Influence of water temperature and salinity on Vibrio vulnificus in Northern Gulf and Atlantic Coast oysters (Crassostrea virginica)

Abstract

This study investigated the temperature and salinity parameters associated with waters and oysters linked to food-borne Vibrio vulnificus infections. V. vulnificus was enumerated in oysters collected at three northern Gulf Coast sites and two Atlantic Coast sites from July 1994 through September 1995. Two of these sites, Black Bay, La., and Apalachicola Bay, Fla., are the source of the majority of the oysters implicated in V. vulnificus cases. Oysters in all Gulf Coast sites exhibited a similar seasonal distribution of V. vulnificus: a consistently large number (median concentration, 2,300 organisms [most probable number] per g of oyster meat) from May through October followed by a gradual reduction during November and December to < or = 10 per g, where it remained from January through mid-March, and a sharp increase in late March and April to summer levels. V. vulnificus was undetectable (< 3 per g) in oysters from the North and South Carolina sites for most of the year. An exception occurred when a late-summer flood caused a drop in salinity in the North Carolina estuary, apparently causing V. vulnificus numbers to increase briefly to Gulf Coast levels. At Gulf Coast sites, V. vulnificus numbers increased with water temperatures up to 26 degrees C and were constant at higher temperatures. High V. vulnificus levels (> 10(3) per g) were typically found in oysters from intermediate salinities (5 to 25 ppt). Smaller V. vulnificus numbers (< 10(2) per g) were found at salinities above 28 ppt, typical of Atlantic Coast sites. On 11 occasions oysters were sampled at times and locations near the source of oysters implicated in 13 V. vulnificus cases; the V. vulnificus levels and environmental parameters associated with these samples were consistent with those of other study samples collected from the Gulf Coast from April through November. These findings suggest that the hazard of V. vulnificus infection is not limited to brief periods of unusual abundance of V. vulnificus in Gulf Coast oysters or to environmental conditions that are unusual to Gulf Coast estuaries.

FIG. 1

Weekly densities of V. vulnificus in oysters from Gulf Coast sites in Alabama (+), Florida (○), and Louisiana (▪) during a 14-month period. Each point represents the geometric mean (n = 2) of the MPN of bacteria per gram of oyster meat.

FIG. 2

Seasonal distribution of V. vulnificus in Gulf Coast oysters. Each point represents the geometric mean of the MPN of V. vulnificus organisms per g of oyster meat for all observations recorded within a half-month period. Transitional periods represent periods of change.

FIG. 3

Influence of water temperature on the concentration of V. vulnificus in Gulf Coast oyster meats. Each point represents the geometric mean of all observations recorded within a 3°C temperature range. See Materials and Methods for an explanation of the smoothing technique.

FIG. 4

Residuals (measured log V. vulnificus count minus model predictions) plotted against salinity for three Gulf estuaries.

FIG. 5

Weekly densities of V. vulnificus in oysters from Atlantic Coast sites in North Carolina (○) and South Carolina (▪) during a 14-month period. Each point represents the geometric mean (n = 2) of the MPN of bacteria per gram of oyster meat.