Immunization Aids Disease Fight

By Guy Tebbit.

Large scale intensive culture of fish by State and Federal agencies and the rapidly growing private aquaculture industry have increased our awareness of the prevalence and significance of fish diseases. Economic importance of these diseases has emphasized the need for rapid and accurate disease diagnosis as well as economical and efficacious disease control methods and products.

Intensive rearing of large numbers of fish under conditions of high density and stress has produced ideal situations for the onset and spread of disease. In some cases, fish diseases may be the limiting factor in the success or failure of an aquaculture operation.

Fish cultural management techniques and the strict control of transportation of diseased animals play major roles in preventing the spread of fish diseases. However, in many situations despite concerted efforts to prevent health problems at the management level infectious diseases are prevalent and endemic. Effective control measures are then required to reduce impact of the problem.

First step in fish disease control is accurate diagnosis of the problem. With infectious disease, this must consist of a precise identification and a thorough characterization of the microbial pathogen responsible. Fish health specialists then can use this information to decide the most effective and efficient methods of disease control and prevention.

Antibiotics and chemotherapeutics have been the principal products used to control fish diseases. Under certain conditions these products have been very successful in treating diseased fish, and chemotherapeutics are the only products available to control diseases caused by protozoan parasites.

However, antibiotics and chemotherapeutic treatment provide only short term control of disease. After treatment, fish are susceptible again to the same or other diseases. Strict governmental regulation and limitation of these compounds for use in food fish, and the fact that some microbes causing bacterial diseases have become resistant to antibiotic treatments, produced the need for other methods of disease control.

Vaccination

Immunization is a common method used to control and prevent disease in human and veterinary medicine. Scientists have known for many years that fish can be successfully immunized against certain infectious diseases. The major problem in developing fish vaccines was the requirement for a mass method capable of immunizing large numbers of fish in a safe, expedient and economical manner. Obviously, hand injecting the millions of fish that are reared in large aqua-culture facilities was not practical.

The problem was solved in 1975 with development of the immersion method of vaccination. It consists of simply placing the fish in diluted vaccine and letting them swim in the vaccine for several seconds. This method is now automated and has been used successfully to immunize hundreds of millions of fish. There is evidence that vaccine uptake occurs through the gills and a large lymphatic vessel associated with the lateral line of the fish.

Fish, like humans and other animals, are able to develop a specific acquired immunity and also possess nonspecific defense mechanisms against infectious diseases. Vaccination stimulates specific acquired immunity, and the fish respond to vaccination by developing specific antibodies to components of the vaccine. These components are usually killed preparations of one or more disease micro-organisms.

Lasts About Year

Antibodies produced by the fish after vaccination circulate through their blood and protect them against infection from the specific disease or diseases. Antibodies also are found in the mucus covering the skin of the fish and could be the first line of defense against infection. In general, it is believed that immunity after vaccination lasts about one year. Several factors such as species and size of fish, vaccine doses, water temperatures, stress, and other environmental parameters influence the onset and duration of immunity.

Fish vaccines are considered veterinary biological products and are regulated by the U.S. Department of Agriculture (USDA). In 1976 USDA issued the first two product licenses for fish vaccines. Besides licensing products, USDA also regulates strict quality control measures to insure that each lot of vaccine is tested for safety, potency and purity. Since 1976 three additional product licenses for fish vaccines have been issued.

The five licensed vaccines for fish are administered by immersion, automated immersion, spray and injection methods. A brief description of the administration methods, the five licensed products, and their applications follows.

Methods Used

Immersion. Fish are placed in containers containing aerated and diluted vaccine at a ratio of 1 pound of fish per liter of diluted vaccine for a 5- to 60-second exposure. The same container of diluted bacterin can be reused for 20 consecutive immersions. Usually 20 to 40 pounds of fish weighing from 5 to 15 grams each are vaccinated in each immersion.

Automated Immersion.

Here a machine performs the immersion process. Fish are loaded onto a rubber conveyor belt that transports them through a reservoir of vaccine and back to the holding pond via a return slide. Fish are loaded into the unit by net or automated fish pumps. The auto-immersion unit is lightweight, compact, mobile, and designed for large aquaculture facilities that vaccinate thousands of pounds of fish a day. The auto-immersion unit can vaccinate up to 200 pounds of fish a minute.

Spray. Diluted vaccine is sprayed over fish as they are removed from their holding area. Simple contact with the vaccine applied without significant pressure suffices. The process also has been mechanized, and the aquaculturist can devise several ways of using it. Lack of complete and controlled exposure can lead to operator error during large scale vaccination procedures and result in inconsistent exposure.

Injection. This can be used for large valuable fish such as brood fish. The method is made quicker and easier with automatic repeating syringes. A skilled operator can inject 600 to 800 fish an hour. Before injection the fish are usually anesthetized by swimming in a solution of a recommended anesthetic. The injection is given intraperitoneally, just before the pelvic fins. Injection administration allows the vaccine manufacturer to add adjuvants to the products compounds that stimulate and enhance the fish's immune response to the vaccine.

Products

1) Enteric Redmouth Bacterin, recommended for use in salmonid fishes (salmon and trout) to protect against enteric redmouth disease (ERM) caused by the bacterium Yersinia ruckeri. The product is used primarily by agencies or companies raising rainbow trout.

The bacterin is administered by immersion, auto-immersion, or spray. In a study at a commercial trout farm involving 27 million fish, the fish vaccinated by immersion demonstrated 84 percent reduction in ERM mortality, 77 percent reduction in medicated feed (antibiotic) requirements, and 14 percent improvement in food conversion when compared to nonvaccinated control fish.