The microsporidian Paranucleospora theridion was discovered in Atlantic salmon Salmo salar suffering from proliferative gill disease in a marine farm in western Norway in 2008. The parasite develops in cells of the reticuloendothelial system, cells important for normal immune function. The aim of this study was to see if P. theridion could play a part in some of the diseases with unclear causes in salmon production in Norway, i.e. proliferative gill disease (PGI), pancreas disease (PD), heart and skeletal muscle inflammation (HSMI) and cardiomyopathy syndrome (CMS). P. theridion was present in all areas with salmon farming in Norway, but high prevalence and densities of the parasite in salmon and salmon lice were only seen in southern Norway. This region is also the main area for PGI and PD in Norway. Quantification of pathogens associated with PGI, PD, HSMI and CMS diagnoses showed that P. theridion levels are high in southern Norway, and may therefore play a role in susceptibility and disease development. However, among the different diagnoses, fish with PGI are particularly heavily infected with P. theridion. Therefore, P. theridion appears as a possible primary agent in cases with high mortality in connection with PGI in western Norway. KEY WORDS: Paranucleospora theridion · Atlantic salmon · Microsporidia · Proliferative gill disease · PGI · Pancreas disease · PD Resale or republication not permitted without written consent of the publisherDis Aquat Org 94: [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] 2011 Clavochlamydia salmonicola , Atlantic salmon paramyxovirus (Kvellestad et al. 2005), and salmon gill poxvirus . These agents may all contribute to the gill pathology, hypertrophia, hyperplasia, inflammation and necrosis of gill tissues, but it is not known whether they are primary pathogens or secondary invaders.It is well documented that 'pancreas disease' is associated with the presence of different strains of SAV (Nelson et al. 1995, McLoughlin et al. 1996, Villoing et al. 2000, Hodneland et al. 2005, Fringuelli et al. 2008). However, the observed disease and mortality in salmon and rainbow trout farms are different from what can be seen in challenge experiments using the different SAV strains . The explanations for this could be presence of different pathogens or other stress factors that may trigger the disease or be the primary cause of disease, reducing SAV to a secondary cause of the observed mortality in farms. Lack of comparable mortality and to a certain extent also pathology has also been observed when comparing challenge experiments using CMS and HSMI homogenates with the observed situation in salmon farms affected by these 2 diseases (Kongtorp et al. 2004a, Fritsvold et al. 2009. Hence, the trigger or primary causes for these diseases could be environmental factors or other pathogens.Paranucleospora theridion was first discovered in farmed Atlantic salmon suffering from gill disease in western Norway in (A. Nylund et al. 2009a,b, S. Nylund et al. 2009. Th...

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