Temporal transcriptomic profiling of channel catfish (Ictalurus punctatus) gills after acute copper exposure reveals suppressed immune gene responsiveness to Flavobacterium covae infection

Copper sulfate pentahydrate (CuSO 4 ·5H 2 O) is widely used as a chemical prophylactic in aquaculture, yet its immunomodulatory effects in fish remain poorly understood. Here, we investigated how the timing of CuSO 4 exposure shapes early immune responses and survival outcomes in channel catfish ( Ictalurus punctatus ) challenged with Flavobacterium covae . Channel catfish fingerlings were exposed to 2.1 mg/L CuSO 4 either immediately before F. covae infection or following a 24 h recovery period, and gill transcriptomes were profiled at 1, 4, and 24 h post-challenge. Infection alone induced strong, time-structured innate and adaptive immune activation, including robust cytokine, chemokine, and pattern-recognition receptor (PRR) signaling. In contrast, Cu exposure followed by immediate infection produced no differentially expressed genes at 1 h and only a minimal response at 4 h, indicating a profound early suppression of host immune activation and a shift toward cellular stress-mitigation pathways. Fish that were treated with Cu followed by a 24 h recovery and then infected with F. covae regained immune responsiveness, displaying coordinated innate and adaptive activation by 4–24 h, consistent with significantly improved survival relative to immediate Cu exposure. Together, these results demonstrate that Cu immunological impact is strongly time-dependent. Acute exposure suppresses early immune readiness and increases susceptibility, whereas a recovery interval restores immune competence and enhances resistance to F. covae . These findings provide mechanistic support for incorporating recovery periods into CuSO 4 treatment protocols to avoid unintended immunosuppression in aquaculture.