The initial outbreak was identified in a population of mature cod (2-4kg) held in an enclosed natural seawater basin on the West Coast of Norway. Increased levels of mortality were registered in July 2005 at a water temperature of around 14.5C and peaked in August. Accumulated mortality reached approximately 40% in five months (July-November), and other pathogenic agents were not thought to have contributed significantly to this total.
Clinical and pathological findings
Externally the fish appeared to have a generally emaciated condition. Some individuals displayed raised haemorrhagic nodules in the skin. All moribund fish caught for examination showed extensive internal gross lesions with moderate to massive occurrence of white, partly protruding nodules of various size in the spleen, heart, kidney and liver. The spleen (see photograph) was enlarged and sero-haemorrhagic ascites and thickened intestinal mucosa were observed. Extensive chronic granulomatous inflammation with multiple granuloma in all organs was the main histopathological finding. Few to numerous small Gram-negative bacteria was found intracellularly in the granulomas
Identification of isolated bacteria
The bacterium associated with the infection does not grow on standard microbiological media but grows well in cell culture and on media with a high cysteine content. A nearly complete 16S ribosomal RNA sequence was obtained showing a high degree of homology with Francisella spp. including isolates previously identified in fish in Taiwan (tilapia) and Japan (three-lined grunt). Although the present isolate has yet to be fully described, the phenotypical evidence so far available comprises compelling evidence for its inclusion within the genus Francisella. Of the limited temperatures tested, best growth was identified at 22oC, with very weak growth registered at 30oC and no growth at 37oC, suggesting that the bacterium is probably incapable of surviving within a mammalian host.
So far three outbreaks have been confirmed by bacterial isolation. Histopathological findings consistent with those described above have been identified in several other cases currently under microbiological investigation.
Spleen from cod with Francisella-infection.
(Photo: Anne Berit Olsen, National Veterinary Institute, Norway)
Source: Norways National Veterinary Institute - 14th December 2005