Larisa A. Ford, PhD; Rocco C. Cipriano, PhD
National Fish Health Research Laboratory, US Fish & Wildlife
Service, Kearneysville, WV
Introduction
Currently, adult Atlantic salmon (Salmo salar) returning to the
Connecticut River are removed from the river and anesthetized in order to weigh, measure and
collect scale samples. At this time, parasitic infestations are treated and the fish are given
a combined antibiotic/bacterin injection. The injection consisted of 2.4 mg/kg fish oxolinic
acid and 0.5 ml/kg fish Aeromonas salmonicida-Yersinia ruckeri bacterin. This treatment
is given to reduce the numbers of fish that succumb to stress-induced epizootics of
Furunculosis and Enteric Redmouth Disease.
Clearly, adult salmon reaching the hatchery facilities have been subjected
to the natural stress of spawning migration and to other stressors, such as handling and
transport. The ability of these fish to mount a protective immune response induced by bacterin
injection is questionable because the physiological status of these fish is less than optimum
at the time of capture and transport. The economy of vaccinating fish that may not be
physiologically or immunologically responsive, therefore, is questionable.
Immunity of Atlantic Salmon
In attempts to increase survival of hatchery-release salmon and the
number of adult returns, previous investigations have concentrated on the ability of vaccinated
parr and smolts to survive challenge with specific pathogens. For example, Baum et al. (1982)
vaccinated 2 year old Atlantic salmon smolts with a trivalent Vibrio bacterin. These
fish were not challenged experimentally but were reared in commercial net pens. Neither
survival among vaccinated and control fish nor the number of adult returns differed. Successful
vaccinations of Atlantic salmon parr have been reported by Holm and Jorgensen (1987), Lillehaug
et al. (1990) and Lillehaug (1990). These reports, however, evaluated the vaccination of
Atlantic salmon against "Cold-water Vibriosis", a disease caused by Vibrio
salmonicida.
Only a few studies concerning immunization of Atlantic salmon against
Furunculosis and Enteric Redmouth Disease have been reported. Shieh (1985) reported that
fingerlings immunized with extracellular protease from an avirulent A. salmonicida were
protected against challenge with virulent A. salmonicida. Bruno and Munro (1989)
indicated that fry vaccinated with a Y. ruckeri bacterin were protected against water
borne challenge for up to 165 days. Data compiled since 1986 by the Northeast Atlantic
Salmon Restoration Project and the US Fish & Wildlife, Fish Health Unit
(Lamar, PA) indicated that mortalities were reduced (as compared to non-vaccinated control
groups) in groups of adult salmon given the combined bacterin and antibiotic treatment
mentioned previously (Clifford, 1990). Reduction in mortality among the treated salmon can not
be attributed to the effects of the vaccine alone, because antibiotic treatment may have
contributed (solely or in conjunction with the vaccine).
Objectives
The ability of returning Atlantic salmon (Salmo salar) to mount an
immune response induced by bacterin injection was questioned because of numerous stressors
affecting returning fish. In addition, a health management plan which is both cost-effective
and most beneficial to the adult Atlantic salmon maintained at brood stock facilities is
needed. The first objective, therefore, was to assess the ability of adult Atlantic salmon to
mount an immune response after returning to spawn. If these fish were not capable of mounting
an immune response, the bacterin injection could be eliminated from the current protocol.
Second, if an immune response was demonstrated, the protective nature of the response would be
evaluated.
Present Study
For this study, the immune response of post-spawning males to A.
salmonicida-Y. ruckeri bacterin was evaluated. Females that survive spawning are
reconditioned as a routine procedure of the Atlantic salmon restoration efforts and are too
valuable to be used for research purposes. Males, on the other hand, are normally sacrificed
after spawning and can be used for research. Initial experiments indicate that post-spawning
males produce a strong antibody response to A. salmonicida (mean - 11.1) and that the
agglutinin response to Y. ruckeri serotypes 1 and 2 is weaker (mean 4.5 and 2.7,
respectively). Antibody titers are expressed as the reciprocal of the last log 2 dilution
showing positive agglutination. Brook trout passively immunized with sera from the Atlantic
salmon were not protected from challenge with Y. ruckeri serotype 2. This serotype of
Y. ruckeri, however, was not included in the bacterin used to immunize the salmon.
The post-spawning male Atlantic salmon maintained at the Richard Cronin
National Salmon Station and sampled in this study mounted an immune response to the bacterin
given at the time of capture. The protective nature of this response was not demonstrated by
this study. Brook trout passively immunized with the Atlantic salmon sera and challenged with
A. salmonicida and Y. ruckeri serotype 1, exhibited low levels of protection,
however, the results are equivocal. Two problems occurred in these challenge experiments.
First, low virulence was noted for both challenge isolates, 3.139 and 11.14. Mortality did not
exceed 30% among fish immunized with phosphate buffered saline (pH 7.2) and challenged with
either 3.139 or 11.14. More virulent isolates would have produced higher mortality in the
non-immunized fish and differences between the immunized and control groups may have become
more apparent. Second, mortality among non-challenged control fish further confounds the data.
Pathogens were not detected in these fish and mortalities were attributed to non-specific
causes. These mortalities, however, do not allow for differences to be detected among treatment
groups. Further research is planned in cooperation with Atlantic salmon brood stock facilities
and the US Fish & Wildlife, Fish Health Unit (Lamar, PA) to evaluate the protective nature
of the immune response in adults upon return and after spawning.
Acknowledgements
This work was coordinated through the US Fish & Wildlife, Fish Health
Unit (Lamar, PA) and the cooperation of the staff of the Fish Health Unit and the staff of the
Richard Cronin National Salmon Station (Sunderland, MA) is greatly appreciated.
References
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hatchery-reared Atlantic salmon smolts vaccinated with a Vibrio anguillarum bacterin. N.
Am. J. Fish. Mang. 4: 409-411.
2. Bruno, D.W. and A.L.S. Munro. 1989. Immunity in Atlantic
salmon, Salmo salar L., fry following vaccination against Yersinia ruckeri, and
the influence of body weight and Infectious Pancreatic Necrosis Virus (IPNV) on the detection
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