617
Diagnosis
Molecular Methods A PCR has been successful for the detection of Moritella
viscosa with a detection limit of 6.09 × 10
−14 g of DNA, which is equivalent to 10
bacterial genomes (Grove et al. 2008 ).
Pathogenicity
The pathogen demonstrated the ability to adhere to mucus from Atlantic salmon
epithelial surfaces, i.e. foregut, gills, hindgut, pyloric caeca and skin (Knudsen et al.
1999 ). Epidermal keratocytes have been shown to remove the pathogen from surfaces, but the engulfment capacity was considered to be ineffi cient. Immersion challenge revealed a signifi cant relationship between the area of exposure and mortality.
Moreover, better invasion capability and mortality resulted with exposure of the
head and gill region rather then elsewhere on the body surface. The development of
ulceration corresponded to the area of the surface exposed to the pathogen, which
suggested that the formation of ulcers resulted principally from skin surface colonization (Karlsen et al. 2014b ).
Experimental infections were achieved in juvenile cod (minimum lethal
dose = 4.0 × 10
4 CFU/fi sh by i.m. or i.p.) and to a lesser extent in halibut (minimum
lethal dose = 6.5 × 10
4 CFU/fi sh by i.m. or i.p). Cod was infected by immersion challenge in 2 × 10
7 CFU/ml for two separate periods of 1 h with 37.5 % mortalities
ensuing within 22 days at 9 ± 1 °C (Gudmundsdóttir et al. 2006 ). ECPs, which contain varying levels of esterase, metallopeptide, cytotoxic and haemolytic activities,
and a lethal toxic factor, have been associated with virulence (Bjornsdottir et al.
2011 ). Three putative toxins have been identifi ed as increasing their transcription
over time during the infection process, and include a cytotoxic necrotizing factor
( cnf ), a haemolysin and a putative repeat in toxin gene ( rtxA ) with the outcome
being rounding of fi sh cells (Tunsjø et al. 2011 ). Also, there was an upregulation of
putative lateral fl agellin and a protease in Atlantic salmon ulcer tissues, which may
refl ect their involvement in colonization and subsequent tissue damage (Tunsjø
et al. 2011 ). A type VI secretion system has now been described. This system,
which is important in virulence, is involved in the transportation of bacterial proteins from the cell into the host cell or into the surrounding environment (Bjornsdottir
et al. 2012 ).
Two major genetic clades have been recognized, one of which is mostly restricted
to Atlantic salmon. Highly virulent isolates from Norwegian Atlantic salmon were
less pathogenic to rainbow trout. Other less typical isolates were somewhat pathogenic to both salmonid species (Karlsen et al. 201 4a , b ). All isolates revealed DNA
sequences encoding aerolysin, bacterioferritins, cytotoxic necrotizing factor
(mostly), haemolysin coregulated protein, invasion, lectin, MARTX, Type VI secretion ATPase, and phospholipase. A putative insecticidal toxin complex was detected
mostly in Atlantic salmon isolates (Karlsen et al. 2014a , b ).
Moritellaceae Representatives
Diagnosis
Molecular Methods A PCR has been successful for the detection of Moritella
viscosa with a detection limit of 6.09 × 10
−14 g of DNA, which is equivalent to 10
bacterial genomes (Grove et al. 2008 ).
Pathogenicity
The pathogen demonstrated the ability to adhere to mucus from Atlantic salmon
epithelial surfaces, i.e. foregut, gills, hindgut, pyloric caeca and skin (Knudsen et al.
1999 ). Epidermal keratocytes have been shown to remove the pathogen from surfaces, but the engulfment capacity was considered to be ineffi cient. Immersion challenge revealed a signifi cant relationship between the area of exposure and mortality.
Moreover, better invasion capability and mortality resulted with exposure of the
head and gill region rather then elsewhere on the body surface. The development of
ulceration corresponded to the area of the surface exposed to the pathogen, which
suggested that the formation of ulcers resulted principally from skin surface colonization (Karlsen et al. 2014b ).
Experimental infections were achieved in juvenile cod (minimum lethal
dose = 4.0 × 10
4 CFU/fi sh by i.m. or i.p.) and to a lesser extent in halibut (minimum
lethal dose = 6.5 × 10
4 CFU/fi sh by i.m. or i.p). Cod was infected by immersion challenge in 2 × 10
7 CFU/ml for two separate periods of 1 h with 37.5 % mortalities
ensuing within 22 days at 9 ± 1 °C (Gudmundsdóttir et al. 2006 ). ECPs, which contain varying levels of esterase, metallopeptide, cytotoxic and haemolytic activities,
and a lethal toxic factor, have been associated with virulence (Bjornsdottir et al.
2011 ). Three putative toxins have been identifi ed as increasing their transcription
over time during the infection process, and include a cytotoxic necrotizing factor
( cnf ), a haemolysin and a putative repeat in toxin gene ( rtxA ) with the outcome
being rounding of fi sh cells (Tunsjø et al. 2011 ). Also, there was an upregulation of
putative lateral fl agellin and a protease in Atlantic salmon ulcer tissues, which may
refl ect their involvement in colonization and subsequent tissue damage (Tunsjø
et al. 2011 ). A type VI secretion system has now been described. This system,
which is important in virulence, is involved in the transportation of bacterial proteins from the cell into the host cell or into the surrounding environment (Bjornsdottir
et al. 2012 ).
Two major genetic clades have been recognized, one of which is mostly restricted
to Atlantic salmon. Highly virulent isolates from Norwegian Atlantic salmon were
less pathogenic to rainbow trout. Other less typical isolates were somewhat pathogenic to both salmonid species (Karlsen et al. 201 4a , b ). All isolates revealed DNA
sequences encoding aerolysin, bacterioferritins, cytotoxic necrotizing factor
(mostly), haemolysin coregulated protein, invasion, lectin, MARTX, Type VI secretion ATPase, and phospholipase. A putative insecticidal toxin complex was detected
mostly in Atlantic salmon isolates (Karlsen et al. 2014a , b ).
Moritellaceae Representatives