371
Molecular Methods There is evidence that molecular techniques are fi nding use
for the identifi cation of infections caused by Y. ruckeri (e.g. Argenton et al. 1996 ;
Taylor and Winton 2002 ; Sakai et al. 2006 ; Saleh et al. 2008 ). A PCR was successful
in detecting Y. ruckeri in artifi cially and naturally diseased trout tissues, with a sensitivity of 60–65 cells/PCR tube (Gibello et al. 1999 ). The value of PCR was echoed
by Altinok et al. ( 2001 ), who detected the pathogen in the blood of rainbow trout
within 1 h of immersion in a suspension containing 4.5 × 10
6 CFU of Y. ruckeri /l.
Indeed, the approach was more reliable than culturing at detecting the organism
(Altinok et al. 2001a , b). Detection of the yruR/yru1 genes involved with quorum
sensing has been regarded as sensitive (i.e. 1 pg; 12 CFU) specifi c for the 6 isolates
of Y. ruckeri tested but not to representatives of 5 other Yersinia species (Temprano
et al. 2001 ). Others have proposed a PCR and RFLP targeting the aroA gene
(Yugueros et al. 2001 ). A nested PCR had a detection limit of 1.4 × 10
5 CFU/reaction. However, use of species-specifi c primers improved detection to <14 CFU/
sample (Taylor and Winton 2002 ). LAMP, amplifying the yruI/yruR gene, which
encodes the quorum sensing system, was regarded as tenfold more sensitive that
PCR, detecting 1 pg of genomic DNA (Saleh et al. 2008 ). Real-time PCR (RT-PCR),
which targeted the glutamine synthetase (glnA) gene, was developed and proposed
for the detection of Y. ruckeri (Keeling et al. 2012 ). The RT-PCR was determined to
be absolutely specifi c for the pathogen, and detected 5 fg of bacterial DNA and
3 × 10
3 CFU/g of seeded kidney tissue (Keeling et al. 2012 ). RT-PCR based on the
r ecA gene was developed, and determined to be specifi c, sensitive and reproducible.
The sensitivity was given as 1.7 CFU/ ml for purifi ed DNA 3.4 CFU/g in the case of
tissues, i.e. liver, kidney, that had been seeded with the pathogen, and 0.34 CFU/100 μl
for blood (Bastardo et al. 2012 ).
Spectroscopy Fourier transform infrared spectroscopy has been regarded to be
extremely reliable and effective for the recognition of Y. ruckeri including the newer
non-motile varieties (Wortberg et al. 2012 ).
Epizootiology
In rainbow trout, ERM most commonly affects fi sh of approximately 7.5 cm in
length. The disease is less severe but more chronic in larger fi sh, i.e. of 12.5 cm in
length. Severity peaks with a water temperature of 15–18 °C, and decreases when it
drops to 10 °C or below. Overly fat or debilitated (stressed) fi sh are thought to be
more susceptible to severe epizootics (Rucker 1966 ). Moreover, occurrence of ERM
may be cyclic, suggesting the presence of asymptomatic carrier fi sh in the population (Busch and Lingg 1975 ). Such fi sh would be capable of shedding the organism
in the faeces. Alternatively, it may be argued that the pathogen is present normally
in the water and/or sediment (Rucker 1966 ). From the results of some fascinating
experiments, Hunter et al. ( 1980 ) demonstrated that unstressed carrier fi sh did not
Yersinia ruckeri
Molecular Methods There is evidence that molecular techniques are fi nding use
for the identifi cation of infections caused by Y. ruckeri (e.g. Argenton et al. 1996 ;
Taylor and Winton 2002 ; Sakai et al. 2006 ; Saleh et al. 2008 ). A PCR was successful
in detecting Y. ruckeri in artifi cially and naturally diseased trout tissues, with a sensitivity of 60–65 cells/PCR tube (Gibello et al. 1999 ). The value of PCR was echoed
by Altinok et al. ( 2001 ), who detected the pathogen in the blood of rainbow trout
within 1 h of immersion in a suspension containing 4.5 × 10
6 CFU of Y. ruckeri /l.
Indeed, the approach was more reliable than culturing at detecting the organism
(Altinok et al. 2001a , b). Detection of the yruR/yru1 genes involved with quorum
sensing has been regarded as sensitive (i.e. 1 pg; 12 CFU) specifi c for the 6 isolates
of Y. ruckeri tested but not to representatives of 5 other Yersinia species (Temprano
et al. 2001 ). Others have proposed a PCR and RFLP targeting the aroA gene
(Yugueros et al. 2001 ). A nested PCR had a detection limit of 1.4 × 10
5 CFU/reaction. However, use of species-specifi c primers improved detection to <14 CFU/
sample (Taylor and Winton 2002 ). LAMP, amplifying the yruI/yruR gene, which
encodes the quorum sensing system, was regarded as tenfold more sensitive that
PCR, detecting 1 pg of genomic DNA (Saleh et al. 2008 ). Real-time PCR (RT-PCR),
which targeted the glutamine synthetase (glnA) gene, was developed and proposed
for the detection of Y. ruckeri (Keeling et al. 2012 ). The RT-PCR was determined to
be absolutely specifi c for the pathogen, and detected 5 fg of bacterial DNA and
3 × 10
3 CFU/g of seeded kidney tissue (Keeling et al. 2012 ). RT-PCR based on the
r ecA gene was developed, and determined to be specifi c, sensitive and reproducible.
The sensitivity was given as 1.7 CFU/ ml for purifi ed DNA 3.4 CFU/g in the case of
tissues, i.e. liver, kidney, that had been seeded with the pathogen, and 0.34 CFU/100 μl
for blood (Bastardo et al. 2012 ).
Spectroscopy Fourier transform infrared spectroscopy has been regarded to be
extremely reliable and effective for the recognition of Y. ruckeri including the newer
non-motile varieties (Wortberg et al. 2012 ).
Epizootiology
In rainbow trout, ERM most commonly affects fi sh of approximately 7.5 cm in
length. The disease is less severe but more chronic in larger fi sh, i.e. of 12.5 cm in
length. Severity peaks with a water temperature of 15–18 °C, and decreases when it
drops to 10 °C or below. Overly fat or debilitated (stressed) fi sh are thought to be
more susceptible to severe epizootics (Rucker 1966 ). Moreover, occurrence of ERM
may be cyclic, suggesting the presence of asymptomatic carrier fi sh in the population (Busch and Lingg 1975 ). Such fi sh would be capable of shedding the organism
in the faeces. Alternatively, it may be argued that the pathogen is present normally
in the water and/or sediment (Rucker 1966 ). From the results of some fascinating
experiments, Hunter et al. ( 1980 ) demonstrated that unstressed carrier fi sh did not
Yersinia ruckeri