173
hydrophila in mixed infections. A sensitivity limit of 10
4 –10
5 cells/well was detailed
for the Aer. hydrophila ELISA devised by Sendra et al. ( 1997 ). Monoclonal antibodies to Aer. hydrophila were used in a dot blotting method with a resultant sensitivity
of 10
5 –10
7 CFU/ml although this could be improved considerably to 10
2 –10
3 CFU/
ml by incubating the infected material in TSB for 3–6 h fi rst (Longyant et al. 2010 ).
Molecular Methods
PCRs have been developed for Aer. hydrophila (Chu and Lu 2005 ).
Epizootiology
The epizootiology of Aer. hydrophila has not been considered in any great detail,
although it has been concluded that the organism is rife in freshwater (HeuschmannBrunner 1978 ; Allen et al. 1983a , b ), aquatic plants and fi sh (Trust and Sparrow
1974 ; Ugajin 1979 ) and fi sh eggs (Hansen and Olafsen 1989 ), and may be associated with invertebrates, such as the ciliated protozoan Tetrahymena pyriformis
(King and Shotts 1988 ), from where it will be readily available for infection of fi sh.
Some isolates have been demonstrated to exhibit chemotactic responses to the
mucus of freshwater fi sh (Hazen et al. 1982 ). The chemotaxic substance, which is
heat stable at 56 °C, has been reported to have a molecular weight of 100 kDa.
The evidence points to a stress-mediated disease condition (Bullock et al. 1971 ),
in which mortalities, if indeed they occur at all (Huizinga et al. 1979 ), are infl uenced
by elevated water temperatures (Groberg et al. 1978 ; Nieto et al. 1985 ). Thus,
Groberg and co-workers determined that deaths among fi sh, which were challenged
via i.p. injection, occurred only at water temperatures in excess of 9.4 °C. This
implies that the disease is not associated with cold water. In addition to water temperatures, the presence of pollutants, notably nitrite at 6 mg/l, increased the susceptibility of channel catfi sh to infection (Hanson and Grizzle 1985 ). It is interesting to
note that survivors possess high serum titres of IgM-like antibody (Hazen et al.
1981 ).
During an examination of likely portals of entry of the pathogen into walking
catfi sh, Lio-Po et al. ( 1996 ) found evidence of localisation in the muscle.
Pathogenicity
Most of the information concerning pathogenicity mechanisms of Aer. hydrophila
appertains to isolates of medical importance and will not be considered further here.
The value of using cultures, grown on nutrient rich media, has been cast into doubt
following the observation that starved cells (NB: this is akin to the natural state of
bacteria in the aquatic environment) are more virulent than their counterparts from
Aeromonas hydrophila
hydrophila in mixed infections. A sensitivity limit of 10
4 –10
5 cells/well was detailed
for the Aer. hydrophila ELISA devised by Sendra et al. ( 1997 ). Monoclonal antibodies to Aer. hydrophila were used in a dot blotting method with a resultant sensitivity
of 10
5 –10
7 CFU/ml although this could be improved considerably to 10
2 –10
3 CFU/
ml by incubating the infected material in TSB for 3–6 h fi rst (Longyant et al. 2010 ).
Molecular Methods
PCRs have been developed for Aer. hydrophila (Chu and Lu 2005 ).
Epizootiology
The epizootiology of Aer. hydrophila has not been considered in any great detail,
although it has been concluded that the organism is rife in freshwater (HeuschmannBrunner 1978 ; Allen et al. 1983a , b ), aquatic plants and fi sh (Trust and Sparrow
1974 ; Ugajin 1979 ) and fi sh eggs (Hansen and Olafsen 1989 ), and may be associated with invertebrates, such as the ciliated protozoan Tetrahymena pyriformis
(King and Shotts 1988 ), from where it will be readily available for infection of fi sh.
Some isolates have been demonstrated to exhibit chemotactic responses to the
mucus of freshwater fi sh (Hazen et al. 1982 ). The chemotaxic substance, which is
heat stable at 56 °C, has been reported to have a molecular weight of 100 kDa.
The evidence points to a stress-mediated disease condition (Bullock et al. 1971 ),
in which mortalities, if indeed they occur at all (Huizinga et al. 1979 ), are infl uenced
by elevated water temperatures (Groberg et al. 1978 ; Nieto et al. 1985 ). Thus,
Groberg and co-workers determined that deaths among fi sh, which were challenged
via i.p. injection, occurred only at water temperatures in excess of 9.4 °C. This
implies that the disease is not associated with cold water. In addition to water temperatures, the presence of pollutants, notably nitrite at 6 mg/l, increased the susceptibility of channel catfi sh to infection (Hanson and Grizzle 1985 ). It is interesting to
note that survivors possess high serum titres of IgM-like antibody (Hazen et al.
1981 ).
During an examination of likely portals of entry of the pathogen into walking
catfi sh, Lio-Po et al. ( 1996 ) found evidence of localisation in the muscle.
Pathogenicity
Most of the information concerning pathogenicity mechanisms of Aer. hydrophila
appertains to isolates of medical importance and will not be considered further here.
The value of using cultures, grown on nutrient rich media, has been cast into doubt
following the observation that starved cells (NB: this is akin to the natural state of
bacteria in the aquatic environment) are more virulent than their counterparts from
Aeromonas hydrophila