In intensive large scale aquacultures where tilapia are reared at high densities, the tilapia is prone to various health problems since pathogens can be so easily transmitted between individuals. The risk is elevated if the keeper of the aquaculture fails to provide the tilapia with optimal conditions, e.g. when it comes to water quality, temperature and salinity. A proper diet is also highly important when it comes to boosting the immune system of tilapia and other fish. Disease can however break out even when tilapia is kept in a suitable environment and fed a proper diet and this is why so much effort have been focused on developing vaccines for fish in aquacultures during the last decades. Vaccines are not only helpful for the individual fish; they can also lead to a significantly decreased use of antibiotics in aquacultures. Extensive use of antibiotics is problematic since it promotes the development of bacteria resistant to antibiotics. Many consumers are also concerned about possible antibiotic residue in fish meat.   

Commercial vaccines are today available for several of the most commonly farmed food fishes, including Atlantic cod, European seabass and seabream, Japanese amberjack and yellowtail, channel catfish, salmon, trout, and tilapia.   

Vaccines and how they work

Vaccines are used to improve the body's immunity against particular diseases. Successfully surviving an infectious disease will often lead to immunity since the immune system will recognize the pathogen if it enters the body again. (In viruses, it will for instance recognize the protein coat on the virus.) A vaccine is a relatively safe way of exposing the body to a pathogenin order to let the immune system learn. When the body is exposed to the pathogen later on, the immune system will recognize and neutralize the pathogen before it gets a chance to enter the cells. The immune system will also be able to rapidly identify cells that have already been infected and destroy them in order to prevent the pathogen from multiplying in the body.

The vaccines we use today are based on the ancient Chinese practise of inoculation. In China, physicians prevented severe cases of smallpox in their patients by making them snort powdered scabs of smallpox victims or scratch the powder into their skin. During early the 18th century, inoculation was introduced to Europe by Lady Mary Wortley Montagu. Lady Mary witnessed inoculation in Constantinople where her husband served as the British ambassador to the Ottoman Empire. Impressed with the results, Lady Mary had her four-year-old daughter inoculated after returning to England. In 1722, the Prince of Wales had his daughters inoculated and the practise began to spread among the royal families of Europe.

During the late 18th century, English scientist Edward Jenner realised that milkmaids who had been exposed to cowpox never fell ill with smallpox. The notion that cowpox provided protection against smallpox was not an uncommon observation among European farmers at that period and several English and German people have been recorded to have vaccinated with cowpox in order to ward off smallpox. In 1796, Edward Jenner introduced a vaccine made from cowpox. Using cowpox was of course much less dangerous than using real smallpox.

Herd immunity

Vaccination will protect not only the vaccinated individuals but also decrease the risk of non-vaccinated individuals from falling ill. As long as a high enough percentage of a population is vaccinated, it will be hard for a disease to spread since it will be difficult for it to find suitable hosts. This protective effect is called her immunity. This means that even if a vaccine fails to reach 100% of your tilapia population it can still decrease the risk of disease in non-vaccinated fish as long as the percentage of vaccinated fish is high enough.

Fish vaccines

As explained above, a vaccine will not kill any pathogens on its own; it will only help the immune system of the host to effectively protect against the disease. Vaccines are therefore only as effective as the immune system of the host. Crustaceans such as shrimps, molluscs such as oysters and aquatic plants such as seaweed are all being grown in commercial aquacultures, but they can not be vaccinated using traditional vaccines since they do not have an adapted or specific immune system. Fishes are however equipped with an immune system that can make use of vaccines and can therefore be vaccinated just like humans. In general, vaccines based on inactivated bacteria have turned out to be highly efficient when used in fish. When it comes to vaccines against viruses, the number of available fish vaccines is much more limited. As of today, there are no fish vaccines available against parasites.   

In the 1970s, immersion vaccines based on formalin-inactivated broth cultures was proven effective against vibriosis. Vibriosis is caused by bacteria of the genus Vibrio, a group of gram-negative bacteria typically found in saltwater. Many species of bacteria in the genus Vibrio are zoonotic (affecting animals) and a common cause of mortality among domestic marine life. Once the immersion vaccines based on formalin-inactivated broth cultures had been proven effective, similar vaccines were developed against Vibrio-disease in salmons. As a result, the use of antibiotics decreased dramatically for farmed salmon.  

In the early 1990s, injectable vaccines containing adjuvants became available to combat Aeromonas salmonicida caused furunculosis in fish. Furunculosis is an acute skin disease where a large number of boils appear on the skin of infected fish. Aeromonas salmonicida has been recognized as a pathogen of fish for over a century and is one of the most studied fish pathogens in the world. It can lead to mass losses for fish farmers and salmonids seem to be particularly sensitive to the bacterium. The first authentic report of its isolation was made by Emmerich and Weibel 1894 when they studied an outbreak at a Bavarian brown trout hatchery.

In the 1990s, five multinational animal health companies began to dominate the fish vaccine market through acquisitions and joint ventures. Earlier, most fish vaccines had been developed and marketed by small local companies, but during this decade they were purchased by larger corporations or commenced collaborations with them through joint venture companies. Today, the five major producers of fish vaccines are Norwegian Pharmaq (once a part of  Alpharma Animal Health), Swiss Novartis Animal Health, Dutch Intervet International, German-Canadian Bayer Animal Health (Bayotek)/Microtek. Inc and United States based Schering Plough Animal Health.

The most important markets for fish vaccines are salmon, trout and catfish farms, but commercial vaccines are also available for other species, particularly European seabream, seabass and tilapia. If you want to obtain vaccines for less commonly farmed species of fish, animal health companies operating in Germany, Spain, Russia and China are known to have locally developed vaccines for sale. For fish species commonly farmed in Japan, Japanese animal health companies are normally your best bet since fish vaccines used in Japan tend to be both developed and marketed by domestic companies. It is naturally important to make sure that the fish vaccine you wish to use is legal in your particular part of the world.

Fish vaccine administration

When a dog or a cat needs a shot we simply bring it to the vet, but this option is not available for large scale fish farmers who needs to vaccinate thousands or even millions of fishes. Oral vaccines hidden in food are convenient, but conventional oral vaccines are not very efficient since the antigens tend to be broken down in the gut of the fish. Several methods are currently being investigated that might be able to increase the efficiency of oral vaccines. It might for instance be a good idea to neutralize the gastric secretions of the fish when the vaccine is administered or trap the antigens in liposomes or alginate beads. Biofilm vaccines have also showed promising results. 

How to achieve optimal effects of vaccines

Using efficient vaccines and administering them correctly is not the only factors affecting the effects of vaccines. A fish farmer that wishes to achieve optimal vaccine effects must also engage in proper fish management. Proper conditions and adequate nutrition are very important and one must also strive to expose the fish to as little stress as possible. The efficiency of a vaccine largely depends on the condition of the immune system and exposing fish to factors that might harm their immune system are therefore highly unadvisable.

A pathogen is a biological agent that causes disease or illness to its host. It can for instance be a virus or bacterium. 

Adjuvants are pharmacological or immunological agents capable of modifying the effect of other agents, such as drugs or vaccines. Adjuvants are virtually useless if given alone, but can serve to make a vaccine much more effective. When given together with a vaccine, the adjuvant will stimulate the immune system and increase its response to the vaccine. Exactly how adjuvants work remains unknown. Aluminium salt, virosomes and certain oils are all commonly used as adjuvants for vaccines.