Background: Ammonia toxicity is a critical environmental challenge in aquaculture that compromises fish health through oxidative stress and inflammation.

Objective: This study investigated the potential of dietary thymol, a phytogenic bioactive compound, to mitigate the adverse effects of chronic unionized ammonia exposure in common carp (Cyprinus carpio).

Methods: Juvenile fish (initial weight: 10.41 ± 0.23 g) were distributed into 12 aquaria (10 fish per tank) and assigned to four triplicate treatments for eight weeks: a negative control (NC; clean water + basal diet), a positive control (PC; 0.06 mg/L unionized ammonia + basal diet), and two ammonia-exposed groups receiving diets supplemented with 100 mg/kg (T1) or 400 mg/kg (T2) of thymol. At the end of the trial, hepatic mRNA expression of stress, inflammatory, and antioxidant-related genes was analyzed.

Results: Chronic ammonia exposure significantly suppressed the expression of heat shock protein 70 (hsp70) and the anti-inflammatory cytokine il10, while up-regulating pro-inflammatory markers (il1-b, tnfa, and nf-kb). Furthermore, the ammonia-exposed group exhibited an up-regulation of keap1 and a downward trend in nrf2 expression, indicating an impaired antioxidant response. Conversely, dietary thymol supplementation, particularly at 400 mg/kg (T2), successfully restored the expression of hsp70 and il10 and neutralized the ammonia-induced up-regulation of il1b and tnfa. Both thymol treatments significantly enhanced the Nrf2/Keap1 pathway by suppressing keap1 and markedly boosting nrf2 transcript levels compared to the PC group. Although thymol attenuated the up-regulation of nf-kb, levels remained higher than the NC group.

Conclusion: Our findings demonstrate that dietary thymol effectively mitigates ammonia-induced hepatic stress and inflammatory responses in common carp. By modulating the Nrf2/Keap1 and NF-κB pathways, thymol enhances the physiological resilience of fish, suggesting its potential as a functional feed additive in aquaculture systems facing environmental stressors.