VACCINATION IMPACT ON STREPTOCOCCAL IMMUNITY IN TILAPIA

Streptococcosis is one of the most important bacterial diseases in tilapia and other warm water fish species. To control Streptococcus infection, inactivated vaccine has been used in tilapia culture in Thailand. Like higher vertebrates, teleost fish posses the innate and adaptive immunity, thus allowing them to develop specific memory response after repeat antigen exposure. The main objective of vaccination is to protect individual animals and better improve herd immunity. After vaccination, most fish develop specific immune response with homogenous immunological status. The application of Streptococcus vaccine provides benefit to the tilapia farmers and improves food safety through reduction of antibiotic usage.

Nile tilapia is one of the most cultured fresh water species in Thailand and Southeast Asia countries. In Thailand, tilapia have been stocked in the open waters such as reservoirs, lagoons, and rivers. This practice allows fish to be exposed to various pathogens. Streptococcal infections by S. agalactiae and S. iniae are one of the most important bacterial diseases that affect tilapia culture in Thailand. To control Streptococcosis, inactivated vaccines have been developed and commercialized as a tool for disease prevention. Like higher vertebrates, the fish immune system relies on the innate and adaptive immunity to fight pathogens. The appearance of adaptive immunity and lymphocyte allows application of vaccine in fish. It is noticeable that fish develop memory response after antigen exposure. As such, the second response to a similar antigen is faster, higher magnitude and lasts longer than the primary antigen exposure.

The specific purpose of vaccination is to provide protection to individual animals, but it can benefit entire population by improving herd immunity. The concept of herd immunity is to vaccinate all animals in the same population to improve herd immunological status (Fig 1). Prior to vaccination, herd immunity is quite low with heterogenous immunological status. After vaccination, most fish develop immunity e.g. against Streptococcus bacteria.  Interestingly, it has been shown that Nile tilapia develop specific immunoglobulin against Streptococcus that last up to 180 days post vaccination. This duration of immunity could stay up to the entire tilapia production period. In summary, the applicable of Streptococcus vaccine has benefit to the tilapia farmers by reducing fish losses from infection and to improve food safety through reduction of antibiotic usage and drug residue.