Purpose: The study objective was to compare the levels of V. cholerae (Vc), V. vulnificus (Vv), and V. parahaemolyticus (Vp) in oysters and clams concurrently harvested from Long Island Sound (LIS), USA.
Methods: Shellfish (68 oyster and 30 clam samples) were collected between July 16 and September 24, 2012 following Vp illnesses associated with shellfish from LIS. MPN-real-time PCR methods were used to enumerate of Vc, Vv, and total and pathogenic (tdh+ and/or trh+) Vp.
Results: Total Vc was detected in 8.8% and 3.3% of oyster and clam samples, respectively. In oysters, mean Vc levels in samples above the LOD was 0.20 log MPN/g; the one positive clam sample had 0.48 log MPN/g. V. vulnificus was detected in 97% and 90% of oyster and clam samples, with mean levels of 1.3 and 0.10 log MPN/g, respectively. Vp was detected in all samples with mean levels of 2.2 and 1.1 log MPN/g in oysters and clams, respectively. The differences between vibrio levels in the shellfish types was significantly different (P < 0.001). However, the differences in detectable levels of pathogenic Vp (tdh, P = 0.329 and trh, P = 0.292) in oysters and clams were not significant.
Significance: The data indicate that although levels of total vibrios are higher in oysters than clams, the potentially pathogenic variants are found in similar levels, albeit less frequently. This may explain why vibriosis is associated with clams, when total vibrio levels are lower than in oysters. However, additional data is still needed to ascertain the reason(s) that clams are increasingly associated with vibriosis.