Outbreak of dengue in national capital territory of Delhi, India during 2003.

Dengue and dengue haemorrhagic fever continue to be major infectious diseases of public health importance in countries of the western Pacific and Southeast Asia regions. These regions are experiencing a geographical spread, both in terms of distribution of the virus and the mosquito vector, with an increase in the frequency of epidemics. Since 1963, outbreaks of dengue/DHF have been recorded in almost all parts of India except the northeastern region. In all the outbreaks the main mosquito involved in transmission was Aedes aegypti. However, during the outbreak in Kerala state during 2004, the vector involved in the transmission was Aedes albopictus. The first outbreak of dengue fever in India with hemorrhagic manifestation was reported in Calcutta City. An increasing trend of dengue outbreaks accompanied by dengue hemorrhagic fever poses a problem of utmost importance to public health in India (WHO 1999). Dengue fever outbreaks have been reported from various parts of the country during the past 30-40 years (Yadav and Narsimham 1992). A severe outbreak of dengue hemorrhagic fever swept National Capital Territory, Delhi in 1996 where 10,252 cases and 423 deaths due to DHF were recorded in various parts of Delhi (Kaul et al. 1998, Sharma et al. 1999), and dengue has remained endemic in Delhi for several years.The first DHF outbreak was reported in 1988 with 33% mortality among children admitted in hospitals (Kabra et al. 1992). The principal vector of dengue fever, Ae. aegypti, is prevalent in all cities and towns of India. The gangetic plain of North India is also infested with Ae. aegypti (Rao, 1967). Krishnamurthy et al. (1965) and Katyal et al. (1996) carried out a comprehensive survey of Ae. aegypti populations in Delhi. Sharma et al. (2001) reported the degree of Aedes prevalence in hospitals and schools in Delhi. Although Ae. aegypti has been known to be widely distributed in several countries of Southeast Asia and its importance as potential dengue fever vector has been long recognized, the information on its prevalence and shifting trend of breeding places is still fragmentary (Kalra et al. 1998). Vector surveillance is an important tool for generating entomological data for suggesting appropriate control strategies and developing an early warning system (Pant and Self 1993). These studies on the incidence of dengue and prevalence of Ae. aegypti were conducted in Delhi during 2003. Delhi, the capital of the Republic of India, is situated on the bank of the river Yamuna at approximately 77.15E and 26.15 N. It occupies a 1,485 km2 area of which 900 km2 is classified as urban. The city is the center of vast economic opportunities, attracting many migrants and registering a phenomenal population growth. The population of Delhi, now estimated to be above 10 million, has grown at the rate of 64.2%, 54.6%, and 57.1% in the decades 1951-1961, 19611971, and 1971-1981 respectively. In Delhi, three agencies, namely the Municipal Corporation of Delhi (MCD), New Delhi Municipal Committee (NDMC), and Defense are responsible for dengue control activities inside their own areas. The MCD, with its 12 zones, covers the largest part. Larval surveys were carried out in all the localities of the city irrespective of the risk of dengue/DHF in that locality. Searches were made for Aedes breeding in all types of breeding habitats during 2003 in the areas covered by the MCD, NDMC, Railways, and Delhi Cantonment. During the past several outbreaks of dengue/DHF, the initial cases have originated mainly from hospital and school premises. Stratification of Delhi was according to areas that differed from one another for Aedes breeding potential. Entomological indices including House Indices (HI), Container Index (CI), and Breateau Index (BI) were used for measuring the larval population: