The study indicated that the S-layer protein antigen of A.
hydrophila is able to confer protection in common carp against a
range of different isolates of the bacterium, although the RPS values
obtained for the carp did vary between the different challenge isolates. No mortalities occurred in any of the groups of fish after
Day 8 post-challenge and no colonies of A. hydrophila grew from
the kidney swabs taken from surviving fish at the end of experiment
except for two fish. This suggests that most of the surviving
fish in the control group had managed to clear the bacterium. It
is known that a healthy fish can produce an antibody response
against different components of the bacterium and clear it from
its circulatory system within 7 days post-infection, if the level of
infection caused by the pathogen is not sufficient to kill the fish
Other proteins of A. hydrophila have been produced as recombinant
antigens for use in vaccination studies. For example, Fang et al.
[1] found significant protection against two isolates of A. hydrophila
in blue gourami, Trichogaster trichopterus (75 and 87.5% RPS) immunised
with a recombinant 43 kDa OMP, while a recombinant 37 kDa
OMP of A. hydrophila has been shown to be immunogenic in rohu
carp [45]. Fish vaccinated with this recombinant OMP had a RPS
value of 57% after challenging the fish with a virulent isolate of
A. hydrophila [46]. However, cross-protection of these vaccines
against a range of A. hydrophila isolates has not been reported.
Amend [33] proposed that a RPS value of more than 60 with vaccinated
and experimentally infected fish was necessary to ensure
protection from natural infection in field. The author also recommended
aminimum mortality of 60% in the control group using two
replicate groups of 25 fish for both the vaccinated and the control
groups. Though not all the criteria suggested by Amend were followed
in the present study, the level of protection obtained with the
recombinant protein against six different isolates of A. hydrophila,
suggests that it is able to protect against a range of different A.
hydrophila isolates despite the fact that two of the challenge isolates
resulted in low RPS values due to slightly increased mortalities in
vaccinated groups (15 and 20%).
In summary, the results of this study, and the smaller preliminary
study with goldfish mentioned above, suggest that the S-layer
protein of A. hydrophila may be an important antigen for conferring
protection in common carp against a variety of virulent isolates of
this pathogenic bacterium. Efficacy testing of this vaccine is currently
in progress in the aquarium and in the field to establish if it
can protect a variety of fish species against different isolates of this
bacterium.