Graduate Thesis Or Dissertation

A bacterial disease of the American shad (Alosa sapidissima)

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  • For the past six years a bacterial infection has been the cause of large losses of adult, spawning, American shad (Alosa sapidissima) in the Coos, Millicoma and Smith Rivers of Oregon. There was a sizable commercial fishery for shad in these rivers and losses of fish due to this infection represented an important economic loss to the industry. This study was undertaken to determine the causative agent of the disease and describe the pathology of the disease. Fifteen strains of bacteria were isolated from diseased shad in the Coos, Millicoma, and Smith Rivers. Cultural and morphological studies, physiological reactions, animal infection, serological tests, and determination of the mole percent guanine + cytosine showed that the causative agent belonged in the genus Aeromonas. Since the shad disease isolates could cause "red leg" disease in frogs and "red mouth" disease in trout, they were named Aeromonas hydrophila. The disease was shown to be a bacterial hemorrhagic septicemia. Externally, the diseased shad had large hemorrhagic areas on their sides and reddening of the head and fins. Very little pathology could be seen internally. The causative organism was recovered from the spleen, liver, kidney, heart, blood and external lesions of the diseased animals. A review of the literature revealed that there was confusion in the nomenclature of the three motile species of bacteria included in the genus Aeromonas. Many authors felt that the three species (A. liquefaciens, A. hydrophila, A. punctata) were one distinct species. Comparative tests performed on the shad disease isolates and known cultures of Aeromonas failed to reveal major differences between these organisms. Attempts were made to separate the three species on the basis of the mole percent guanine + cytosine and thermal denaturation temperature (Tm) of the bacterial DNA. Three shad disease isolates had Tm values of 94.3, 94.6, and 94.2°C with corresponding mole percent guanine + cytosine values of 60.9, 61.7, and 60.7. An isolate of Aeromonas liquefaciens obtained from the Communicable Disease Center (CDC) in Atlanta, Georgia had a Tm of 94.1°C and a mole percent guanine + cytosine of 60.3. Aeromonas hydrophila (CDC) had a Tm of 95.4°C and a mole percent guanine + cytosine of 63.4 while A. punctata (CDC) had a Tm of 93.5°C and a mole percent guanine + cytosine value of 59.0. The mole percent guanine + cytosine results indicated that the shad disease bacterium was closely related to A. liquefaciens. The three known cultures of Aeromonas each had a distinct mole percent guanine + cytosine value.
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