RESUMO
In Atlantic salmon, vaccines have failed to control and prevent Piscirickettsiosis, for reasons that remain elusive. In this study, we report the efficacy of two commercial vaccines developed with the Piscirickettsia salmonis isolates AL100005 and AL 20542 against another two genogroups which are considered highly and ubiquitously prevalent in Chile: LF-89 and EM-90. Two cohabitation trials were performed to mimic field conditions and vaccine performance: (1) post-smolt fish were challenged with a single infection of LF-89, (2) adults were coinfected with EM-90, and a low level coinfection of sea lice. In the first trial, the vaccine delayed smolt mortalities by two days; however, unvaccinated and vaccinated fish did not show significant differences in survival (unvaccinated: 60.3%, vaccinated: 56.7%; p = 0.28). In the second trial, mortality started three days later for vaccinated fish than unvaccinated fish. However, unvaccinated and vaccinated fish did not show significant differences in survival (unvaccinated: 64.6%, vaccinated: 60.2%, p = 0.58). Thus, we found no evidence that the evaluated vaccines confer effective protection against the genogroups LF-89 and EM-90 of P. salmonis with estimated relative survival proportions (RPSs) of -9% and -12%, respectively. More studies are necessary to evaluate whether pathogen heterogeneity is a key determinant of the lack of vaccine efficacy against P. salmonis.
RESUMO
INTRODUCTION: Piscirickettsia salmonis (P. salmonis) is the aetiological bacterium of the contagious disease piscirickettsiosis or salmonid rickettsial septicaemia (SRS) and causes significant economic losses to aquaculture production in Chile. Current strategies to control infection are i) indiscriminate antibiotic use and ii) vaccination with predominantly P. salmonis bacterin vaccines that do not provide acceptable levels of protection against piscirickettsiosis. Areas covered: This review covers the basic biology of P. salmonis, clinical piscirickettsiosis and disease control, the development of current P. salmonis vaccines, innate and adaptive immunity and a 5-year plan to develop new piscirickettsiosis vaccines. Expert commentary: Fundamental knowledge is lacking on the complexities of P. salmonis-host interactions, relating to bacterial virulence and host innate and adaptive immune responses, which needs to be addressed. The development of new P. salmonis vaccines needs the application of comprehensive 'omics' technologies to identify candidate vaccine antigens capable of stimulating long-lasting protective immune responses.