Tale Marie Karlsson Drangsholt defended her thesis on August 25th 2011

Ane Gro Siri Skjelfjord

Quantitative genetics of traits related to disease resistance and effects of vaccination in Atlantic salmon (Salmo salar)

Summary from the thesis - PhD thesis 2011:39

Disease resistance is of major importance to the fish farming industry as disease outbreaks have negative effects on the industry's economy, its reputation and on fish welfare. Today, almost all fish in the Norwegian salmon industry are vaccinated against a number of diseases, while selection for increased resistance to specific diseases is based on survival of unvaccinated fish in challenge tests. The main aim of this doctoral thesis was to obtain a better understanding of how to select for increased resistance to furunculosis in Atlantic salmon and how this relates to side effects of vaccination, taking into account that most fish in the industry are currently vaccinated. Resistance to furunculosis (survival) was recorded by challenge testing fish from 150 families (unvaccinated and vaccinated fish). Vaccine-induced side effects (adhesions of internal organs and melanin deposits) were recorded on samples of the 150 families at three points in time: after three months in freshwater (high temperature), and six and 12 months after sea transfer.

The first objective was to estimate the magnitude of the genetic (co)variation in survival of unvaccinated and vaccinated Atlantic salmon challenged with A. salmonicida, the bacteria causing furunculosis. The results showed a low genetic correlation between resistance to furunculosis in unvaccinated and vaccinated fish. The second objective was to estimate the magnitude of genetic variation of the negative side effects of vaccination. Intermediate heritabilities were obtained for adhesions and melanin deposits. However, the results also showed that an alternative vaccine reduced the side effects compared to the standard vaccine. The third objective was to estimate the magnitude of the genetic correlation between disease resistance, side effects of vaccination, and harvest body weight. These traits were not genetically correlated; though a possible exception is harvest body weight and survival of vaccinated fish, where a weak and unfavorable correlation was reported.

Today's breeding strategy of testing unvaccinated fish is optimal if the long term goal is a reduced need for vaccination. Selection based on vaccinated fish is likely to be the most effective short term strategy, as all fish in the industry today are vaccinated. However, this strategy is not very relevant for furunculosis as the vaccine is highly effective. Vaccine-induced side effects (adhesions and melanin deposits) could be reduced through selective breeding, but it is likely to be more appropriate to focus on other measures such as vaccine development. Selection for increased disease resistance, vaccine-induced side effects, or harvest body weight are not expected to lead to unfavorable correlated responses in any of these traits, with the possible exception of survival of vaccinated fish and harvest body weight.

Updated: 07.09.11
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