Epidemiological usefulness of changes in hemolytic activity of Vibrio cholerae biotype El Tor during the seventh pandemic

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Abstract
Hemolytic V. cholerae biotype eltor strains were isolated in the USA in 1973 and 1978 after they had supposedly disappeared worldwide during the 1960s and 1970s. The change in prevalence of hemolytic eltor strains since the beginning of the 7th pandemic was examined. The usefulness of hemolytic activity as an epidemiological marker was evaluated. A total of 48 isolates of V. cholerae biotype eltor isolated in the Eastern Hemisphere between 1960-1979, along with 1 Texas (1973) and 38 Louisiana (1978) isolates, were tested for hemolytic activity by each of 4 methods. One method (utilizing heart infusion broth with 1% glycerol) was slightly superior for detecting hemolytic activity. Titers obtained with this method ranged from < 2-1024. Of 13 (76.9%) strains from the earliest part of the current pandemic, 10 were hemolytic, compared with 1 of 26 (3.8%) strains isolated from 1966-1979 in the Eastern Hemisphere, indicating that nonhemolytic eltor strains have replaced the hemolytic variety there. In contrast, all 38 Louisiana isolates and the Texas isolate were strongly hemolytic. Hemolytic activity was concluded to be a useful epidemiological marker.