aquaculturehealth - Montana Water Center - Montana State University

NEW DISEASE IDENTIFIED
IN NORWAY’S FARMED COD
BY DR ANNE-BERIT OLSEN, JARLE MIKALSEN AND DR DUNCAN COLQUHOUN, (NATIONAL VETERINARY INSTITUTE, NORWAY)
FINFISH
FIGURE 1: SPLEEN LESIONS IN AN INFECTED FISH
Apreviously undescribed granulomatous disease in Atlantic
cod (Gadus morhua L) related to the presence of a Gramnegative,
facultative intracellular bacterium belonging to
the genus Francisella has recently been identified in Norway.
The initial outbreak was identified in a population of mature
cod weighing 2-4kg held in an enclosed natural seawater “basin”
on the western coast of Norway. Increased levels of mortality
were registered in July 2005 at a water temperature of around
14.5˚C and peaked in August. Accumulated mortality
reached approximately 40 percent in the five months
from July to November. 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 sizes in the spleen (see Figures 1 and 2), the heart
(Figure 3), kidney and liver.
The spleen 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 were 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
FIGURE 3: HEART LESIONS ARE ANOTHER SYMPTOM OF THE CONDITION
FIGURE 2: SEVERE LESIONS IN
AN EXCISED SPLEEN
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.Ofthe
limited temperatures tested, the best growth was identified at 22˚C,
with very weak growth registered at 30˚C and no growth at 37˚C,
suggesting that the bacterium is probably incapable of surviving
within a mammalian host.
OCCURRENCE
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.
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FEBRUARY 2006 AQUACULTURE HEALTH INTERNATIONAL 7