Occurrence of Francisella piscicida in farmed and wild Atlantic cod, Gadus morhua L., in Norway K F Ottem 1 , A Nylund 1 , T E Isaksen, E Karlsbakk 1,2 and Ø Bergh 2 1 Department of Biology, University of Bergen, Bergen, Norway 2 Institute of Marine Research, Nordnes, Bergen, Norway Abstract Francisellosis, caused by the bacterium Francisella piscicida, has become one of the most serious diseases in Atlantic cod production in Norway. The major aim of this study was to determine the distribution of F. piscicida in farmed and wild fish in areas with cod farming along the Norwegian coast, and its occurrence in cod from areas without cod farming. Two real-time PCR assays, targeting the 16S rRNA gene and the FopA gene of F. pi- scicida, were developed since sensitive and specific diagnostic tools are required for detecting asymp- tomatic carriers of the bacterium. A total of 422 wild cod from 13 sampling areas and 955 farmed cod from 10 areas along the coast of Norway were examined. Using the real-time polymerase chain reaction (PCR) assays, F. piscicida was detected in wild populations of cod from all counties exam- ined south of Sogn og Fjordane in southern Norway (overall prevalence 13%, n = 221). Wild cod north of Sogn og Fjordane were negative for the bacterium (n = 201). Farmed cod from most parts of Norway were F. piscicida positive. The apparent absence of the bacterium in wild popu- lations of cod in the northern parts of Norway and its widespread occurrence in wild cod from southern parts of Norway is believed to relate to differences in seawater temperatures. Keywords: Atlantic cod, Francisella piscicida, Gadus morhua, Norway, occurrence, PCR. Introduction In the autumn of 2004, a previously unknown bacterium, Francisella piscicida, was isolated from Atlantic cod, Gadus morhua L., in a farm in Rogaland County, western Norway (Nylund, Ot- tem, Watanabe, Karlsbakk & Krossøy 2006; Ottem, Nylund, Karlsbakk, Friis-Møller & Krossøy 2006; Ottem, Nylund, Karlsbakk, Knappskoy & Krossøy 2007b). The fish showed loss of appetite, reduced swimming performance and dark pigmentation. The most prominent internal signs were a swollen spleen, kidney and heart, which were covered with, and penetrated by, white granulomas. The closest relative to F. piscicida is Francisella philomiragia (Nylund et al. 2006; Ottem et al. 2006). Other isolates of the genus Francisella had previously been isolated from tilapia, Oreochromis spp., in Taiwan and three-line grunt, Parapristipoma trilineatum L., in Japan (Fukuda, Okamura, Nishiyama, Kawaka- mi, Kamaishi & Yoshinago 2000; Kamaishi, Fukuda, Nishiyama, Kawakami, Matsuyama, Yoshinaga & Oseko 2005). After the reports of these isolates, other Francisella isolates have been obtained from several fish species worldwide (Hsieh, Wu, Tung & Tsui 2007; Kay, Peterson, Duus, Perry & Vinogradev 2006; Ostland, Stannard, Creek, Hedrick, Ferguson, Carlberg & Westerman 2006). The unknown Agent 2 (UA2) described from Atlantic salmon, Salmo salar L., parr in fresh water in Chile (Cvitanich, Garate, Silva, Andrave, Figueroa & Smith 1995) was also recently identi- fied as a member of the genus Francisella (Birkbeck, Bordevik, Frøystad & Baklien 2007). Since the first isolation of F. piscicida, several new cases of francisellosis have been recorded in Norway, mainly in the southwestern counties Journal of Fish Diseases 2008, 31, 525–534 doi:10.1111/j.1365-2761.2008.00930.x Correspondence K F Ottem, Department of Biology, University of Bergen, Thormohlensgt 55, 5020 Bergen, Norway (e-mail: karl.ottem@bio.uib.no) 525 Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Publishing Ltd