As oyster farming has expanded across the United States, new, economically-beneficial methods of culturing oysters are being developed and used by growers. The expansion of oyster aquaculture in the Gulf region using these new methods has led to concern about the concentration of vibrios, harmful bacteria that can potentially cause illness or even death when raw oysters are consumed. Scientists Bill Walton and Covadonga Arias of Auburn University will work with Jessica Jones of the Food and Drug Administration on a series of experiments to determine the effect of various methods used in the industry to control bio-fouling upon vibrio concentrations. The data generated in this study can be used to influence oyster growers’ best practices and help public health agencies write informed regulations.
Abstract
The increase in oyster aquaculture in the U.S., particularly in the Gulf of Mexico, has raised broad concerns about the effects of aquaculture practices on the abundances of both Vibrio vulnificus and V. parahaemolyticus in oysters and the re-immersion time required to return the vibrio levels to ambient levels. We will test the following hypotheses: 1) there is a significant effect of tested aquaculture practices (mud worm treatment, routine bio-fouling treatment, or never desiccated) and time of re-immersion (measured from 1-28 days after re-immersion) upon the abundance of V. vulnificus and V. parahaemolyticus, and 2) oysters subjected to either aquaculture practice will not significantly differ from untreated oysters in abundance of V. vulnificus and V. parahaemolyticus after a predictable amount of days of re-immersion. We propose a two-year experimental field study (at Auburn University’s research field site in Portersville Bay, Coden, Alabama) with two 1-month long replicate runs/year (July 15th-September 1st). At the onset of any given run, six randomly selected baskets will be subjected to either 1) a 3-hr freshwater dip, and then allowed to air dry for 24 hours, 2) 27 hours of desiccation at ambient air temperatures, or 3) left, untreated, in the water. After treatment, all baskets will be re-immersed, adjacent to the unhandled control. Samples will be collected prior to treatment, immediately post-treatment/before re-immersion, and then 1, 2, 3, 7, 10, 14, 21 and 28 days after re-immersion. Samples will be processed immediately following standard NSSP protocols and pathogenicity of strains will be evaluated. We will provide a report of our results to relevant end-users, present our work at regional and national meetings, and track the number of oyster farmers that have adopted the recommended days of re-immersion (either as required by regulatory agencies or adopted voluntarily).
Objectives
The overall goal of the proposed work is to provide the oyster industry and public health agencies scientifically rigorous and relevant data to guide regulations and best management practices that protect public health while minimizing the costs to the oyster aquaculture industry.
Our research objectives are as follows:
1. By 2015, we will determine the effect of the aquaculture practice used to control mud worms (3-hr freshwater dip followed by 24-hr desiccation) on abundances of V. vulnificus and V. parahaemolyticus after re-immersion over time.
1. By 2015, we will determine the effect of the aquaculture practice used to control bio-fouling (27-hr desiccation) on abundances of V. vulnificus and V. parahaemolyticus after re-immersion over time.
Throughout the research and once it is concluded, our outreach objectives are as follows:
2. By 2016, at least 100 industry and 25 public health officials will be aware of the results of this study to inform best management practices and regulatory decisions in Alabama and other states.
3. By 2016, at least 25 oyster farmers will have adopted the recommended days of re-immersion as a standard practice.
Methodology
We will test the following hypotheses. 1. There is a significant effect of tested aquaculture practices (mud worm treatment, routine bio-fouling treatment, or never desiccated) and time of re-immersion (measured from 1-28 days after re-immersion) upon the abundance of V. vulnificus and V. parahaemolyticus. 2. Oysters subjected to either aquaculture practice will not significantly differ from untreated oysters in abundance of V. vulnificus and V. parahaemolyticus after a predictable amount of days of re-immersion. We propose to conduct a two-year experimental field study of the effects of two different aquaculture practices used to control fouling (one targeting mud worms and other shell borers and another targeting routine bio-foulers) that rely on desiccation at ambient air temperatures (relative to a never desiccated control) during the summers of 2014 and 2015. We will conduct two 1-month long replicate runs per year (four replicate runs total over the study), between July 15th and September 1st to ensure that testing is done under conditions that are most favorable to proliferation of Vibrio spp. The fieldwork will be conducted at Auburn University’s research field site in Portersville Bay, Coden, Alabama (Mississippi Sound), a shallow (1-2 m) firm mud bottom site with a small tidal range (0.5-1.0 m). Within any single replicate run, we will deploy six of these baskets for each of three treatments (with 600 oysters for each treatment and a total of 18 baskets per replicate run): 1) 3-hr freshwater dip with 24-hr desiccation, 2) 27-hr desiccation and 3) an unhandled control treatment. At the onset of any given run, six randomly selected baskets will be subjected to a 3-hr freshwater dip, and then allowed to air dry for 24 hours. Another six randomly selected baskets will be subjected to 27 hours of desiccation at ambient air temperatures, while the last six baskets will be left, untreated, in the water. After treatment, all baskets will be re-immersed, adjacent to the unhandled control. Samples will be collected prior to treatment, immediately post-treatment/before re-immersion, and then 1, 2, 3, 7, 10, 14, 21 and 28 days after re-immersion. Upon collection, each batch of oysters will be labeled, bagged and put into cold coolers and shipped to the appropriate laboratory (with the analysis spread across two labs to ease the logistic burdens on any single lab, with some parallel processing to provide a cross-check on results). Upon arrival to the analytical laboratories, samples will be processed immediately following standard protocols according to NSSP guidelines (http://www.issc.org/2009GuidePDF.aspx). Additionally, pathogenicity of strains will be evaluated using standard methods. For each Vibrio species, results will be analyzed as MPN/g of oyster. A two-way ANOVA will be used to determine the effects of day and salinity (both treated as fixed factors). Tukey’s HSD post-hoc tests will be run to determine which pairs differ if significant effects are found. We will provide a report of our results to relevant end-users, and present our work at regional and national meetings. In the medium-term, we will provide a peer-reviewed publication of our results to these same end-users, as well as a broader audience, including the East Coast Growers Association and Sea Grant Extension agents. We will track the number of oyster farmers that have adopted the recommended days of re-immersion (either as required by regulatory agencies or adopted voluntarily) through regular communication with state public health agents and industry members.
Rationale
To reduce the risks from infections by the naturally occurring bacteria associated with consumption of raw oysters, Vibrio vulnificus and V. parahaemolyticus, public health agencies require rapid refrigeration of harvested oysters (including the Eastern oyster, Crassostrea virginica) destined to be sold live, in the shell, for raw consumption (referred to as shellstock), with strict time/temperature requirements as control plans (NSSP, 2011). The calculation of risks is based on the presumption that all harvested oysters from a given body of water come out of the water with the same background levels of V. vulnificus and V. parahaemolyticus. This presumption may be violated when the oysters are manipulated in some manner that exposes them to high ambient air temperatures or disrupts their typical filter feeding activity. Recently, concerns have been raised about the effects of various oyster aquaculture practices upon the levels of V. vulnificus and V. parahaemolyticus. This study in particular was prompted by a decision by the Alabama Department of Public Health (ADPH) in 2012 to require a commercial oyster farmer to re-submerge his oysters for > 30 days prior to sale after the oysters were subjected to a treatment to remove mud worms, Polydora websterii, which exposed them to ambient air temperatures for a period of ~27 hours. This prolonged period of re-immersion was problematic for the grower who was concerned that this period of immersion without desiccation would lead to increased bio-fouling on his product, with increased labor costs to clean the oysters prior to sale and/or potential loss of premium markets. Similar decisions have recently been made in other states; e.g., in Massachusetts, under a 2012 control plan, oyster farmers on portions of Cape Cod were prohibited from offsite culling of market sized oysters between June 17th and September 8th (MDPH, 2012). This case will set a precedent within Alabama, the Gulf Coast region and nationally, and provide data that can be used to help guide public health agencies’ decision making and influence industry best management practices within each of those regions.