New Disease Poses Big Threat to Norwegian Farms

The increasing number of outbreaks of this disease in recent years indicates that it poses a significant threat to Norwegian salmon farming.

HMSI was discovered in 1999, and has since been found in disease outbreaks at many fish farms along the entire Norwegian coastline. In her doctorate, Kongtorp described the disease and compared the pathological findings to those of a number of known diseases such as pancreas disease (PD) and cardiomyopathy syndrome (CMS). Both field sampling and contact infection trials indicate that HMSI is a new disease caused by an unknown organism.

Kongtorp and her colleagues also investigated the progression of the disease in field and infection trials. HMSI primarily produces inflammation and cell death in the heart. This damage arises early on in the disease, and can continue for many months. The earliest changes found were inflammatory cells lining the blood vessels of the compact heart muscle layer, and in the lining layers of the heart muscle (the epicardium outside the heart and the endocardium inside). With time the heart muscle itself also becomes inflamed.

Salmon may develop significant damage in their heart musculature without showing obvious signs, but as a rule, outbreaks of disease with increased mortality start to occur when a significant amount of the heart is inflamed. HSMI produces heart changes in nearly 100% of affected fish. In this phase, inflammation and cell death spread to the red skeletal muscles, cell death to the liver, and oedema (swelling) and disturbed blood circulation to multiple organs of many fish.

Using infection trials, Kongtorp and her colleagues showed that several organs harbour the infectious organism during the acute phase of the disease. Samples of heart that contained the organism were taken two months before an outbreak, under the actual outbreak, and two months after the outbreak had subsided. The samples were capable of transmitting the disease to healthy experimental fish.

Cand. med. vet. Ruth Torill Kongtorp defended her Ph. D. thesis, entitled "Heart and skeletal muscle inflammation (HSMI) in Atlantic salmon, Salmo salar: pathology, pathogenesis and experimental infection", at the Norwegian School of veterinary Science, on January 23, 2009.

The work was carried out at the National Veterinary Institute, Oslo, ably assisted by fish health services and fish farmers. The project was financed by the Norwegian Research Council and by the Fishery and Aquaculture Industry Research Fund.