Comparative evaluation of booster vaccine efficacy by intracoelomic injection and immersion with a whole-cell killed vaccine against Lactococcus petauri infection in rainbow trout (Oncorhynchus mykiss)

Authors: DE RUYTER, TRYSSA - University Of California, Davis; LITTMAN, ERIC - University Of California, Davis; YAZDI, ZEINAB - University Of California, Davis; ADKISON, MARK - California Department Of Fish & Game; CAMUS, ALVIN - University Of Georgia; YUN, SUSAN - University Of California, Davis; Welch, Timothy - Tim; KELEHER, WILLIAM - Kennebec River Biosciences; SOTO, ESTEBAN - University Of California, Davis

Submitted to: Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2023
Publication Date: 4/22/2023
Citation: De Ruyter, T., Littman, E., Yazdi, Z., Adkison, M., Camus, A., Yun, S., Welch, T.J., Keleher, W.R., Soto, E. 2023. Comparative evaluation of booster vaccine efficacy by intracoelomic injection and immersion with a whole-cell killed vaccine against Lactococcus petauri infection in rainbow trout (Oncorhynchus mykiss). Pathogens. (12)5:632. https://doi.org/10.3390/pathogens12050632.
DOI: https://doi.org/10.3390/pathogens12050632

Interpretive Summary: Picine lactococcosis, caused by Lactococcus petauri, is a rapidly emerging disease of farmed rainbow trout in the United States. Recent outbreaks in California caused severe losses and ultimately required the depopulation of three trout hatcheries. Effective disease control methods are critically needed to control this disease. This report describes the development and validation of a safe and effective L. petauri vaccine consisting of autologous killed cells combined with an adjuvant. The vaccine provided suitable protection from experimental challenge when delivered by injection and moderate but significant protection when delivered by immersion. These results suggest that injection vaccination of this vaccine should aid it the control of picine lactococcosis and reduce the likelihood of further pathogen dissemination in the United States.

Technical Abstract: Lactococcus petauri is an important emergent bacterial pathogen of salmonids in the USA. The purpose of this study was to evaluate protection conferred to rainbow trout (Oncorhynchus mykiss) against L. petauri by formalin-killed vaccines in immersion and injectable forms, as well as enhanced protection afforded by booster vaccination. In the first challenge, fish were immunized via immersion (IM) or intracoelomic injection (ICi) routes alone. Approximately 418 degree days (dd) IM, or 622 dd ICi post-vaccination, fish were challenged via ICi with wild-type L. petauri. In the second experiment, initial IM vaccination was followed by booster vaccination via IM or ICi routes 273 dd post-immunization along with appropriate PBS controls. The various vaccination protocol efficacies were evaluated by challenging fish with L. petauri by cohabitation with diseased fish 399 dd post-booster administration. A relative percent survival (RPS) of 28% and 89.5% was recorded in the IM and ICi single immunization treatments, respectively. In the second study, an RPS of 98%, 14%, 3%, and -8% plus approximately 0%, 50%, 20%, and 30% bacterial persistence were recorded in the IM immunized + ICi boosted, IM immunized + mock ICi boosted, IM immunized + IM boosted and IM immunized + mock IM boosted treatments, respectively. Only the IM immunized + ICi injection boosted treatments provided significant protection when compared to unvaccinated and challenged treatments (p<0.05). In conclusion, although both IM and ICi vaccines appear to be safe for trout, inactivated IM vaccines seem to provide only mild and temporary protection against lactococcosis; whereas ICi-immunized trout develop a significantly stronger protective response in both challenges.