salmonicida has wider potential for causing disease problems than has been

Salmonicida has wider potential for causing disease

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Aer. salmonicida has wider potential for causing disease problems than has been hitherto suspected. To study the epizootiology of Aer. salmonicida diseases in aquatic environments other than freshwater, again, as in so many other aspects of the pathogen, demands additional attention to unravel its complexities. Aeromonas salmonicida the summary In summary, although substantial efforts have been made to reach an understanding of the natural disease cycle in the environment, there still remain many issues to be
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268 Bacterial Fish Pathogens resolved. Some of the data provide partial answers, some are contradictory. Lack of appropriate methodology appears to constitute a major hindrance to progress. In the face of the several unresolved questions concerning Aer. salmonicida epizootiology, McCarthy (1980) has valiantly put forth several hypotheses aimed at deriving an overall view of Aer. salmonicida epizootiology. The first of these is that furunculosis is introduced to a fish farm by importation of healthy carrier fish or that non-carrier resident fish may be infected from a water supply contaminated by wild or imported carrier fish. Accordingly, if contact with carriers of Aer. salmonicida can be prevented, furunculosis should not arise in the population. The second hypothesis McCarthy (1980) advanced is that the mechanisms by which carriers disseminate the disease to non-carriers is dependent upon environmental parameters at a given fish farm site. He postulated that if stressful conditions are absent, resident fish may become carriers without developing clinical signs of disease, and that under such conditions a high carrier rate could be maintained without cHnical evidence of furunculosis. This, in the opinion of other workers, would be an arguable premise, as some believe all carriers will eventually succumb to the disease. It is reasonable to suppose that once carrier fish are present on a fish farm they are responsible for initiation of epizootics, although infection also occurs by the water-borne route. The third hypothesis is that the bacterium in the carrier state provides a measure of protection to the fish in return for shelter from the ravages of the aquatic environment. Were this the case, the observation by McCarthy (1980) that non-carrier and carrier fish exhibit marked differences in their susceptibility to furunculosis would be explained. Finally, it is our opinion that, while a firm understanding of the interactions of the pathogen in its milieu and in its fish host is lacking, fisheries scientists will remain at a serious disadvantage when tackling the problems of infectious diseases of Aer. salmonicida aetiology that currently beset fish cultivation. Aeromonas sobria It may be assumed that the natural reservoir is eutrophic freshwater.
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