Growth performance and respiratory burst activity in rainbow trout treated with growth hormone and vaccine

Size-selected (167±11 mm) female rainbow trout were: given a high (1·8 μg g−1body weight per week) or low (0·35 μg g−1) dose of recombinant bovine growth hormone (GH), delivered by cholesterol pellet implant; injected with a commercial bivalent (vibriosis–furunculosis) vaccine; provided with each dose of GH plus vaccine; implanted with a pellet alone; or left untreated (N=20 per group). Growth in each treatment group was monitored over a 7 week period. Four weeks into the trial, and at trial termination, head kidney neutrophils were quantified using a histochemical method, and their respiratory burst (CL) response evaluated (N=10 per group). By week 4 of the trial, high dose GH treatment was observed to enhance the CL response of vaccinated fish (P<0·05) above that of control and other groups. High dose GH treated animals also returned greater, although relative to week 4 data significantly lower, CL response at trial termination. Vaccination had an overall negative effect (P<0·05) upon trout growth performance during the trial, although this consequence was annulled when vaccine was co-administered with high dose GH. When considered conjointly with the findings of others, the results of the present investigation provide evidence in support of the existence of a hypothalamo-pituitary-immune axis in teleosts.