Lund 1967 however contended that since the conclusions were based on the

Lund 1967 however contended that since the

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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 confirm this point. In a detailed series of experiments aimed at clarifying the situation regarding transmission of Aer. salmonicida at fertihsation, Lund (1967) examined ovaries, testes and ova of experimentally infected fish for the presence of the pathogen. Aer. sal- monicida was recovered in pure culture from the ovaries and testes of infected fish. Results of isolation of the pathogen from ova were decisive as, of 500 ova sampled, only 3 obtained from the same fish yielded the bacterium in pure culture from the interior of the ovum. In further experiments, using wild spawners, ova were con- taminated 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, broodstock 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 artificially infected broodstock were stripped as soon as signs of cHnical furunculosis had developed, both fish organs and fertilised eggs were positive for Aer. salmonicida. However, the high numbers of viable cells initially present, i.e. 10^ cells/ml of egg macerate, rapidly decHned 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 significant means of disseminating the disease, and, moreover, in the improbable event that overtly infected fish 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 broodstock, 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. Aeromonas salmonicida transmission in seawater A remaining aspect of the epizootiology of Aer. salmonicida diseases which requires consideration is the transmission of the infection in seawater. This is an important topic for the aquaculture industry, as salmonids are not infrequently placed in sea- water for on-growing. In addition, early in the study of the pathogen, the possibiHty that migratory Salmonidae could spread the infection was considered. Lund (1967)
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