Aetiology of cholera in Tamil Nadu: recent observations

Vibrio cholerae was isolated from 1008 of 3496 stool samples (28.8%) examined in Tamil Nadu State, India, between November 1992 and December 1995. During November and December 1992, 363 of the 370 isolates serotyped (98%) were V. cholerae O139 (Bengal). The epidemic predominantly affected adults (91%; 597/656). Both V. cholerae O1 and O139 serotypes were sometimes isolated in the same locality from different individuals. From January 1993 onwards, the rate of isolation of V. cholerae O139 declined, and in 1995 V. cholerae E1 Tor (serotype O1) was isolated from most of the cases (85.6%; 131/153). V. cholerae E1 Tor has clearly not been replaced by serotype O139, but can survive during inter-epidemic periods and reappear at an opportune moment. The decline of serotype O139 may be due to the development of immunity as a result of repeated exposure.