Previous research demonstrated that bacterial and
viral vaccines delivered via the nasal route in rainbow trout (Oncorhynchus mykiss) at 7 and 28 days post-vaccination are highly protective (>95% protection). Long-term protection following nasal vaccination in teleosts has not been evaluated. The goal of this study was to assess efficacy and immune responses at 6 months (mo) post-vaccination (mpv), and long-lasting immune responses at 12 mpv of two different
vaccines: an inactivated enteric red
mouth disease (ERM) Yersinia ruckeri
bacterin and a live attenuated infectious hematopoietic necrosis virus (IHNV)
vaccine. Juvenile rainbow trout were vaccinated for Y. ruckeri via intraperitoneal (I.P.) and intranasal (I.N.) routes, and for IHNV by intramuscular (I.M.) and I.N. routes, then challenged at 6 mpv. Immune responses were determined at 6 and 12 mpv. ERM
vaccine I.P. delivery elicited significantly higher serum
IgM-specific titers that remained elevated compared to mock-vaccinated fish at 6 mpv. By 12 mpv, antibody titers to Y. ruckeri were not significantly different across all treatments. Following Y. ruckeri challenge at 6 mpv, a significant difference in cumulative percent mortality (CPM) was found for I.P.-vaccinated fish but not I.N.-vaccinated fish. I.M. and I.N. vaccination with live attenuated IHNV did not result in significant specific serum
IgM titers at 6 or 12 mpv. Yet, I.N.-vaccinated fish showed the lowest CPM 6 mpv indicating long-term protection that does not correlate with systemic
IgM responses. Repertoire analyses confirmed unique expansions of VH-JH rearrangements in the spleen of rainbow trout 12 mpv that varied with the type of
vaccine and route of vaccination. Combined, these data demonstrate that I.N. vaccination with a live attenuated
viral vaccine confers long lasting protection, but I.N. ERM vaccination does not and booster before 6 mpv is recommended.