Non‐Specific Immune Response in Large Yellow Croaker ( Larimichthys crocea ) Experimentally Infected With Pseudomonas plecoglossicida by Intraperitoneal Injection or Immersion

The bacterial pathogen Pseudomonas plecoglossicida causes visceral white nodules disease (VWND) in the large yellow croaker (LYC; Larimichthys crocea ), a commonly farmed species in China, resulting in substantial economic losses in the aquaculture industry. To better understand host innate immune mechanisms during P. plecoglossicida infection, LYCs were infected either by intraperitoneal (IP) injection or immersion, and their non‐specific immune responses were compared. Fish were subjected to serial concentrations of the pathogen in both challenged groups, and the median lethal dose (LD 50 ) was determined for subsequent infection trials. The results show that the respiratory burst activity, antioxidant activity (SOD, CAT and MDA), and non‐specific immune responses (ACP, ALP and LZM) of LYCs were stimulated, and the expression levels of inflammatory cytokines ( il‐1β , tnfα1 , il‐8 and il‐10 ) were considerably upregulated compared with mock‐infected fish during bacterial infection periods via both challenge routes. Furthermore, the non‐specific response in the immersion group started later and appeared to last longer, often exceeding the response in IP‐injected fish. These results will contribute to a further understanding of the pathogenesis and host defence system in fishes following infection with P. plecoglossicida , and may inform the future development of preventive strategies for controlling VWND.