Enhancing farmed fish welfare: Evaluating the effectiveness of plant-based stress mitigating agents as sedatives in sea bass (Dicentrarchus labrax) following intraperitoneal vaccination

The effectiveness of vaccines may be compromised by the stress response induced by intraperitoneal/intramuscular (IP/IM) vaccination due to an intimate interaction between the neuroendocrine and immune systems. Essential oils (EOs), known for their antibacterial, antioxidant, and sedative properties, are potential candidates to mitigate this stress response. This study investigates the short-term sedative effect of two essential oil-based products, FishEase-C (FEC) and FishEase-L (FEL) in sea bass prior to intraperitoneal vaccination (IP). Physiological stress indicators (plasma cortisol, glucose, lactate) increased 1-h post-vaccination (hpv) as expected. Cortisol remained elevated up to 24 hpv in the N.P. vaccine and FEC + vaccine groups but decreased with FEL treatment (FEL + vaccine group). However, FEC at the tested concentration appeared to induce stress. The transcription of stress (gr, hsp70, cox2), immune (il1β, il6, tnfα), and antioxidant (gpx, sod, catalase) genes confirmed the vaccination-induced stress response, with mc2r transcription indicating increased cortisol production in vaccinated groups (N.P. vaccine and FEC + vaccine). FEL reduced stress at both physiological (e.g., cortisol) and transcriptional levels (e.g., hsp70, cox2 and il6) at either 1 or 24 hpv. It is worth noting that, from an inflammatory perspective, there was a big difference between tissues in terms of magnitude and pattern (treatment and time effects). The brain was more resistant to inflammation, while the head kidney and spleen showed heightened il1β expression (860-2100-fold). These findings support the use of FEL as a sedative before IP/IM vaccination in sea bass.