Previous work using rainbow trout which received dietary garlic
for 14 day treatment periods revealed protection against challenge
with A. hydrophila and enhanced innate defence mechanisms, such
as high oxidative radical production by serum neutrophils, proliferation
of lymphocytic cells and phagocytic activity of the head
kidney macrophage [10]. However, modulation of non-specific
defence mechanisms in treated fish may have been chiefly by
activation of the release of reactive oxygen species (ROS) by
immune cells. This might explain the significant increase
(P < 0.05%) in the respiratory burst activity of the neutrophils,
measured by the reduction of NBT to formazan as indicator of
superoxide anion (O2) production. This reactive oxygen species
include superoxide radicals and hydrogen peroxide, which are
known to be toxic to pathogenic bacteria [25,26]. Moreover, the
significant difference between the treatment and control groups
was similar to the finding of [16], who observed a high NBT activity
in rohu Labeo rohita juveniles fed with 0.4% dietary yeast RNA.
Comparable results were also obtained by [27], in Cyprinus carpio
which received dietary nucleotide derived from yeast RNA.