Simultaneous classical and El Tor cholera in Bangladesh.

Cholera appears annually in Bangladesh, with peak incidence from September through December and April through June. After being absent for a decade, the classical biotype of Vibrio cholerae reappeared in Commilla and Dhaka on September 1 and 3, 1982, and soon thereafter appeared in many other districts. Cases due to the newly emerged classical biotype of V. cholerae rapidly surpassed those due to the El Tor biotype. Before 1970, classical V. cholerae had been responsible globally for both endemic and epidemic cholera. In Bangladesh, El Tor was first isolated in 1964. By 1973, it has replaced the classical strain. Subsequently until 1981, only 6 classical isolates were noted. The new classical strains were almost identical to those isolated a decade earlier, except for their ability to successfully displace El Tor in an epidemic setting. Investigation of the first 10 cholera cases suggests a multiple source outbreak. The age distribution of the classical and El Tor was similar, suggesting a lack of important new antigenic determinants, which could circumvent existing immunity in the population. Both strains followed known seasonal patterns.