Dengue serosurveillance in Kolkata, facing an epidemic in West Bengal, India.

Background & objectives: A dengue outbreak occurred throughout West Bengal, India starting from August 2005. The objectives of this study were: (i) documentation of suspected cases, confirmed by MAC ELISA; (ii) a longitudinal serosurveillance of dengue in Kolkata by detecting denguespecific IgG and IgM antibodies in suspected cases and to note the seasonal incidence; (iii) to get information about proportion of primary, secondary and old dengue cases and persons free from dengue in the population tested; and (iv) to get some idea to forecast advent of dengue and dengue epidemic/precipitation of DHF. Methods: The epidemiological data were collected from different sources including Calcutta School of Tropical Medicine and Government of West Bengal. For serosurveillance in Kolkata, serum of each suspected patient was examined for detection of IgG and IgM antibodies using the microwell ELISA dengue fever kit. Results: Altogether, 6293 ELISA IgM reactive cases were documented out of 12,059 persons (52.18%) tested in the whole of West Bengal, involving 18 districts with 34 deaths. Serosurveillance of 1668 persons from August 2005 to December 2007 in Kolkata showed that only 18.1% (302) suffered from dengue, 6.03% (101) from primary, 12.03% (201) from secondary dengue, 20.44% (341) were not sufferers of dengue and 61.45% (1025) were old dengue cases. Though stray dengue cases were found perennially, most of the cases occurred in the post-monsoon season, with a peak in September. Of dengue patients, 63.24% (191) were males. Secondary dengue cases outnumbered primary dengue cases. So, possibility of DHF would remain in all age groups and in those persons who suffered from dengue on earlier occasion. Dengue cases drastically reduced in two subsequent post-epidemic years (2006 and 2007). Interpretation & conclusion: Dengue serosurveillance studies may give some idea about advent, intensity, transmission season, seasonal incidence, waxing and warning, and impending epidemic of dengue and DHF. A large-scale active longitudinal serosurvey along with the study of vector capacity and vector competence would provide more correct information.