An outbreak of disease resembling heart and skeletal muscle inflammation in Scottish farmed salmon, Salmo salar L., with observations on myocardial regeneration

Keywords: Anitschkow-like nuclei, cardiac regener-ation, heart, myositis, salmon, Scotland.Heart diseasein farmedAtlantic salmon, Salmo salarL., is becoming a frequently encountered diagnosticproblem. Examples include cardiomyopathy syn-drome (CMS), a condition of unknown aetiology inwhich spongy myocardium of both ventricle andatriumistargeted,andwhichmaybecomesoseverelyaffectedthattheatriumruptures(Ferguson,Poppe&Speare 1990). Myocardium is also affected in theviral condition salmon pancreas disease (SPD),although in that case both compact and spongylayers are involved, and lesions are not as severe as inCMS. In addition, striated muscle elsewhere isaffected, including axial skeletal muscle and oeso-phagus,and,asmaybeinferredfromthenameofthedisease,exocrinepancreasisalsoinvolved(Ferguson,Roberts, Richards, Collins & Rice 1986).Heart and skeletal muscle inflammation (HSMI)is an infectious condition of undetermined causethat has recently been described in Norwegianfarmed salmon (Kongtorp, Kjerstad, Taksdal, Gutt-vik & Falk 2004). Once again, major lesions areencountered in myocardium, in both spongy andcompact layers, but cardiac lesions are more severethan those typically seen in SPD and in additionthere is often an epicardial infiltrate. Skeletal muscleis the other major location for lesions, red fibresbeing mostly affected.The present report documents an outbreak ofdisease in sea-caged salmon in Scotland in whichthe lesions mirror those described for HSMI. If it isindeed the same as HSMI, this outbreak wouldrepresent the first time the condition has beendescribed outside Norway.A marine salmon farm on the west of Scotlandexperienced significant mortality of second yeargrower fish from the third week in June 2004 untilthe site was cleared in September. Cumulativemortality for this period approached 9% and at theheight of the outbreak was escalating sharply,precipitating the decision to slaughter out. Priorto this, health status had been good and the onlysignificant health problem had been clinical infec-tious pancreatic necrosis (IPN) as post-smoltsduring summer 2003, resulting in approximately7% mortalities.Some cages had been held at a relatively highstocking density, and as a result had been split usinga well boat in June 2004. Mortality started in thesecages approximately 2 weeks after the handling buta rising trend subsequently developed in all cageswithin 5 weeks of the early losses. Mortalities wereinitially mostly large fish in good condition andwith no gross lesions. As the problem developed,however, an increasing number of dead fish showedpoor condition, scale loss, skin and fin erosion andabraded or ruptured eyes. Some cages showed areduction in feed intake prior to mortalities