Health risks associated with the migration of Atlantic salmon (Salmo salar L.): an epidemiological surveillance programme in Northern Spain

The authors present the results of an epidemiological surveillance programme involving Atlantic salmon (Salmo salar) populations in north-eastern Spain. The study investigates the risk factors critical in disease transmission, which include the presence of potentially pathogenic micro-organisms, the innate cell-mediated immune response of the salmon, and the potential interactions between the two. Also taken into account are the biological and migratory cycles of these salmon populations. The results showed that Aeromonas salmonicida and Saprolegnia parasitica were the micro-organisms that played the most important role in the transmission of the disease by virtue of their pathological characteristics, their occurrence in large numbers, and their capacity to transmit disease. With regard to the cell-mediated immune response, the authors observed a wide variability between individuals in some cell groups, as well as certain seasonal and age differences. These may be factors that predispose the animals to infection or to becoming asymptomatic carriers, with the attendant risk of transmitting infection in the course of migration. This could be especially relevant in the case of salmon infected with A. salmonicida and S. parasitica, since it has been observed that such salmon have a decreased number of leucocytes compared to non-infected salmon.