Vaccination in fish farms: how far are we?

Currently, the number and variety of approved aquatic vaccines worldwide are relatively limited, and they cannot fully cover the major diseases of relevant farmed species. Inadequate supply of efficient vaccines limits the overall effect of immunization against infectious agents and reduces farmers’ trust in the industrial application of vaccines. Still, the “in farm” vaccination of fish has become a standard operative procedure in most countries in Europe and North America. In Asia, Africa, and South America, with some exceptions like Japan and Chile, vaccines are not commonly used for fish disease control. This is apparently not consistent with the relevant amounts of fish that are produced in these areas. There are several reasons explaining the present situation. Most of the farms operate on a small scale with little technical support. Farmers are increasingly oriented toward prevention; despite this, treatment with antibiotics remains a necessary tool, even if with important differences between countries in terms of availability and local regulations that authorize their use. Moreover, the commercialization and field application of vaccines require a great investment of time and resources. Important issues have to be considered in order to guarantee the effective full “in field” delivery of vaccination protocols worldwide: the level of technology and good management reached in the target farm; the prompt identification of the disease causing agent or agents and, of course, the availability of adequate vaccine formulations; the identification of earliest time and of available windows for vaccination; the efficacy and safety of the product, preferably applicable to multiple fish species; a good return on the investment by the fish farmer; the public perception of consumers favoring vaccine use versus antibiotics (aiming at the limitation of antibiotic residues and of antimicrobial resistance (AMR) phenomena). The present chapter will provide a critical outlook on what is currently applied, in terms of in-field vaccination, by countries more or less developed, where aquaculture has a relevant role in the economy. The vaccination approach may include technologically advanced protocols, such as those applied in North America or in northern Europe, where farmers and vets dispose of multivalent formulations including several antigens, administered automatically with very efficient machinery, but also very simple protocols in which formulations are bacterins including a single pathogen, administered by syringes or injection guns managed by individual operators. Also, the formulations designed may deeply vary accordingly to the level of refinement achievable (inactivated pathogens versus DNA- or RNA-based vaccines) or accordingly to single countries’ current regulations and approved formulas. Several European countries, such as Italy, where the application of fish vaccination is quite common, even if not always technologically advanced, are promoting the production and periodical use of autogenous vaccines. These formulations are specifically tailored on the needs of single farms since they are developed using strains isolated from the same farm to which the vaccine will be addressed, and we think that this approach might be advantageous both in extended farms and in small-scale farms, in order to protect freshwater and marine species from bacterial/viral infections. The beneficial aspects as well as the limits of multimodal approaches of vaccination will be discussed in the light of the novel formulations development and on the modes of administration to fish.