Resistance against HSMI

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Updated documentation, per February 2017


Heart and skeletal muscle inflammation (HSMB) is one of the most common diseases in salmon farming with 130-180 annual outbreaks in Norway. The disease is associated with Piscine orthoreovirus (PRV) and causes tissue damage and inflammation of the heart and red skeletal muscle that can lead to cardiac and circulatory failure. The virus is also associated with dark spots in the fillet. QTL eggs selected for resistance to HSMI are documented to significantly reduce heart damage and have higher survival. QTL-innOva® HSMI will be available from the egg season 2017/2018.

Figure 1. Histopathology (tissue damage) performed on 240 fish 10 weeks after infection with the virus (PRV) associated with HSMI. The experimental fish was 80-90 grams smolt from the AquaGen breeding nucleus. Fish with two (QQ) or one (Qq) copy of the favourable variant Q, which is included in QTL-innOva® HSMI, had less heart damage compared to the unfavourable variant (qq).

Test results from the laboratory

Since infection in laboratory trials does not lead to mortality, other analyzes such as histopathology (tissue damage), viral consentration and immunological responses (amount of T cell marker CD8 and protein RTP3) are used as an indirect measure of susceptibility to HSMI.

After PRV infection of 541 families and subsequent
genomic analysis of 887 fish, two significant QTLs for HSMI resistance were found. The QTL status (QQ, Qq or Qq) of the most important QTL correlated well with both tissue damage and immunological responses. The histopathological studies showed that tissue damage for fish with two or one copy of the favorable variant Q (QQ and Qq) was less than in the unfavorable variant (qq) with average scores of 2.1, 2.05 and 2.6 of maximum 3.0 (Figure 1).

This QTL has low incidence in investigated brood fish populations, which means that there is a great potential for significant progress by using it in selection. For the second QTL, the frequency of the favorable variant is already high.

Results from a natural HSMI outbreak

In July 2016, during a natural HSMI outbreak, studies of 187 salmon(105 live and 82 dead) of approx. 1.2 kg were carried out.

Figure 2. Distribution of genotypes (QQ, Qq or Qq) in 105 live fish and 82 dead fish during a natural HSMI outbreak in July 2016. The salmon was transfers to sea in autumn 2015 at a location in Central Norway and weighed about 1.2 kilos. Fish with the unfavorable variant (qq) was overrepresented among fish that died. It is QQ and Qq that are included in QTL-innOva® HSMI.

The fish was transfered to sea in autumn 2015 at a location in Central Norway.

Among live fish histopathology and genotyping for the same QTL as in the infection test were performed. The results showed again that tissue damage was less the more copies of the favorable variant Q the fish had. Fish that had two, one and no copies of the favourable variant had an average histopathology score of 2.0 (QQ), 2.2 (Qq) and 2.5 (qq).

Analyzes of QTL frequencies among live and dead fish in the natural HSMI outbreak showed a clear overweight of fish with the unfavorable variant (qq) among fish that died. The proportion of the favourable variants (QQ and Qq) was higher (35%) in live fish compared to dead fish (19%). The most favourable QTL (QQ) was not recorded among fish that died while 4% with QQ were found among live fish. The proportion of the unfavourable variant (qq) was lower in live fish (65%) than in dead fish (81%) (Figure 2).

Products with HSMI resistance:

  • AquaGen®Atlantic QTL-innOva® HSMI
    (can be chosen in addition to specific products)

Benefits of QTL-innOva® HSMI:

  • Less heart damage both during HSMI infection in the laboratory and HSMI outbreaks in the field
  • Higher survival during HSMI outbreaks in the field