Background & objective: In addition to Vibrio cholerae O1, a recently emerged non-O1 serotype, V. cholerae O139 has also been found to be responsible for epidemics of cholera. In the context of the changing etiology of cholera, the occurrence of different serotypes, biotypes and phage types of V. cholerae in Tamil Nadu is presented.
Methods: Between 1980 and 2001, 26,502 stool/rectal swabs from patients suspected to have cholera were processed for the presence of V. cholerae at the King Institute of Preventive Medicine, Chennai (Madras), Tamil Nadu. Serotypes, biotypes and phage types (Basu & Mukerjee and new phage typing method) of the isolates were determined. Antimicrobial susceptibility determination was included from 1997 onwards.
Results: Of the 26,502 stool/rectal swabs tested, 6035 (22.8%) specimens yielded V. cholerae. 4,046 (67%) of them were O1 serotype; followed by 1529 (25.3%) O139 serotype and 448 (7.4%) non-O1 serotypes other than O139 were encountered. Twelve specimens yielded rough strains of V. cholerae. All V. cholerae O1 were the biotype ElTor and the predominant serotype was Ogawa (95.9%). After the initial explosive epidemic by O139 serotype in late 1992 and early 1993 occurrence of this serotype had declined. Resurgence of this new serotype was observed in 1997. O1 domination was observed during 1995 and 1996. The new phage typing scheme for V. cholerae O1 was able to differentiate the isolates into 22 types, type 27 (68.2%) being the predominant one followed by type 26 (12.3%). In Basu & Mukerjee scheme, the isolates were clustered between two types--type 2 (34%) and type 4 (61%). Further, about 5 per cent of them were untypeable. All but a few of the isolates irrespective of serotype, were susceptible to chloramphenicol, tetracycline, gentamicin and ciprofloxacin. 97 per cent of the O1 strains and 48.9 per cent of the O139 strains were resistant to cotrimoxazole.
Interpretation & conclusion: V. cholerae O139 might have the ability to survive during interepidemic periods. Both V. cholerae O1 and O139--the two serotypes with epidemic potential seem to be firmly established in this region. This situation warrants constant surveillance.