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

Hemolytic Vibrio cholerae biotype El Tor strains were isolated in the United States in 1973 and 1978 after they had supposedly disappeared worldwide during the 1960s and 1970s. We decided to examine the change in prevalence of hemolytic El Tor strains since the beginning of the seventh pandemic and evaluate the usefulness of hemolytic activity as an epidemiological marker. A total of 48 isolates of V. cholerae biotype El Tor isolated in the Eastern Hemisphere between 1960 and 1979, along with 1 Texas (1973) and 38 Louisiana (1978) isolates, were tested for hemolytic activity by each of four methods. One method (utilizing heart infusion broth with 1% glycerol) was slightly superior for detecting hemolytic activity. Titers obtained with this method ranged from less than 2 to 1,024. 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 in the period from 1966 to 1979 in the Eastern Hemisphere, indicating that nonhemolytic El Tor 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.