Molecular Epidemiology of Streptococcus iniae Isolated from Fresh Fish Purchased in the Greater Toronto Area (GTA).

E. T. H. LIU, R. FINE, S. MATSUMURA, B. M. WILLEY, S. PONG-PORTER, J. FULLER, J. DE AZAVEDO, THE FISH TEAM, D. E. LOW, A. MCGEER. Mount Sinai and Princess Margaret Hospitals, University of Toronto, Ontario, Canada.

Objective: S. iniae is an emerging fish pathogen associated with invasive disease in humans. During 1995-96, 9 cases of bacteraemia caused by a single clone of S. iniae occurred in the GTA. All patients had handled live or freshly killed fish, 6/9 were tilapia. Our objective was to identify the frequency and types of S. iniae colonizing fish purchased from stores across the GTA.

Methods: During the summer of 1997, 66 fresh, whole fish (31 tilapia, 11 bass, 5 catfish, 4 each of cod/trout, and 11 other species) were purchased from a random sample of 19 retailers in the GTA. 45 fish were selected alive, then killed at time of purchase. 5 surface swabs from different parts of the fish were obtained on the day of purchase, again after storage for 48h at 4oC, and after additional freezing for 24h at –10oC. Isolates were identified as S. iniae by standard microbiologic methods. One isolate per fish with S. iniae was typed by SmaI pulsed field gel electrophoresis (PFGE). PFGE parameters were 5-60s, 200V, 12oC, for 20h.

Results: 330 isolates of S. iniae were cultured from 42/66 (64%) fish purchased from 8/19 retailers. Fish positive and negative for S. iniae were purchased from the same retailer. 35/45 (78%) fish selected live were culture positive compared to 7/12 (58%) fish killed sometime prior to purchase (P<0.001). Storage for 48h at 4oC did not affect the amount of S. iniae isolated (mean colonies/fish 26 fresh vs. 42 stored, P=0.08); however, a decrease was noted after freezing (mean cols/fish 42 vs. 21, P=0.002). S. iniae colonized all except 3 species of fish sampled. PFGE of 37 strains showed 5 major patterns: 23 type A (62%), 7 type B (19%), 4 type C (11%), 2 type D (5%) and 1 type E (3%). 15/23 isolates of type A were found on tilapia compared to 5/14 isolates of other PFGE types (P=0.055). No PFGE type matched that of the clone associated with human disease.

Conclusion: More S. iniae was found from cultures of fish purchased live than those kept on ice in stores. Freezing also reduced colony counts. Different clones of S. iniae may preferentially colonize different species of fish. The clone associated with human disease was not detected in this sample.

Presented at:

66th CANADIAN ASSOCIATION FOR CLINICAL MICROBIOLOGY AND INFECTIOUS DISEASES (CACMID), Conjoint Meeting on Infectious Diseases, Toronto, ON, Canada, Nov 8-12, 1998.