428
Epizootiology
Although the precise reservoir of Fla. psychrophilum has not been established, we
have recovered some organisms, with the key characteristics of Fla. psychrophilum,
from fresh water in England. However, it is uncertain whether or not the organisms
were native, or merely transient, in the aquatic environment. Cells may be able to
enter a VBNC state following 2-days incubation at 28 °C although the relevance to
pathogenicity is questionable insofar as the cells did not cause clinical disease
(Sugahara et al. 2010b ). Starvation over 14-days resulted in morphological changes
with cells maintaining their length but the majority (80 %) changed into coiled
forms that remained culturable, and subsequently developed an extracellular matrix
that covered the cells; starved cells were avirulent. Revival in the presence of nutrients led to cells returned to their original shape (Arias et al. 2012 ). The organism
forms biofi lms, and of signifi cance is that attached cells are less susceptible to
inhibitory agents (Sundell and Wiklund 2011 ). Recovery has been achieved from
overwintering ayu in a Japanese river, with the suggestion that the fi sh may be the
source of infection during the following spring (Miyazaki 2008 ). Fla. psychrophilum was isolated from the internal organs (spleen) and/or sexual products (gonad
but not the eggs at the eyed stage) in 7/50 (=14.0 %) Baltic salmon (Salmo salar)
brood fi sh sampled at capture and 63/272 (=23.2 %) of fi sh examined at stripping.
Also, the pathogen was recovered from the spleen or gonads in 2/19 (=10.5 %) of
the fi sh with abnormal wiggling behaviour. Overall, the possibility exits that brood
fi sh could transfer the pathogen to their progeny (Ekman et al. 1999 ). Additionally,
Kumagai et al. ( 2011 ) found the organism in wild ayu from four Japanese rivers that
had been artifi cially stocked with ayu. In this study, the isolation rate from apparently healthy fi sh was in the range of 58–100 % with the isolates demonstrating
virulence to juvenile ayu (Kumagai et al. 2011 ). Kumagai and Nawata ( 2011a )
reported the presence of the pathogen in the ovarian fl uid [544 out of 3276 fi sh; but
less so intra-ovum (Kumagai and Nawata 2011b )] and milt [248 out of 1434 fi sh] of
farmed salmonids in Japan, and suggested that the organism is widely distributed.
There is other evidence that Fla. psychrophilum may be transmitted within salmonid eggs (Brown et al. 1997 ), possibly entering during water hardening (Kumagai
et al. 2000 ). This same team reported the presence of the organism on the surface of
but not inside ayu eggs but not after surface sterilization with 5 mg/l of povidoneiodine for 10 min or 150 mg/l hydrogen peroxide for 30 min (Kumagai et al. 2004b ).
Also, the organism has been found in wild fi sh, namely perch and roach, from the
ovarian fl uids and milt of rainbow trout broodstock, and from waters (in close proximity to farmed trout or their eggs) in rainbow trout farms in Denmark (Madetoja
et al. 2002 ; Madsen et al. 2005 ), in the eggs, gill or kidney tissue of wild ayu and
other feral fi sh in Japan (Amita et al. 2000 ) and from water in Finnish trout farms as
determined by nested PCR and iFAT but not culturing which undoubtedly lacked
sensitivity (Madetoja and Wiklund 2002 ). More than likely, wild fi sh and broodstock are a possible source of infection.
7 Flavobacteria and Cytophagas
Epizootiology
Although the precise reservoir of Fla. psychrophilum has not been established, we
have recovered some organisms, with the key characteristics of Fla. psychrophilum,
from fresh water in England. However, it is uncertain whether or not the organisms
were native, or merely transient, in the aquatic environment. Cells may be able to
enter a VBNC state following 2-days incubation at 28 °C although the relevance to
pathogenicity is questionable insofar as the cells did not cause clinical disease
(Sugahara et al. 2010b ). Starvation over 14-days resulted in morphological changes
with cells maintaining their length but the majority (80 %) changed into coiled
forms that remained culturable, and subsequently developed an extracellular matrix
that covered the cells; starved cells were avirulent. Revival in the presence of nutrients led to cells returned to their original shape (Arias et al. 2012 ). The organism
forms biofi lms, and of signifi cance is that attached cells are less susceptible to
inhibitory agents (Sundell and Wiklund 2011 ). Recovery has been achieved from
overwintering ayu in a Japanese river, with the suggestion that the fi sh may be the
source of infection during the following spring (Miyazaki 2008 ). Fla. psychrophilum was isolated from the internal organs (spleen) and/or sexual products (gonad
but not the eggs at the eyed stage) in 7/50 (=14.0 %) Baltic salmon (Salmo salar)
brood fi sh sampled at capture and 63/272 (=23.2 %) of fi sh examined at stripping.
Also, the pathogen was recovered from the spleen or gonads in 2/19 (=10.5 %) of
the fi sh with abnormal wiggling behaviour. Overall, the possibility exits that brood
fi sh could transfer the pathogen to their progeny (Ekman et al. 1999 ). Additionally,
Kumagai et al. ( 2011 ) found the organism in wild ayu from four Japanese rivers that
had been artifi cially stocked with ayu. In this study, the isolation rate from apparently healthy fi sh was in the range of 58–100 % with the isolates demonstrating
virulence to juvenile ayu (Kumagai et al. 2011 ). Kumagai and Nawata ( 2011a )
reported the presence of the pathogen in the ovarian fl uid [544 out of 3276 fi sh; but
less so intra-ovum (Kumagai and Nawata 2011b )] and milt [248 out of 1434 fi sh] of
farmed salmonids in Japan, and suggested that the organism is widely distributed.
There is other evidence that Fla. psychrophilum may be transmitted within salmonid eggs (Brown et al. 1997 ), possibly entering during water hardening (Kumagai
et al. 2000 ). This same team reported the presence of the organism on the surface of
but not inside ayu eggs but not after surface sterilization with 5 mg/l of povidoneiodine for 10 min or 150 mg/l hydrogen peroxide for 30 min (Kumagai et al. 2004b ).
Also, the organism has been found in wild fi sh, namely perch and roach, from the
ovarian fl uids and milt of rainbow trout broodstock, and from waters (in close proximity to farmed trout or their eggs) in rainbow trout farms in Denmark (Madetoja
et al. 2002 ; Madsen et al. 2005 ), in the eggs, gill or kidney tissue of wild ayu and
other feral fi sh in Japan (Amita et al. 2000 ) and from water in Finnish trout farms as
determined by nested PCR and iFAT but not culturing which undoubtedly lacked
sensitivity (Madetoja and Wiklund 2002 ). More than likely, wild fi sh and broodstock are a possible source of infection.
7 Flavobacteria and Cytophagas