Aeromonas salmonicida the role of carriers The second mechanism by which fish

Aeromonas salmonicida the role of carriers the second

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Aeromonas salmonicida the role of carriers The second mechanism by which fish may provide a source of infection is by becoming carriers of Aer. salmonicida. The existence of such fish and their role in transmission of the disease was recognised early in the history of furunculosis. In a study of furunculosis in trout in the river Kennett, Home (1928) found that Bacter- ium salmonicida was 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 fish from fish farms and found them to be generally healthy and thus contended that these results indicated that trout farms were not harbouring the disease. Home (1928) additionally commented that knowledge of carrier rates in fish populations before, during and after the furunculosis season would be of great epizootiological value.
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262 Bacterial Fish Pathogens Because of the obvious importance carrier fish 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 difficulties concerned with detecting such fish 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 mortahty, the pathogen could not be recovered from the majority of test fish 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 fish. In further experiments, he assessed the prevalence of carriers in fish populations, examining 1-year-old brown trout from four different commercial fish farms. The fish 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 appHed by McCarthy (1975a) to non-salmonids, e.g. silver bream (Blicca bjoerkna), infected with a non-pigmented aberrant Aer. salmonicida strain.
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