Short communication Pasteurellosis in Atlantic salmon, Salmo salar L.: immunohistochemistry of the naturally-occurring disease L Foyle 1 , T Turnbull 2 , A Ellis 3 , A Barnes 3 , A Adams 1 and H W Ferguson 1 1 Institute of Aquaculture, University of Stirling, Scotland 2 Scottish Seafarms, Argyll, Scotland 3 Marine Laboratory, Aberdeen, Scotland Keywords: Atlantic salmon, immunohistochemistry, pasteurellosis, pathology. In aquaculture, pasteurellosis caused by Photobacte- rium damselae subspecies piscicida (Ph. d. subsp. p.) is a globally significant disease. Typically it presents as a granulomatous inflammatory response in a range of organs. The causative bacterium was originally named Pasteurella piscicida (Janssen & Surgalla 1968) when it was isolated from a dead white perch, Morone americana (Gmelin). This bipolar staining, rod-shaped pathogen has since been recorded in many countries, and in Europe, bacteria that share with it biochemical and serolog- ical characteristics have been isolated since 1990, particularly from cultured sea bass, Dicentrarchus labrax (L.), gilthead sea bream, Sparus aurata L., and Dover sole, Solea solea (L.). The bacterium was subsequently renamed Ph. d. subsp. p. after phylo- genetic analysis of its small subunit rRNA sequences (Gauthier, Lafay, Ruimy, Breittmayer, Nicholas, Gauthier & Christen 1995). Pasteurellosis with typical lesions has not been recorded in salmonids, but in the last few years, there have been losses at several commercial Atlantic salmon, Salmo salar L., farms in Scotland, all associated with a Pasteurella-like microorganism, suggesting a causal relationship. This bacterium was tentatively named Pasteurella salmonicida (Jones & Cox 1999) and more recently, as Pasteurella skyense sp. nov. (Birkbeck, Laidler, Grant & Cox 2002). Pathological changes associated with these out- breaks were primarily bacteraemic with various signs of systemic inflammation. In addition, however, an unusual lesion (Ôvolcanic rimÕ) was noted in the spleen and liver (Jones & Cox 1999). This communication reports preliminary data on the immunohistochemical (IHC) co-localization of P. skyense antigens with lesions in the naturally- occurring disease. In particular the Ôvolcanic rimÕ lesions were examined to see if, as suggested, they are an unusual host response to this bacterial antigen. A bacterial isolate of P. skyense (LN45X) from one of the Scottish outbreaks in question was grown on blood agar base with 5% added defibrinated blood (Oxoid Horse Blood, Oxoid Ltd, Basing- stoke, England). A Ôground-glassÕ growth was seen after 48–72 h at 15 °C (normal atmosphere), consisting of Gram-negative, non-motile bacteria, forming small, grey, round, convex colonies. Midi Labs (Newark, NJ, USA) compared the 16S rRNA alignment profiles of (mainly terrestrial) Pasteurella sp. with P. skyense. The Pasteurella sp. with the most similar profile was P. trehalosi, which was 8.33% different, although Haemophilus parahaemolyticus was the same, and H. paracuniculus was closer with 7.58% difference. Ph. d. subsp. d. revealed a 17.55% difference. While no identification was made, it was shown by rRNA profiling that the same bacterium was isolated from two of the outbreaks involved in this discussion. It was felt, therefore, that the isolate used in these experiments was representative of the disease overall. Journal of Fish Diseases 2003, 26, 373–376 Correspondence Professor H W Ferguson, Institute of Aquaculture, University of Stirling, Stirling, FK9 4LA, Scotland (e-mail: hwf1@stir.ac.uk) 373 Ó 2003 Blackwell Publishing Ltd