Disease interaction and pathogens exchange between wild and farmed fish populations with special reference to Norway

Multilocus sequence typing (MLST) was proposed in 1998 as a portable, universal, and definitive method for characterizing bacteria, using the human pathogen Neisseria meningitidis as an example. In addition to providing a standardized approach to data collection, by examining the nucleotide sequences of multiple loci encoding housekeeping genes, or fragments of them, MLST data are made freely available over the Internet to ensure that a uniform nomenclature is readily available to all those interested in categorizing bacteria. At the time of writing, over thirty MLST schemes have been published and made available on the Internet, mostly for pathogenic bacteria, although there are schemes for pathogenic fungi and some nonpathogenic bacteria. MLST data have been employed in epidemiological investigations of various scales and in studies of the population biology, pathogenicity, and evolution of bacteria. The increasing speed and reduced cost of nucleotide sequence determination, together with improved web-based databases and analysis tools, present the prospect of increasingly wide application of MLST.

Vaccination plays an important role in large-scale commercial fish farming and has been a key reason for the success of salmon cultivation. In addition to salmon and trout, commercial vaccines are available for channel catfish, European seabass and seabream, Japanese amberjack and yellowtail, tilapia and Atlantic cod. In general, empirically developed vaccines based on inactivated bacterial pathogens have proven to be very efficacious in fish. Fewer commercially available viral vaccines and no parasite vaccines exist. Substantial efficacy data are available for new fish vaccines and advanced technology has been implemented. However, before such vaccines can be successfully commercialized, several hurdles have to be overcome regarding the production of cheap but effective antigens and adjuvants, while bearing in mind environmental and associated regulatory concerns (e.g., those that limit the use of live vaccines). Pharmaceutical companies have performed a considerable amount of research on fish vaccines, however, limited information is available in scientific publications. In addition, salmonids dominate both the literature and commercial focus, despite their relatively small contribution to the total volume of farmed fish in the world. This review provides an overview of the fish vaccines that are currently commercially available and some viewpoints on how the field is likely to evolve in the near future.

Sea lice, especially Lepeophtheirus salmonis and Caligus spp., have the greatest economic impact of any parasite in salmonid fish farming and are also a threat to wild salmonids. Here, I review how the biology and ecology of various louse and host species influence their pathogenicity and epidemiology. Recent discoveries of new species and genotypes emphasize the need for more basic research on louse taxonomy and host preferences. Louse development rates are strongly dependent on temperature, and increasing mean sea temperatures are likely to increase infestation pressure on farms and wild fish, as well as affecting the geographical distribution of hosts and parasites. Despite progress in finding L. salmonis larvae in the plankton and in modelling louse production in several countries, more data on larval behaviour and distribution are required to develop dispersal and transmission models for both L. salmonis and Caligus spp. This knowledge could be used to take measures to reduce the risks of lice affecting farmed and wild fish.