Thymol-carvacrol essential oil extract mitigates the mucosal stress-immune response induced by intraperitoneal immunization in sea bass (Dicentrarchus labrax)

The present study investigates the mitigation of the stress response induced by a bivalent vaccine against vibriosis and pasteurellosis in sea bass through treatment with thymol-carvacrol oil extracts, known as FishEase-C (FEC) and FishEase-L (FEL). Plasma and skin mucus cortisol as well as mucosal transcript stress-immune response were measured at 1 and 24 h post-vaccination (hpv). Our results indicate that FEL but not FEC was capable of reducing plasmatic cortisol secretion by 24 hpv, in contrast to vaccinated and vaccinated plus treated with FEC groups. A similar pattern was noted in the skin mucus though secretion of cortisol was prevented by the FEL. The expression of stress-immune related gene transcripts (gr1, gr2, crh, hsp70, cox2, il1β, il6, il8, il10, tgfβ1, lysozyme, igm, igt and igd/igt) revealed that stress-immune gene regulation was not homogenous among mucosae. Gills were more responsive to the stress/inflammation induced by FEC, as shown by crh, hsp70, cox2, il1β and il6 enhanced expression, suggesting higher absorption of FEC via this mucosae. The gills also appeared to be more resilient to the stressful effect of the vaccine, while higher regulation was observed in skin and intestine. In summary, administration of FEL was able to mitigate the stress response at physological (e.g., cortisol) and the stress-immune genes activation in skin and intestine(hsp70, il1β, il6), mainly at 24 hpv. We recommend FEL as a customized formulation to farmers for use on sea bass prior to vaccination, in order to mitigate negative effect of vaccination procedure.