Environmental determinants of the occurrence and distribution of Vibrio parahaemolyticus in the rias of Galicia, Spain

Abstract

Infections associated with Vibrio parahaemolyticus on the coast of Galicia (in northwestern Spain) were reported to be linked to large outbreaks of illness during 1999 and 2000. Little information is available about the ecological factors that influence the emergence of V. parahaemolyticus infections in this temperate region. We carried out a 3-year study to investigate the occurrence and distribution of V. parahaemolyticus at 26 sites located in the four main rias of Galicia in association with environmental and oceanographic variables. V. parahaemolyticus was detected in all the areas investigated and throughout the complete period of study with an overall incidence of 12.5%. Salinity was the primary factor governing the temporal and spatial distribution of V. parahaemolyticus, whereas seawater temperature had a secondary effect and only modulated the abundance in periods and areas of reduced salinities. Higher occurrence of V. parahaemolyticus was observed during periods of lower salinity in autumn, with a total of 61 positive samples (18%) and a mean density of 1,234 most probable number/100 g. V. parahaemolyticus was primarily detected in areas of reduced salinity close to freshwater discharge points, where it was found in up to 45% of the samples. Characterization of the isolates obtained from the study resulted in the first identification of two pathogenic tdh-positive strains of V. parahaemolyticus recovered from the marine environment in Galicia. These isolates showed serotypes identical to and DNA profiles indistinguishable from those of the clinical clone of V. parahaemolyticus dominant in infections in Spain in the last 10 years.

FIG. 1.

Area of study and locations of the sampling stations in the coastal areas of Galicia.

FIG. 2.

Variations in salinity and seawater temperature throughout the period of study (A) and distribution of the presence and abundance of V. parahaemolyticus during the same period (B).

FIG. 3.

GIS maps showing the spatial distributions of V. parahaemolyticus, salinity, and seawater temperatures in the rias of Muros-Noia, Arousa, Pontevedra, and Vigo. Note the different values associated with the color scales in the maps.

FIG. 4.

Predicted probabilities for the detection of V. parahaemolyticus in association with the values of salinity (A) and seawater temperature (B) and both variables simultaneously (C).

FIG. 5.

Dendrogram generated by Bionumeric software showing the relationship of PFGE patterns for V. parahaemolyticus isolates included in the comparison. The numbers at the top of the figure indicate molecular sizes in kbp. ≪, pandemic O3:K6 isolates.