FISH HEALTH

Fish age and vaccination route influence IHNV vaccine effectiveness in rainbow trout

Trucha arcoíris viva

The success of vaccination strategies, particularly against infectious haematopoietic necrosis virus (IHNV) in rainbow trout (Oncorhynchus mykiss), largely depends on selecting the appropriate age for first immunisation and the route of vaccine administration.

IHNV is one of the most significant viral pathogens affecting salmonid aquaculture, capable of causing mortalities of up to 90% in juvenile fish and generating substantial economic losses for fish farms.

In a study published in the journal Vaccine, researchers led by Irene Salinas from the Center for Evolutionary and Theoretical Immunology at the University of New Mexico (United States) examined how fish age at first vaccination and the administration route influence the effectiveness of vaccination strategies against the virus.

The researchers evaluated several prime–boost vaccination combinations using a live attenuated IHNV vaccine in rainbow trout vaccinated at 1000 and 1500 degree-days, representing two different developmental stages. Three administration routes were compared: immersion, intranasal delivery and intramuscular injection, applied in different priming and booster combinations.

The results showed that intranasal vaccination used both for priming and boosting provided the highest levels of protection, achieving survival rates of around 97–98% following experimental viral challenge.

The study also revealed that the route used for the initial vaccination determines the effectiveness of the booster dose, highlighting the importance of carefully designing vaccination protocols in aquaculture.

When fish were first vaccinated by immersion — a method commonly used in early life stages because of its practical application — boosting through intramuscular injection produced better outcomes than repeating immersion vaccination.

The researchers also observed age-related differences in immune responses. Fish vaccinated at earlier developmental stages produced higher levels of virus-specific antibodies than those vaccinated later in the production cycle.

According to the authors, this phenomenon may be linked to morphological changes that occur as fish grow. As trout develop, the skin and mucosal tissues become thicker, which may reduce antigen uptake when vaccines are administered through mucosal routes such as immersion or intranasal delivery.

Overall, the findings suggest that optimising both vaccination timing and administration routes could significantly improve protection against IHNV, a disease that continues to represent an important health challenge for salmonid aquaculture.

Reference:

Abouismail, A., Sapayeva, S., Erhardt, E. B., Maxwell, R., Cox, M., Ma, J., & Salinas, I. (2026). Priming route and age at first vaccination shape prime and boost vaccine responses in rainbow trout (Oncorhynchus mykiss). Vaccine, 78, 128403. https://doi.org/10.1016/j.vaccine.2026.128403