Introduction

Adult Atlantic salmon (Salmosalar) returning to New England river systems are removed from their respective river and transported to centralized broodstock facilities. Parasitic infestations are treated at this time, and each salmon is injected with 2.4 mg oxolinic acid, an antibiotic, and 0.5 ml Aeromonas salmonicida-Yersinia ruckeri bacterin/kg of fish. This treatment is given to reduce the numbers of fish that succumb to epizootics of Furunculosis and Enteric Redmouth Disease.

The adult salmon reaching the broodstock facilities have not only been subjected to the natural stress of spawning, but also, to other stressors, such as handling and transport. The ability of these salmon to mount an immune response to the bacterin was questioned because the general health of fish may be less than optimum at the time of capture and transport to the broodstock station. Previous data indicated that post spawning Atlantic salmon could respond to the bacterin given at the time of capture, but the protective nature of this response was not demonstrated (Ford and Cipriano, 1991).

Data compiled since 1986 by the Northeast Atlantic Salmon Restoration Project and the US Fish and Wildlife, Fish Health Unit (Lamar, PA) indicated that mortalities were reduced (as compared to non-treated, control groups) in groups of salmon given the combined antibiotic/bacterin treatment (Barbish, 1991). The reduction in mortality among treated fish can not be attributed to the effects of the vaccine alone, because the antibiotic treatment may have contributed (solely or in conjunction with the vaccine) to reduced mortality.

Objectives

The primary objective of this study was to determine the kinetics of the immune response of Atlantic salmon to the bacterin, and to determine if the fish had any natural titers to the pathogens of interest prior to immunization. Second, differences in agglutinin responses between fish given both the antibiotic and bacterin versus the bacterin alone were evaluated.

Present Study

Serum agglutinin to A. salmonicida and Y. ruckeri antigens were determined by standard agglutination procedures. Serum from each salmon was tested against four antigen preparations. Two antigens chosen for agglutination tests included formalized whole cell preparations of Y. ruckeri isolates 11-40 (serotype 1) and 11.29 (serotype 2). The other two antigens included sonicated whole cell preparations from A. salmonicida isolates, 3.10 and 3.64. Isolate 3.10 possesses a typical lipopolysaccharide (LPS) profile for A. salmonicida but does not produce a surface layer protein (A-layer). Isolate 3.64, on the other hand, produces the typical A-layer but not the 0-antigen portion of the LPS.

In June 1991, thirty grilse that were being maintained in isolation at the Craig Brook National Fish Hatchery (ME) were transported to the National Fish Health Research Laboratory (Leetown, WV). These fish were maintained throughout the spawning cycle in conditions similar to the broodstock stations. Nineteen fish were given the bacterin injection, and nine fish were injected with saline. Two fish died from stress during transportation. Biweekly serum samples were taken from each fish in order to determine the kinetics of the agglutinin response to the bacterin. As previously mentioned, agglutination titers were determined to four antigens, 3.10, 3.64, 11.40 and 11.29. Within six weeks, the fish injected with bacterin only (Leetown) had agglutinin titers to 3.10 that reached 15.2 + 1.6, and continued to be > 12 for the duration of the study. Control fish injected with only PBS remained much lower throughout the study.

In addition to determining the kinetics of the humoral response, the response of these fish were compared to broodstock agglutinin responses taken from post-spawning adults, at Richard Cronin National Salmon Station (males only) and Nashua National Fish Hatchery (males and females). Fish at these two broodstock stations received the antibiotic/bacterin combination.

Control fish were sampled at Richard Cronin National Salmon Station (6 fish) and Nashua National Fish Hatchery (10 fish) in June, 1991. All control fish at both brood stock facilities received no injection and had agglutinin titers of < 1. In November, post-spawning fish were sampled at both broodstock facilities. The surviving males (48) at Cronin had a mean agglutinin titer to 3.10 of 14.0 + 5.6, similar to the fish given the bacterin injection alone at Leetown. The Nashua fish had lower titers to 3.10 antigen (9.9 + 6.3) but were experiencing a fungal epizootic and in some cases the fish were also exposed to Aeromonas salmonicida.

The titers in all three groups of fish (Leetown, Cronin, and Nashua) were lower to 3.64 antigen, but were significantly higher than that of control fish. Titers to 11.40 and 11.29 were low (< 3) for all groups of fish.

As evidenced by the similar titers to 3.10 among Cronin and Leetown fish, the oxolinic acid appears to have no dramatic effect on the salmon's ability to produce a humoral response to the vaccine. This study measured the agglutinin response, the increase in specific antibody stimulated by antigens in a bacterin. These results do not, by themselves, provide evidence for the protective nature of the humoral response evoked in these fish. In fish, increased agglutinin responses have rarely been correlated with protection to disease. In fact, preliminary studies at this laboratory, using passive immunization protocols have indicated that the agglutinin response is not protective.

The success of the antibiotic/bacterin injection therapy; therefore, is most likely to be attributed at some level to both components. Even high levels of agglutinins, may not be protective, but may evoke non-specific immune reactions that contribute to the success of the treatment.

Acknowledgments

This work was coordinated through the US Fish & Wildlife, Fish Health Unit (Lamar, PA) with the cooperation of the staff at Richard Cronin National Salmon Station (MA), Nashua National Fish Hatchery (NH), and the Craig Brook National Fish Hatchery (ME), which is greatly appreciated.

References

1.  Barbash, P. 1991. Control of Aeromonas salmonicida in Connecticut and Merrimack Rivers Atlantic Salmon Broodstock Hatcheries. Proceedings of the Atlantic Salmon Workshop, Rockport, ME pp.173-177.

2.  Ford, L.A. and R.C. Cipriano. 1991. Evaluation of the Immune Status of male Atlantic salmon Broodstock at the Richard Cronin National Salmon Station. Proceedings of the Atlantic Salmon Workshop, Rockport, ME. pp. 160-172.