Transmission and reservoirs of Francisella piscicida. Implications for wild and farmed cod.

A new species of Francisella (F. piscicida), causing mortality in farmed cod (Gadus morhua), was isolated autumn 2004 from farmed cod in Rogaland. In 2005 this bacterium was isolated from several cod farms in Rogaland, Hordaland, Møre og Romsdal, Nordland and Denmark and from Atlantic salmon in Rogaland. A closely related bacterium has also been isolated from tilapia and isaki in Taiwan and Japan and from salmon in Chile. F. piscicida is closely related to another aquatic member of this genus, F. philomir agia; which may cause human diseases. However, biochemical and genetic characteristics clearly shows that the bacterium isolated from cod, F. piscicida, is a distinct species and not a subspecies of F. philomiragia. F. piscicida is known to replicate in both salmon and cod, it may survive for more than a year outside the host. The optimal temperature for replication of F. piscicida is 20 ºC, which is much higher than the optimal temperature for the welfare of cod. This bacterium has emerged as one of th e most serious pathogens affecting cod production in Norway. Hence, there is a strong need for fast, specific and sensitive diagnostic tools that can be used for diagnosis of disease and screening of possible carriers. It is also of vital importance to id entify natural and farmed reservoirs for F. piscicida and to identify the major transmission routes (vectors, horizontal and transgenerational routes). It is also important, for future production of cod in western Norway that information is obtained about the effects of temperature on cod during infection with F.piscicida.