species. C. piscicola can be isolated from seemingly healthy fish
and probably part of the normal microbiota of the gastrointestinal tract. Isolation has been reported from salmonids which are
over a year old, and up to and including broodstock held in fresh
water. Handling, post-spawning and other forms of stress appear
to be predisposing factors to what may become a chronic condition and although infrequently, there are reports of significant
outbreaks. The bacterium has also been reported as causing
autolytic changes in cold-smoked salmon.
Clinical signs are somehow inconsistent but overall indicative of a septicaemia, including general darkening, sub
dermal blisters, abdominal distension, pronounced bilateral
exophthalmia with periocular haemorrhaging (Fig. 6.42).
At necropsy, peritonitis, splenomegaly and diffuse
haemorrhaging involving the musculature, liver and swim
bladder, may be observed. Histologically, focal necrosis and
vacuolation of the kidney tubular endothelium, liver sinusoid congestion, hyaline droplet degeneration and pancreatic
acinar cell necrosis are reported.
Diagnosis is based on clinical signs, tissue sections and
isolation of the aetiological agent. C. maltaromaticum is
Gram positive, non-motile bacilli which grows well on
TSA at 22
C occurring singly and in short chains. A differential diagnosis would include Renibacterium.
6.17 Streptococcus phocae
Streptococcus phocae is reported as an emerging pathogen
for farmed Atlantic salmon smolts and adult fish in Chile
located in estuaries and marine waters. Outbreaks occur
during the summer when temperatures are above 15
C,
reaching in some occasions a cumulative mortality up to
25 % of the affected population. Infected fish show
exophthalmia with accumulation of purulent and
haemorrhagic fluid around eyes, and ventral petechial
haemorrhage. At necropsy, haemorrhage in the abdominal
fat, pericarditis and enlarged liver, spleen and kidney are
expected pathological changes.
The bacteria are Gram positive and beta-haemolytic.
Additional studies will be necessary to determine the clinical
significance of this species for the salmon industry.
6.18 Mycobacterium spp.
Mycobacterium spp. in teleosts is largely represented by
three species, M. chelonae, M. fortuitum and M. marinum.
They can infect and cause disease in a wide range of fish
species including salmonids, developing as a typically
chronic condition which may take several years to progress
into clinical disease.
Common clinical signs are emaciation, ocular lesions
with exophthalmia, change in skin colour, fin rot and ulceration of the skin. At the external examination, a subcutaneous
erythema and granulomatosis may occur on the ventral
aspect of the cranial abdomen. At necropsy, ascites and
greyish-white visceral nodules are seen in many organs but
particularly, the heart, kidney, liver and spleen, becoming
progressively swollen and fused by white membranes
(Fig. 6.43). Histologically, chronic or proliferative forms of
focal granulomas composed of various types of immune
Fig. 6.42 Carnobacterium maltaromaticum infection in farmed rainbow trout; dark coloured fish with splenomegaly and petecchia in liver
90
6 Bacterial Diseases
and probably part of the normal microbiota of the gastrointestinal tract. Isolation has been reported from salmonids which are
over a year old, and up to and including broodstock held in fresh
water. Handling, post-spawning and other forms of stress appear
to be predisposing factors to what may become a chronic condition and although infrequently, there are reports of significant
outbreaks. The bacterium has also been reported as causing
autolytic changes in cold-smoked salmon.
Clinical signs are somehow inconsistent but overall indicative of a septicaemia, including general darkening, sub
dermal blisters, abdominal distension, pronounced bilateral
exophthalmia with periocular haemorrhaging (Fig. 6.42).
At necropsy, peritonitis, splenomegaly and diffuse
haemorrhaging involving the musculature, liver and swim
bladder, may be observed. Histologically, focal necrosis and
vacuolation of the kidney tubular endothelium, liver sinusoid congestion, hyaline droplet degeneration and pancreatic
acinar cell necrosis are reported.
Diagnosis is based on clinical signs, tissue sections and
isolation of the aetiological agent. C. maltaromaticum is
Gram positive, non-motile bacilli which grows well on
TSA at 22
C occurring singly and in short chains. A differential diagnosis would include Renibacterium.
6.17 Streptococcus phocae
Streptococcus phocae is reported as an emerging pathogen
for farmed Atlantic salmon smolts and adult fish in Chile
located in estuaries and marine waters. Outbreaks occur
during the summer when temperatures are above 15
C,
reaching in some occasions a cumulative mortality up to
25 % of the affected population. Infected fish show
exophthalmia with accumulation of purulent and
haemorrhagic fluid around eyes, and ventral petechial
haemorrhage. At necropsy, haemorrhage in the abdominal
fat, pericarditis and enlarged liver, spleen and kidney are
expected pathological changes.
The bacteria are Gram positive and beta-haemolytic.
Additional studies will be necessary to determine the clinical
significance of this species for the salmon industry.
6.18 Mycobacterium spp.
Mycobacterium spp. in teleosts is largely represented by
three species, M. chelonae, M. fortuitum and M. marinum.
They can infect and cause disease in a wide range of fish
species including salmonids, developing as a typically
chronic condition which may take several years to progress
into clinical disease.
Common clinical signs are emaciation, ocular lesions
with exophthalmia, change in skin colour, fin rot and ulceration of the skin. At the external examination, a subcutaneous
erythema and granulomatosis may occur on the ventral
aspect of the cranial abdomen. At necropsy, ascites and
greyish-white visceral nodules are seen in many organs but
particularly, the heart, kidney, liver and spleen, becoming
progressively swollen and fused by white membranes
(Fig. 6.43). Histologically, chronic or proliferative forms of
focal granulomas composed of various types of immune
Fig. 6.42 Carnobacterium maltaromaticum infection in farmed rainbow trout; dark coloured fish with splenomegaly and petecchia in liver
90
6 Bacterial Diseases