Etiology of Juvenile Oyster Disease (JOD) of Cultured Crassostrea Virginica

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NON-TECHNICAL SUMMARY:<br>
The Eastern oyster is the primary species of bivalve cultured in the northeastern United States. However, since 1988, many producers in this region have experienced annual mortalities of juvenile oysters (up 90% of total production) due to a syndrome of unknown etiology. Juvenile oyster disease (JOD) is now prevalent in several areas in Maine, Massachusetts, and New York, and there is currently no way to predict the impact of JOD or test for its presence. We have accumulated compelling evidence that this is a bacterial disease, and that a novel species of marine bacterium (designated CVSP) is the etiological agent.</p>
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We will attempt to reproduce JOD-signs and mortalities in laboratory-maintained C. virginica by exposure to CVSP-bacteria. A potential probiotic bacterium will also be investigated for its ability to protect juvenile oysters from JOD. Fluorescence microscopy will be used to discern the location and dynamics of colonization by both bacterial species. </P>
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Conclusive identification of the JOD-agent will result in detection methods that will provide immediate economic benefits for the industry, and be invaluable for long-term management of the problem. New information about the factors mediating bacterial colonization of the animals will also be forthcoming. </p>
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As intensive aquaculture of oysters and other bivalves continues to expand in national and economic importance, this research will provide a framework and the tools to better understand, diagnose, and minimize mortalities that result from bacterial infections of cultured shellfish.</p>
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APPROACH: <br>
<ol> <li>Laboratory-maintained juvenile Crassostrea virginica will be exposed to CVSP-bacteria by injection and immersion routes. Animals in the control groups will be exposed to either a non-pathogenic bacterium or media alone. All animals will be observed for development of JOD-signs and mortalities, and the cumulative effect ofeach treatment will be monitored by weekly bacteriological and histological analyses.
<li>Experimental challenge with CVSP-bacteria will also be conducted using animals that have been precolonized by the potential probiotic strain. The protective ability of the potential probiotic will be assessed by weekly bacteriological and histological sampling and by comparing instantaneous and cumulative mortality rates to those of control animals.
<li>To facilitate the investigations of bacterial colonization dynamics, both bacterial species under investigation will be modified to express a different fluorescent protein. In this way, these bacteria will be easily identifiable among the resident microbial populations of the animals and ambient water. The majority of colonization assays will involve exposing healthy juvenile oysters to the fluorescently modified bacteria and following the abundance and distribution of the bacteria as a function of time by standard plate counts of animal and water samples. The effects of temperature, salinity, nutrient concentration, and the presence of competing bacteria on host-colonization, will be examined for both bacterial species. Further, both fluorescence and electron microscopy will be utilized to identify the nature and location of colonization ofeach species within the animal.</ol> </p>
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PROGRESS: 1999/10 TO 2000/09<br>
In the early Fall of this year, juvenile oyster disease (JOD) resulted in a 30 to 60% loss of total production for several Maine shellfish operations. Epizootics were documented in the Damariscotta River (a known JOD-enzootic site), and two other coastal sites where JOD had not previously been seen. All affected animals that we examined were colonized by the suspected etiological agent (a bacterium designated CVSP), and these strains were identical to those isolated from previous epizootics on the Damariscotta River. The impact of JOD had been minimal in the past couple of years, but it has now reemerged as the single greatest threat to the sustainability of oyster aquaculture in Maine. The CVSP bacterium is a member of the Roseobacter group of the marine alpha-proteobacteria, and we are currently working on a method that would render these bacteria self-fluorescent. The fluorescent cells will facilitate experiments designed to follow the colonization process of these bacteria in juvenile oysters.</p>
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Further, a study designed to directly demonstrate the ability of Roseobacter bacteria to induce JOD-signs and mortalities are planned for next Spring. </p>
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IMPACT:1999/10 TO 2000/09
<br>Oyster growers in Maine may lose up to 90% of their annual total production to juvenile oyster disease (JOD). Previous efforts to control JOD and prevent its spread have proven ineffective, and there is currently no way to predict the occurrence of JOD-epizootics. </p>