256
recovered in the blood of 17 % (3 out of 18) of trout examined, and it was concluded
that carriers provided a source of infection in the river. He also sampled fi sh from
fi sh farms and found them to be generally healthy and thus contended that these
results indicated that trout farms were not harbouring the disease. Horne ( 1928 )
additionally commented that knowledge of carrier rates in fi sh populations before,
during and after the furunculosis season would be of great epizootiological value.
Because of the obvious importance carrier fi sh have in the epizootiology of
furunculosis, it is essential that the methods used to detect their presence are effective. However, research into the carrier state has been hampered by technical diffi -
culties concerned with detecting such fi sh with certainty, as present culture isolation
methods appear to be too insensitive (McCarthy 1980 ). Blake and Clark ( 1931 )
reported that raising the temperature of the water in which suspected carriers (usually survivors which had been previously exposed to infection) were maintained
from 5 to 18 °C induced the disease. At present, a combination of increasing the
water temperature to 18 °C and the injection of corticosteroids is employed to activate the carrier state. This method is based on the work of Bullock and Stuckey
( 1975b ), who tested corticosteroid injection and heat stress as means of producing
overt furunculosis in carrier trout. They reported that although heat stress alone
produced mortality, the pathogen could not be recovered from the majority of test
fi sh which died. Direct kidney cultures to isolate Aer. salmonicida were found to be
ineffective for carrier detection. McCarthy ( 1980 ) reported success with the method
of Bullock and Stuckey ( 1975b ), although he stated that prednisolone acetate was
the most effective corticosteroid of the several he tested, for inducing furunculosis
in fi sh. In further experiments, he assessed the prevalence of carriers in fi sh populations, examining 1-year-old brown trout from four different commercial fi sh farms.
The fi sh had a high carrier rate of 40–80 %; however, similar populations of rainbow
trout tested had a very low incidence of <5 % or were possibly free of carriers. The
technique has also been applied by McCarthy ( 1975a ) to non-salmonids, e.g. silver
bream ( Blicca bjoerkna ) , infected with a non-pigmented aberrant Aer. salmonicida
strain.
The ELISA technique has also been assessed for its effectiveness in the detection
of carrier fi sh. Rose et al. ( 1989 ) compared a commercial ELISA kit (obtained from
Stirling Diagnostics Ltd.), the commonly used corticosteroid/ heat protocol of
McCarthy ( 1980 ) and plating of rectum and kidney samples from Atlantic salmon
onto BHIA and TSA. The Atlantic salmon were obtained from a site where outbreaks of furunculosis had previously occurred. From the results, it was apparent
that the ELISA was the most successful technique, with 56.17 % (14/26) of the fi sh
shown to be carriers. This compared with a carrier rate of 26.4 % as determined by
means of the corticosteroid/heat test. Yet, plating techniques failed to reveal Aer.
salmonicida in any of the fi sh. However, some disadvantages of ELISA are apparent, namely the inability to distinguish living from dead cells, and, for that matter,
to differentiate viable, pathogenic cells from those resulting from use of living,
attenuated or dead, virulent cells from vaccines. Moreover, ELISA does not enable
the provision of cultures, which could be used for additional investigation, such as
5 Aeromonadaceae Representative (Aeromonas salmonicida)
recovered in the blood of 17 % (3 out of 18) of trout examined, and it was concluded
that carriers provided a source of infection in the river. He also sampled fi sh from
fi sh farms and found them to be generally healthy and thus contended that these
results indicated that trout farms were not harbouring the disease. Horne ( 1928 )
additionally commented that knowledge of carrier rates in fi sh populations before,
during and after the furunculosis season would be of great epizootiological value.
Because of the obvious importance carrier fi sh have in the epizootiology of
furunculosis, it is essential that the methods used to detect their presence are effective. However, research into the carrier state has been hampered by technical diffi -
culties concerned with detecting such fi sh with certainty, as present culture isolation
methods appear to be too insensitive (McCarthy 1980 ). Blake and Clark ( 1931 )
reported that raising the temperature of the water in which suspected carriers (usually survivors which had been previously exposed to infection) were maintained
from 5 to 18 °C induced the disease. At present, a combination of increasing the
water temperature to 18 °C and the injection of corticosteroids is employed to activate the carrier state. This method is based on the work of Bullock and Stuckey
( 1975b ), who tested corticosteroid injection and heat stress as means of producing
overt furunculosis in carrier trout. They reported that although heat stress alone
produced mortality, the pathogen could not be recovered from the majority of test
fi sh which died. Direct kidney cultures to isolate Aer. salmonicida were found to be
ineffective for carrier detection. McCarthy ( 1980 ) reported success with the method
of Bullock and Stuckey ( 1975b ), although he stated that prednisolone acetate was
the most effective corticosteroid of the several he tested, for inducing furunculosis
in fi sh. In further experiments, he assessed the prevalence of carriers in fi sh populations, examining 1-year-old brown trout from four different commercial fi sh farms.
The fi sh had a high carrier rate of 40–80 %; however, similar populations of rainbow
trout tested had a very low incidence of <5 % or were possibly free of carriers. The
technique has also been applied by McCarthy ( 1975a ) to non-salmonids, e.g. silver
bream ( Blicca bjoerkna ) , infected with a non-pigmented aberrant Aer. salmonicida
strain.
The ELISA technique has also been assessed for its effectiveness in the detection
of carrier fi sh. Rose et al. ( 1989 ) compared a commercial ELISA kit (obtained from
Stirling Diagnostics Ltd.), the commonly used corticosteroid/ heat protocol of
McCarthy ( 1980 ) and plating of rectum and kidney samples from Atlantic salmon
onto BHIA and TSA. The Atlantic salmon were obtained from a site where outbreaks of furunculosis had previously occurred. From the results, it was apparent
that the ELISA was the most successful technique, with 56.17 % (14/26) of the fi sh
shown to be carriers. This compared with a carrier rate of 26.4 % as determined by
means of the corticosteroid/heat test. Yet, plating techniques failed to reveal Aer.
salmonicida in any of the fi sh. However, some disadvantages of ELISA are apparent, namely the inability to distinguish living from dead cells, and, for that matter,
to differentiate viable, pathogenic cells from those resulting from use of living,
attenuated or dead, virulent cells from vaccines. Moreover, ELISA does not enable
the provision of cultures, which could be used for additional investigation, such as
5 Aeromonadaceae Representative (Aeromonas salmonicida)