The pathogenic agent and transmission
The disease HSMI appeared in salmon farming in Norway in 1999. In 2010, a reovirus, Orthoreovirus piscis (PRV, previously Piscine orthoreovirus), was found in tissue from HSMI-affected salmon. This is a virus with a double protein capsid without lipid membrane, with segmented, double-stranded RNA as its genetic material. The virus is widespread and found also in healthy farmed salmon, wild salmon, rainbow trout (Oncorhynchus mykiss), and sea trout (Salmo trutta). There are several variants (genotypes) of the virus, with PRV-1 most commonly found in salmon, while the genotype common in rainbow trout and sea trout is called PRV-3. The PRV-2 variant has not been found in Norway, but in Canada and Japan. These genotypes are genetically about 80-90 % similar.
The disease HSMI has so far only been detected in farmed salmon and is associated with large amounts of PRV-1 in the heart and all blood-filled organs. In 2014, experiments showed that PRV initially infects the red blood cells of the fish before the virus is transferred to the heart and muscle. The viral RNA genome can be detected in blood long before and even longer after the disease, often persisting until slaughter. In salmon, it is not common for the number of red blood cells to decrease (anemia) during infection.
The disease is transmitted through water contact between infected and non-infected salmon. Transmission occurs mostly in the period before heart inflammation develops, but the virus can also spread afterward. Fish can recover from HSMI but may carry the virus for life after a disease outbreak. PRV-1 is found in almost 90 % of farmed salmon in the sea phase in Norway. Studies suggest that there is a low prevalence of the virus in migrating wild salmon, indicating that farmed salmon are more susceptible.
The disease
HSMI in farmed salmon causes inflammation in the heart. Such inflammatory damage can occur without signs of clinical disease or fish death and can be present for 1-2 months. The fish can also have inflammation in the red skeletal muscle, especially if the fish is also clinically ill. In the early phase of inflammation, the epicardium (outer membrane of the heart) thickens due to influx of inflammatory cells. This develops into further infiltration of inflammatory cells into the compact (outer) and eventually into the spongy (inner) muscle layer of the heart chamber and atrium, and the heart muscle fibers can die. Similar changes can be seen in the red (dark) skeletal muscles. In some HSMI outbreaks, the fish can also have necrotic cell death in the liver, changes in the spleen morphology, and circulatory disturbances. Autopsy findings in fish victim to HSMI suggest death from circulatory failure. Blood clots in the heart cavities are not uncommon, and sometimes the heart itself can be pale. The fish may have fluid in the abdominal cavity (ascites), a yellowish liver, enlarged spleen, and pinpoint bleeding in the fatty tissue.
Salmon can have significant heart inflammation without showing external signs of disease, but in outbreaks with increased mortality the inflammation is usually found to be extensive. Fish death in affected pens can vary from negligible to about 20 %, and many individuals can be affected (high morbidity). It has been shown that low oxygen levels in the water during infection can lead to a lower tolerance for the disease, and it is expected that stressful handling, like crowding of the fish during treatments in well boats, can have a similar effect.
Diagnosis
HSMI is diagnosed by microscopy of prepared heart (and muscle) samples (histopathology). This method diagnoses HSMI by detecting the characteristic inflammation in the heart, and sometimes also in the red skeletal muscles. A PCR method is commonly used to detect the PRV genome. Several antibodies (immunohistochemistry) and gene sequence probes (in situ hybridization) have also been developed to detect the virus by staining infected tissue.
Occurrence
Since first detected in Nord-Møre (mid-Norway west coast area) in 1999, HSMI has gradually become one of the most common diseases in aquaculture facilities on the entire Norwegian coast, along with the disease cardiomyopathy syndrome (CMS) or "heart rupture". HSMI is particiularly dominant in Mid and Northern Norway.
HSMI was a notifiable disease until 2014, and there was therefore a good overview of disease cases until then. In the following years, when the service of private diagnostics of fish diseases increased, the extent of HSMI outbreaks was not fully reported to the Norwegian veterinary institute. Since 2020, improved data exchange showed that HSMI outbreaks had not decreased in number.
So far, HSMI has been diagnosed in Norway, Scotland, Ireland, and Chile - and – to a more limited extent- also on the west coast of Canada. PRV-1 is found in most countries that farm salmon, but in many cases without disease being detected. There seem to be fewer HSMI cases in salmon farmed in Iceland and Canada, even though infection with PRV is equally common. It has previously been shown that PRV-1 from Canada does not cause as severe disease in experiments compared to Norwegian PRV-1 isolates, even though the viruses are more than 95% similar. Also, it appears that the salmon genetics plays a role in susceptibility. Also in Norway, there seem to be several genetic subvariants of PRV-1 that cause more or less disease, and it is believed that the variants that today cause the most disease cases developed in the 1990s when the disease first appeared in Norway.
Salmon usually get HSMI in their first half year at sea, but there are also several cases of disease in hatcheries. The survey made for the annual Fish Health Report shows that HSMB is most significant for the mortality rates at sea, but also considered a problem in hatcheries.
Heart disease and anemia in rainbow trout caused by PRV-3 is also an occurring problem in Norway, see fact sheet.
Disease control
There is no current effective treatment against HSMB, but with careful handling, mortality can be at least partly avoided. One of the most important preventive measures is to avoid stressing the fish in the period when it suffers from a weakened heart due to HSMI. This can pose challenges in connection with treatments against sea lice, relocations, and other operational routines.
There is no vaccine against HSMI on the market, but several vaccines have been tested experimentally, and some have a certain effect without completely blocking the virus. A vaccine made from the whole virus is difficult to produce since PRV cannot be cultured in cells. It has been shown in experiments that salmon infected with PRV-3 do not get any disease, but are well protected against secondary infection with PRV-1. PRV-3 cannot be used as a vaccine to date, since the virus can cause disease in rainbow trout and infect sea trout. A type of HSMI-protective feed is reported to limit the severity of heart inflammation. Salmon are currently also bred for better resistance against HSMI. So far, these measures have not been able to significantly reduce the number of HSMI cases in Norway.
Reports
An overview of emerging diseases in the salmonid farming industry - Technical report 2018
Fish health report 2024
Research projects
Heart health in Norwegian salmon farming
RED FLAG: Salmonid red blood cells – sensors of stress and infection.
