Evaluation of immune effects of a multivalent vector vaccine in turbot, Scophthalmus maximus

Abstract

Vibrio anguillarum and Aeromonas hydrophila are the primary cause of mortality in marine and fresh water aquaculture.Mortality,due to disease from these two pathogens,has resulted in significant economic loss.Historically,infected fish have been treated with antibiotics,leading to widespread drug resistance among patho-genic bacteria.More recently,a number of vaccines against pathogenic bacteria have been developed and ap-proved for use in aquaculture.Although China has a long history of intensive fish culture,there has been little progress in developing/using vaccines for the treatment of fish diseases in this region.Our objective was to de-velop a combined,single application vaccine against V.anguillarum and A.hydrophila.We constructed a multi-valent vector vaccine(MVAV6203A-1) by fusing glyceraldehyde-3-phosphate dehydrogenase(GAPDH) from pathogenic A.hydrophila to attenuated V.anguillarum.We then evaluated the effectiveness of this vaccine against the two pathogens.Turbot,body length(8.99±0.73) cm,were immunized with the vaccine(MVAV6203A-1) by intraperitoneal injection(i.p.) or immersion.The fish were then challenged with virulent strain V.anguillarum MVM425 and A.hydrophila LSA34 by i.p injection and held for 30 d.We measured the relative percent survival(RPS) and antibody titers in the plasma against V.anguillarum by indirect ELISA at the end of the experiment.Fish that were given the vaccine by i.p.injection had high protections against MVM425 and LSA34 with RPS of 80.6% and 77.0% respectively.Similarly,the fish that were immersed in the vaccine also had a protection against MVM425(RPS 62.2%),but had a low protection against LSA34(RPS 11.7%).The plasma antibody titers were higher in the i.p.vaccination group(max: 2048,mean: 891) than in the control(mean: 97).Conversely,there was no difference in the mean antibody titer between the immersion treatment group and the control(mean: 64).Thus,our data suggest that the multivalent vector vaccine MVAV6203A-1 is effective in protecting turbot against bac-terial disease caused by Vibrio anguillarum and/or Aeromonas hydrophila. We discuss the limitations of evaluating vaccine efficiency using RPS and plasma antibody titers.The RPS and antibody titer were lower in the immersion immunization group than in the i.p.immunization group.However,this does not imply that immersion in the vaccine is associated with a weaker immune response in turbot.A num-ber of studies have shown that immersion challenge(not i.p.challenge) may be more suitable for evaluating the efficacy of vaccines that are delivered by immersion.This is because of differences in the antibody titer in the cutaneous mucus relative to the plasma.