Piscirikett siosis caused by P. salmonis is the leading cause of mortality in Atlantic salmon (S . salar) in Chile, where two genotypes of the bacteria (LF–89 and EM–90) have been described. Both genotypes have been associated with outbreaks in S. salar. The current vaccination strategy in Chile uses vaccines that only incorporate the EM–90 genotype in their formulations, this has led to an increase in clinical cases in the field caused by isolates belonging to the LF– 89 genotype. This has led to a decrease in the field efficacy of vaccines for the prevention and control of Piscirikettsiosis. For this reason, the present study proposes to evaluate the establishment of immunity from an inactivated microdose vaccine formulated with isolates of both genotypes of the bacteria.
For the study, three prototypes were evaluated (A) lower dose; (B) medium dose or (C) Higher dose and a control group (PBS) in duplicate and after period of 650-degree days (DD), they were challenged by IP injection with heterologous inoculum of P. salmonis genotype LF–89 and EM–90 (2.5–5.0E+05 CFU/fish). In terms of efficacy, for the challenge with the LF–89 isolate, an RPS60 of 100% was obtained for prototypes A, B and C and RPS end point of 60% for prototype A and 75% for prototype B and C, At 28 days post–challenge. In the case of the challenge with the EM–90 isolate, an RPS60 of 100% was obtained for prototypes A, B and C and an RPS end point of 50% for prototype A, 75% for prototype B and 65% for prototype C, At 28 days post–challenge. In addition, fish vaccinated with prototypes A, B and C showed higher values of Ct compared with control group. In order to validate the long– term protection of the vaccine, prototype B was evaluated at 1600 DD, followed by a IP challenge with of P. salmonis genotype LF–89 and EM– 90 showing a protection of 100% (RPS60 and RPS end point) for EM– 90 challenge, and 89.25% and 90.15% for RPS 60 and RPS end point respectively for LF–89 challenge. The present study provides valuable insights into the development of vaccines against P. salmonis in Atlantic salmon. Our results clearly showed that the P. salmonis microdose vaccine prototype, formulated whit both genotypes (LF–89 and EM–90), is effective in protecting S. salar against Piscirikettsiosis.