Vaccination is a crucial strategy for protecting tilapia from streptococcosis. There are currently various commercial vaccines available for tilapia. However, there are limitations in the use of vaccines, especially concerning the vaccination method and the timing of vaccine administration to fish. While injection is the most effective method, it can be challenging to apply. Therefore, oral and immersion methods have gained attention as they, though less effective, are easier to administer.
The immersion vaccination method is more practical, cost-effective, causes less stress to organisms, requires less labor, and is highly recommended for small-sized fish. However, it presents some disadvantages related to the amount of vaccine to be used and the most appropriate timing for its application.
In this regard, researchers from Kasetsart University, the Asian Institute of Technology, the National Center for Genetic Engineering and Biotechnology (BIOTEC), and Mahidol University sought to determine the best timing for vaccination and how it affects fish’s immune responses.
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Timing is everything
The researchers conducted a comprehensive study with Nile tilapia at different stages of their early development, from 1 to 42 days after yolk sac collapse (DAYC). They immersed these fish in a vaccine composed of formalin-killed Streptococcus agalactiae (FKV-SA) to investigate the effectiveness of vaccination at different ages.
The results were illuminating. It was observed that specific IgM, a crucial antibody in immune responses, was first detected in fish at 21 days, and this response began 336 hours after vaccination. However, in fish aged 28 to 42 days, a faster IgM response was observed, detectable as early as 24 hours after vaccination.
“It is possible that fish in this age group have more developed immune systems, allowing them to respond more effectively to vaccines than fish at 21 days,” the study reports.
The study’s findings suggest that the timing of vaccination plays a crucial role in the strength and speed of the immune response in these fish.
Beyond antibody responses, the researchers also examined the gene expression related to the fish’s immunity. The study included an analysis of TCRβ, CD4, MHCIIα, IgHM, IgHT, and IgHD genes in fish aged 21 to 42 days, immunized with FKV-SA through immersion.
The findings were remarkable, as most of the genes related to the immune system showed significantly higher expression in vaccinated fish across all age groups compared to non-vaccinated fish. This highlights the vaccination’s ability to activate crucial immune genes.
Strengthening immune defense
Immunohistochemistry, a technique for visualizing the distribution of antibodies in tissues, showed that fish vaccinated at 21, 28, and 35 days exhibited stronger signals of IgM in gills, head kidneys, and intestines compared to non-vaccinated fish. This underscores the capacity of vaccination to reinforce immune defenses in these vital tissues.
Enhanced survival rates
Perhaps the most promising discovery was that fish vaccinated with FKV-SA, regardless of their age, showed significantly higher survival rates when challenged with live Streptococcus agalactiae, in contrast to non-vaccinated fish. This effect was particularly pronounced in fish immunized at 168 and 336 hours after vaccination. The increase in relative percent survival (RPS) was notable, demonstrating the real-world benefits of vaccination in improving the fish’s chances of survival against a common pathogen.
This study sheds light on the optimal periods for vaccinating Nile tilapia, with a strong emphasis on the crucial role of timing. “Administering the S. agalactiae vaccine to Nile tilapia larvae through immersion can initially induce specific IgM and the expression of immune-related genes in fish at 21 days after yolk sac collapse,” the researchers conclude.
By adapting vaccination strategies to the age of the fish, we can enhance their immunity and mitigate the impact of diseases. Based on the results of their study, the researchers recommend that the optimal vaccination period for Nile tilapia larvae falls between 28 and 35 days after yolk sac collapse.
The study was funded by Kasetsart University through the Graduate School Fellowship Program and BIOTEC’s Fellow Research Grant.
National Center for Genetic Engineering and Biotechnology (BIOTEC)
Reference (open access)
Kumwan, B.; Bunnoy, A.; Chatchaiphan, S.; Kayansamruaj, P.; Dong, H.T.; Senapin, S.; Srisapoome, P. First Investigation of the Optimal Vaccination Timing for Nile Tilapia (Oreochromis niloticus) Larvae against Streptococcus agalactiae. Preprints 2023, 2023092037. https://doi.org/10.20944/preprints202309.2037.v1