Background & objectives: Dengue fever is an important public health problem in India. In recent years this disease has extended to rural areas also due to rapid urbanization. In Tamil Nadu, fever outbreaks were reported in two villages of Dharmapuri district during May and September 2001 with clinical symptoms suggestive of dengue fever. Epidemiological, virological and entomological investigations were carried out in these two villages to ascertain the etiology of the outbreaks.
Methods: Paired serum samples were collected from febrile patients clinically suspected to have dengue and were tested for the presence of IgM antibodies to dengue virus by MAC ELISA. Samples were also tested by Dengue Duo IgM/IgG rapid strip. Surveys of larval and adult Aedes mosquitoes were carried out before and after anti-larval and anti-adult measures were implemented. Female Ae. aegypti mosquitoes collected in Mampatti village were tested individually for the presence of dengue-2 viral antigen by indirect immunofluorescence assay (IIFA). In addition, two pools of female Ae. aegypti mosquitoes were tested for the presence of dengue viral antigen by ELISA and then subjected to Toxo-IFA system for demonstration of dengue virus.
Results: A total of 124 and 267 fever cases with clinical symptoms and signs suggestive of dengue were reported in Kadumuchandiram and Mampatti villages of Dharmapuri district, respectively. Serodiagnosis revealed that 13 of 31 and 14 of 52 patients tested were positive for dengue-2 virus by MAC ELISA in Kadumuchandiram and Mampatti villages respectively. Dengue Duo rapid strip test also detected 14 (of 31 tested) patients positive for dengue virus specific IgM antibodies in Kadumuchandiram village and 8 (of 12 tested) in Mampatti village. Application of temephos and fogging with pyrethrum 2 per cent extract were found to be effective against immatures and adults respectively. Both the pools of Ae. aegypti tested for the presence of dengue viral antigen were positive by ELISA and one mosquito (tested individually) was positive by IIFA. Supernatants of two pools were found to be positive for dengue-2 virus by Toxo-IFA.
Interpretation & conclusion: Virological and serological investigations confirmed that the outbreaks of fever were due to dengue virus infection. High breeding of Ae. aegypti in the study villages, detection of dengue-2 viral antigen and isolation of dengue-2 virus in Ae. aegypti mosquitoes confirmed the etiology.