Health Challenges

Viral diseases



PD is the viral disease that causes the most loss in farmed salmon in Norway today. PD primarily affects the pancreas, the digestive organ, but also the skeletal muscles and the heart muscles can be damaged by the virus. The virus also attacks the oesophageal muscles, which can contribute to the reduction in appetite that is observed during outbreaks.

Fish affected by PD stop eating abruptly and serious outbreaks can lead to emaciated fish and increased mortality. The consequences of such an outbreak can be severe in terms of both increased wastage and due to lost feeding days. Lost feeding days will lead to less growth, and can lead to the fish having to remain longer at sea to attain the desired slaughter weight. If the fish are weakened by disease and reduced nutrient intake, they can also be more exposed to new diseases.

Two forms of the PD virus result in disease in Norway today. We have an epidemic with SAV-3 south of Hustadvika, and an epidemic of SAV-2 north of Hustadvika. These epidemics are different in some respects, among other things SAV3 often results in a more serious disease and they each have their own strategy to combat them and prevent their spread beyond certain geographical areas.


  • An outbreak of PD generally starts with an abrupt loss of appetite.
  • The fish are often high up in the water, with their head towards the direction of flow.
  • Death generally occurs 2-3 weeks after the appearance of the first symptoms.
  • Emaciation in chronic cases.

Normal behaviour in healthy fish is that they generally swim in the pelagic zone in rings in the cage. A change in behaviour with the fish resting high up in the water column can be a sign of disease. PD can lead to major damage to the muscles and when the fish is against the flow, it can acquire oxygen-rich water via the gills with the least possible use of the muscles.

If the outbreak is prolonged, the fish can suffer chronic changes in their muscles and this can affect the slaughter quality. A correlation has been demonstrated between increased severity of dark spots in the fillet if the fish is suffering from PD near to the time of slaughter.

PD is generally diagnosed using PCR, or using clinical symptoms.

Combating the disease  - preventive health care

Vaccination against PD is widespread. The vaccine will make the fish more resistant but does not provide 100% protection against the disease.

With PCR screening and early detection of the virus in the shoal, operations can be planned so that further spread of the virus can be prevented. If the fish is approaching the point of slaughter, it can be an option to slaughter the fish earlier so as to prevent further infection. You can also arrange for the fish to get through periods with increased risk of a PD outbreak without becoming sick. You can put in place measures to reduce stress, and provide feed that strengthens the immune system of the fish so that the fish can fight against the disease more effectively. 

Feed strategy for reduced harmful effect of a PD outbreak



ILA is a serious infectious disease that can affect salmon in the seawater phase. The disease is listed, which means that we don't want to have the disease in Norway, and we have an extensive programme in place to combat it. Nevertheless, there have been around 10 outbreaks every year over the last 10 years, and the frequency appears to be on the increase. In 2015 there were 5 more cases of ILA than in the preceding two years. Outbreaks of ILA can have enormous economic consequences.

When Chile was struck by the disease between 2007-2009, the whole industry was set back several years, and the outbreak was subsequently estimated to have cost the industry in excess of ten billion kroner

ISA is caused by an orthomyxo virus that is closely related to the influenza virus in humans.  The virus attacks and settles in the layer of cells that covers the inside of the veins, and can lead to these being weakened. If the veins are damaged, they leak and the fish can suffer from internal bleeding. This can lead to anaemia – which means lack of blood. An outbreak of ISA often occurs in sick or weakened individuals, so the focus on keeping the fish robust and healthy is important in preventing outbreaks..

In addition to the most harmful strains of the ISA virus (HPR), Norway has also experienced a more widespread "harmless" strain of the virus, HPR0. This virus can be found in gill tissue in seemingly health fish. We don't know for sure the significance of this virus, but it is thought that it can mutate and become pathogenic.

ISA is often suspected if there are signs of circulatory failure. You can also see bleeding on the skin and internal organs when an autopsy is performed. In serious cases, you can see a dark, blood-filled liver and kidney, a swollen spleen and bleeding in the intestines. The symptoms that appear vary.

A diagnosis is made based on typical disease changes, and upon detection using molecular methods such as immunohistochemistry (viruses of a different colour to the tissue), or PCR.

Combating the disease
To combat ISAinvolves preventing infection first and foremost. By ensuring the maintenance of good hygienic barriers against infection, and laying the groundwork for a healthy and resistant salmon by strengthening the immune system, it is possible to minimise the likelihood of the virus causing disease in salmon. There are vaccines against ISA, but vaccination is not allowed in all areas and the effect is limited. 



IPN is a contagious viral infection in salmonids that was previously very prevalent in Norway. Outbreaks lead to a weakening of the digestive system and affected fish can be undernourished and weakened, and can die in the worst case scenario. The occurrence of the virus was considerably reduced after the introduction in 2009 of QTL roe to combat IPN, but the disease continues to have economic consequences today, especially associated with the handling and sorting of weakened fish subsequent to the disease taking hold in the saltwater phase.

IPN attacks occur in salmonids in both the freshwater phase and saltwater phase. The clinical symptoms are most serious in spawn and post-smolt salmonids in the freshwater phase, and you observe that the fish are especially vulnerable in the period around the transition from the hatchery to the sea. The symptoms are often not as severe in the saltwater phase but the indirect consequences are substantial when the fish are weakened and more susceptible to other more widespread diseases such as HSMB and CMS, and ultimately ILA. Even though the disease is most serious in freshwater, it is also the case that the economic consequences are often at their greatest in seawater as the fish at this point are larger, and fish farmers have used considerable resources on feed and healthcare.

The virus primarily attacks the pancreas, which is a key organ in the digestive system, while there is also often inflammation in the intestines and liver. This weakens the fish, and means that it cannot take in nutrients from the feed. The degree of severity of the disease varies, and the degree of severity is often greater the younger the fish is.  In serious cases, the mortality rate can be up to 100%. 

In 2015, 30 cases of IPN were reported in salmon and rainbow trout. This represents a reduction from 2014, when 48 cases were reported. Of the cases that affected Atlantic salmon, 6 were in hatcheries and 13 in fish farms. All of the 11 outbreaks that affected rainbow trout took place in the hatchery phase.

The symptoms are often most pronounced in hatcheries. Here you can see:

  • "Messing around on the surface": sick fish stay on the surface and spin around their own axes.
  • Dark pigmentation, especially on the back.
  • Exophthalmus: the eyes are bigger and protrude from the head.
  • Distended abdomen
  • Bleeding on the abdomen and at the base of the fins
  • Long white threads from the hole opening – abnormal stools due to lack of intake of nutrients from the intestines.
  • Fluid in the abdomen, pale organs, bleeding in the fatty tissue around the intestinal sacks, and empty intestines when an autopsy is carried out.


  • Detection of pathological changes in the pancreas, intestines and liver.
  • Immunohistochemistry
  • PCR

Combating the disease
IPN is often used as an example of how the joint efforts of the industry can contribute to limited the significance of a disease. IPN was formerly very prevalent, but its occurrence has been considerably reduced after the industry focused on developing methods to prevent the disease. This led to increased knowledge of operational measures to reduce infection in fish farms, the development of vaccines, and the discovery of a gene that could be associated with resistance to IPN. This gene made it possible to breed salmon stock that were not as susceptible to IPN. Today, the occurrence of IPN has been considerably reduced, and in 2009 it was removed from the list of reportable diseases.


HSMB - Heart and skeletal muscle inflammation

HSMI stands for heart and skeletal muscle inflammation and is a disease that is connected with infection with Piscint Ortoreovirus (PRV). The is one of the most frequently diagnosed diseases in Norwegian fish farming with 141 reported cases in 2014. The real figure is probably higher as the figure was removed from the list of reportable diseases midway through the year. 

First year at sea
HSMI often affects salmon in the course of their first year at sea and mortality varies up to 20%. The highest proportion of mortality is often seen in connection with operational procedures, as these expose the circulatory system of the fish to extra stress. If there is an outbreak of HSMI in a collection of livestock, fish farmers and fish health personnel must be especially vigilant of this when planning procedures such as delousing, sorting and similar. For example, this will influence the choice of agent used for delousing. 

Reduced growth
In addition to the increased mortality, an outbreak of HSMI may lead to reduced growth. Fish affected by disease often have a poor appetite, and in addition, some of the energy they receive will go towards fighting the disease. This can therefore lead to poorer utilisation of the feed, an extended production period and therefore increased production costs. 

In addition to the typical changes we see in autopsies on salmon that have died from HSMI, we will often find that the heart wall has thickened due to the infiltration of a large number of inflammatory cells. With an active outbreak of disease, we will also be able to find bleeding and inflammation in the skeletal muscles. 

There is currently no treatment for HSMI, but research indicates that you can influence resistance to and the degree of severity of the disease by changing the composition of fatty acids in the feed. The disease can also be prevented by reducing operational procedures that involve stress. (Source:

  • Where: Entire coastline, especially northern areas
  • When: Often during the first year at sea
  • Occurrence:
    Throughout the production phase – freshwater and seawater. 141 reported cases (-July 2015)
  • Problems of the disease
    Small external changes, typical autopsy findings are consistent with heart failure: fluid in the cardiac and abdominal cavities, pale heart, pale or yellowish liver, swollen spleen, punctiform accumulations of blood in adipose tissues surrounding the organs. Bleeding can be observed in the skeletal muscles.
  • How feed can be used in disease control:
    Research has shown that the course of disease can be influenced by adjusting the level of fatty acids in the feed.
  • Our feed: Qardio


CMS - cardiomyopathy syndrome

CMS is another viral disease that affects the heart, and has also been seen increasingly frequently in recent years. CMS stands for cardiomyopathy syndrome, which translates to ”disease of the heart muscles”. The disease is colloquially called "broken heart syndrome" as the typical autopsy findings are that the heart wall has torn or ruptured.

New virus

In 2010, CMS was associated with a new virus that was named Piscine Myocarditis Virus – PMCV, and the disease is characterised by the infiltration of large numbers of inflammatory cells into the heart muscles.

CMS outbreaks can lead to substantial financial losses for fish farmers as they affect salmon as they approach their harvest size. Then the fish farmer has to use substantial resources to ensure the salmon survive, both in the form of working hours that are used to care for the salmon, and for the use of feed. Mortality is on a par with HSMI primarily related to operational procedures, as the weakened heart is not able to withstand the increased physical stress it is exposed to. (Source:

  • Where: Entire coastline.
  • When:
    All year round, often affects large salmon and salmon close to slaughter size.
  • Occurrence: 107 reported outbreaks in 2014
  • Problems of the disease:
    Few external changes, often large and seemingly healthy salmon die with no visible changes.
  • How can feed can be used in disease control?
    Research has shown that the course of disease can be influenced by adjusting the level of fatty acids in the feed.
  • Our feed: Qardio