Phage cocktail therapy against multidrug-resistant luminescent Vibrio harveyi infection in Pacific white shrimp Penaeus vannamei induces indirect immunostimulatory effect.

Pacific white shrimp, Penaeus vannamei is an economically important aquaculture species, however, production is frequently constrained by luminescent vibriosis caused by Vibrio harveyi. Phage therapy has emerged as a promising alternative to conventional antibiotics in shrimp aquaculture, helping to mitigate the emergence of multidrug-resistant bacteria. This study evaluated the lytic phage cocktail of five Caudoviricetes phages against multidrug-resistant V. harveyi and examined its effects on associated immune responses in P. vannamei. Six experimental treatments were established, comprising a negative control, an infected positive control and four phage cocktail treatments with different phage-bacteria pre-incubation periods. Total Vibrio count (TVC), total haemocyte count (THC), total differential count (TDC), cell viability (CV), phenoloxidase (PO) activity, superoxide dismutase (SOD) activity, survival, and histopathological changes were evaluated. Phage-treated groups generally showed lower TVC than the infected positive control, together with favourable changes in THC, TDC, CV, and PO activity. SOD activity varied among treatments but did not differ significantly, indicating that the antioxidant response was not consistently affected by treatments. The percentage of survival was higher, and hepatopancreatic damage was reduced in phage-treated shrimp compared with the infected control. Because the phage cocktail was pre-incubated with V. harveyi before shrimp exposure, these beneficial outcomes may partly reflect a reduced viable bacterial challenge rather than direct immunostimulation. Overall, the phage cocktail suppressed V. harveyi, improved survival and was associated with favourable changes in selected host defence parameters, supporting its potential as a biocontrol strategy against luminescent vibriosis in shrimp aquaculture.