Vaccination strategies designed to control furunculosis were reported as early as the 1940s (Duff, 1941). Midtlyng (1997) has reviewed the attempts that were made, over the next 50 years, to produce vaccines and vaccine administration methods that would provide adequate control of this disease. Succinctly, and possibly rather unfairly, these attempts can be summarized, at least from the perspective of commercial salmon farmers, as failures. Oral, immersion and injection administrations of a variety of bacterins were developed and a few were commercially produced and marketed. Some, particularly those administered by injection, were cost-effective in that the value of the fish protected probably exceeded the cost of the vaccination. None, however, were sufficiently effective to have a significant influence on the epizootiology of the disease. This situation changed dramatically at the beginning of the 1990s. At this time, oil-adjuvant injection vaccines became available (Bricknell and Ellis, 1993; Midtlyng, 1996; Midtlyng et al., 1996; Anderson et al., 1997).
Simply stated, the availability of oil-adjuvant injection vaccines has transformed the significance of furunculosis in commercial salmon farming. In the 1980s, prior to their introduction, furunculosis was, in Norway and Scotland, one of the most important causes of mortality in sea-reared salmon. The use of these vaccines is now almost universal throughout the European industry and this has resulted in the perceived disappearance of furunculosis as a cause of salmon mortality. Munro and Gauld (1996), for example, reported a reduction in mortality of salmon in sea cages from 35% to approximately 10% that was coincident with the introduction of these vaccines. Similarly Markestad et al. (1995) related the 75% reduction in the use of antimicrobials in Norway that occurred in 1994 due to the increased use of oil-adjuvant vaccines. The development of these vaccines has been accompanied by the development of machinery that allows the injection of a large number of fish in a short time. Not only have these machines substantially minimized the logistical problems associated with the administration of vaccines by injection, but they have also significantly reduced the health risks to farm workers. Accidental self-injection of furunculosis vaccines, particularly those that contain oil adjuvants, can result in significant damage to workers. In any large-scale vaccination, significant thought must be given to worker health issues, and clearly the use of machines to perform the injection will significantly reduce the potential risks. Midtlyng (1997) has also discussed the animal-welfare aspects of the use of oil-adjuvant vaccines. While he accepts that there is evidence that such vaccines do result in suffering for fish, he argues that any such suffering must be balanced against that which would result from a devastating epizootic of furunculosis.
While the importance of oil-adjuvant vaccines for the salmon farming industry is well attested (Markestad et al., 1995), the application of this technology to salmonid farms whose primary function is restocking presents problems. These problems are primarily associated with the persistence of the adjuvant within the vaccinated fish. In restocking operations, there is inherently less control over the time period between vaccination and consumption of fish. In situations where fish are to be restocked into a 'put-and-take' fishery or for the purpose of competition fishing, considerations of the health risks that may be associated with the consumption of highly immunogenic adjuvants may place considerable constraints on the use of oil-adjuvant vaccines. Hiney (1995) has demonstrated that covert infections can persist in oil-adjuvant-vaccinated fish. Thus, there is a clear possibility that such fish can act as 'immune carriers'. The movement of such fish between water bodies may therefore be capable of acting as a vector of the disease. As yet, there are no estimates of the size of this risk, but its possible existence should be borne in mind in any movement of vaccinated fish.
The success of oil-adjuvant vaccines for the control of furunculosis in commercial fish farming has had a number of side-effects, which should also be considered. Stress is the major precipitating factor associated with furunculosis. In many situations, the fear of furunculosis has been a significant factor in the introduction of husbandry practices that were aimed at reducing stress. In particular, furunculosis has exerted a major constraint on stocking densities. The effective removal, by vaccination, of the fear of furunculosis may result in the relaxation of some farm practices, particularly when these place economic constraints on the operation of a farm. It is axiomatic that any increase in stress levels will ultimately result in reduction in fish health. The effective control of furunculosis may, therefore, become the predisposing factor for another disease. A second side-effect of the efficiency of modern vaccination is that funding for research into furunculosis has been drastically reduced. Whether this will have any significant effects outside the research community only time will tell.
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