260
to intraperitoneally injected fi sh (40, 20 and 10 % carrier rates, respectively).
Markwardt and Klontz (1989) also commented that the exposure to clinically diseased fi sh and bathing in broth cultures most probably simulated the natural routes
of infection (Paterson 1982 ).
Aeromonas salmonicida – Transmission; The Role of Eggs Vertical or transovarian transmission of Aer. salmonicida as a possible route of infection has been widely
studied with inconclusive results. Smith ( 1939 ) claimed the pathogen could be carried on the egg surface, in contrast to Plehn ( 1911 ) and Mackie et al. ( 1930 ), who
were of the opinion that the organism was unable to infect fi sh eggs. The possibility
that the pathogen could be transmitted at fertilisation was examined by the
Furunculosis Committee in the UK. The experimental evidence gathered for their
report indicated that furunculosis was not transmitted in such a fashion. Lund
( 1967 ), however, contended that since the conclusions were based on the results of
a sole set of experiments, further work was necessary to confi rm this point. In a
detailed series of experiments aimed at clarifying the situation regarding transmission of Aer. salmonicida at fertilisation, Lund ( 1967 ) examined ovaries, testes and
ova of experimentally infected fi sh for the presence of the pathogen. Aer. salmonicida was recovered in pure culture from the ovaries and testes of infected fi sh.
Results of isolation of the pathogen from ova were decisive as, of 500 ova sampled,
only three obtained from the same fi sh yielded the bacterium in pure culture from
the interior of the ovum. In further experiments, using wild spawners, ova were
contaminated with Aer. salmonicida on the external surface at the time fertilisation
was effected, and the eggs then planted out in a river bed. It was observed that these
ova died quickly and were subjected to Saprolegnia ferax infection. Aer. salmonicida was not isolated from dead or living ova, and Lund ( 1967 ) concluded that the
experiment had been unsuccessful. Ova taken from parents experimentally infected
with Aer. salmonicida also failed to yield the bacterium. Continuing the study of
vertical transmission, McCarthy ( 1980 ) examined the fertilised ova of mature brood
stock brown trout taken from a known carrier population (5/8 proved to be carriers
when challenged with prednisolone acetate). However, Aer. salmonicida was not
recovered from the fertilised egg sample. When artifi cially infected brood stock
were stripped as soon as signs of clinical furunculosis had developed, both fi sh
organs and fertilised eggs were positive for Aer. salmonicida. However, the high
numbers of viable cells initially present, i.e. 10
6 cells/ml of egg macerate, rapidly
declined and could not be detected 5 days after incubation began. Based upon these
experiments, McCarthy ( 1980 ) concluded that vertical transmission of Aer. salmonicida was not a signifi cant means of disseminating the disease, and, moreover,
in the improbable event that overtly infected fi sh were used for stripping, the pathogen was unlikely to survive to the eyed-egg stage at which the eggs are marketed.
Neither Lund ( 1967 ) nor McCarthy ( 1980 ) recovered Aer. salmonicida from fry
derived from experimentally infected parents or of known carrier brood stock,
respectively. However, both these authors pointed out the possibility that the negative results obtained may have been due to the inadequacy of techniques used for
detection and isolation of the pathogen in the face of inhibition or overgrowth by
commensal bacteria, or the presence of only small numbers of Aer. salmonicida.
5 Aeromonadaceae Representative (Aeromonas salmonicida)
to intraperitoneally injected fi sh (40, 20 and 10 % carrier rates, respectively).
Markwardt and Klontz (1989) also commented that the exposure to clinically diseased fi sh and bathing in broth cultures most probably simulated the natural routes
of infection (Paterson 1982 ).
Aeromonas salmonicida – Transmission; The Role of Eggs Vertical or transovarian transmission of Aer. salmonicida as a possible route of infection has been widely
studied with inconclusive results. Smith ( 1939 ) claimed the pathogen could be carried on the egg surface, in contrast to Plehn ( 1911 ) and Mackie et al. ( 1930 ), who
were of the opinion that the organism was unable to infect fi sh eggs. The possibility
that the pathogen could be transmitted at fertilisation was examined by the
Furunculosis Committee in the UK. The experimental evidence gathered for their
report indicated that furunculosis was not transmitted in such a fashion. Lund
( 1967 ), however, contended that since the conclusions were based on the results of
a sole set of experiments, further work was necessary to confi rm this point. In a
detailed series of experiments aimed at clarifying the situation regarding transmission of Aer. salmonicida at fertilisation, Lund ( 1967 ) examined ovaries, testes and
ova of experimentally infected fi sh for the presence of the pathogen. Aer. salmonicida was recovered in pure culture from the ovaries and testes of infected fi sh.
Results of isolation of the pathogen from ova were decisive as, of 500 ova sampled,
only three obtained from the same fi sh yielded the bacterium in pure culture from
the interior of the ovum. In further experiments, using wild spawners, ova were
contaminated with Aer. salmonicida on the external surface at the time fertilisation
was effected, and the eggs then planted out in a river bed. It was observed that these
ova died quickly and were subjected to Saprolegnia ferax infection. Aer. salmonicida was not isolated from dead or living ova, and Lund ( 1967 ) concluded that the
experiment had been unsuccessful. Ova taken from parents experimentally infected
with Aer. salmonicida also failed to yield the bacterium. Continuing the study of
vertical transmission, McCarthy ( 1980 ) examined the fertilised ova of mature brood
stock brown trout taken from a known carrier population (5/8 proved to be carriers
when challenged with prednisolone acetate). However, Aer. salmonicida was not
recovered from the fertilised egg sample. When artifi cially infected brood stock
were stripped as soon as signs of clinical furunculosis had developed, both fi sh
organs and fertilised eggs were positive for Aer. salmonicida. However, the high
numbers of viable cells initially present, i.e. 10
6 cells/ml of egg macerate, rapidly
declined and could not be detected 5 days after incubation began. Based upon these
experiments, McCarthy ( 1980 ) concluded that vertical transmission of Aer. salmonicida was not a signifi cant means of disseminating the disease, and, moreover,
in the improbable event that overtly infected fi sh were used for stripping, the pathogen was unlikely to survive to the eyed-egg stage at which the eggs are marketed.
Neither Lund ( 1967 ) nor McCarthy ( 1980 ) recovered Aer. salmonicida from fry
derived from experimentally infected parents or of known carrier brood stock,
respectively. However, both these authors pointed out the possibility that the negative results obtained may have been due to the inadequacy of techniques used for
detection and isolation of the pathogen in the face of inhibition or overgrowth by
commensal bacteria, or the presence of only small numbers of Aer. salmonicida.
5 Aeromonadaceae Representative (Aeromonas salmonicida)