New Zealand rickettsia-like organism and Tenacibaculum maritimum vaccine efficacy study

A cohort of Chinook salmon juveniles was vaccinated, with an autogenous bivalent vaccine against New Zealand RLOs (NZ-RLO1) and Tenacibaculum maritimum. A proportion of the cohort was not vaccinated to act as controls. At smoltification, the fish were challenged with NZ-RLO1, NZ-RLO2, or T. maritimum. We found that challenge with T. maritimum by immersion in (7.5 × 10⁵  cfu/mL of water) did not yield any pathology. Challenge with RLOs produced clinical signs that were more or less severe depending on the challenge route, dose or vaccination status. Survival was significantly higher for vaccinated fish within the groups challenged with NZ-RLO1 by intraperitoneal injection with a relative percent survival (RPS) of 48.84%. Survival was not significantly different between vaccinated and non-vaccinated fish for groups challenged with NZ-RLO2 by intraperitoneal injection or by NZ-RLO1 by immersion. Yet, anecdotally the clinical disease presentation (manifesting as haemorrhagic, ulcerative skin lesions) was more severe for the non-vaccinated fish. This study demonstrates that autogenous vaccine against NZ-RLO is protective against severe disease and death by NZ-RLO1 challenge which warrants implementation and further evaluation under field conditions. Yet, this study also highlights the importance of the route of administration and dose when evaluating pathogenicity and vaccine efficacy.