999.
Ronald L. Thune, Professor, Departments of Veterinary Science and Veterinary Microbiology and Parasitology, and John P. Hawke, Assistant Professor, Veterinary Microbiology and Parasitology, School of Veterinary Medicine, Baton Rouge, La.