Prevalence of Aedes aegypti and Aedes albopictus– Vectors of Dengue and Dengue haemorrhagic fever in North, North-East and Central India*

The Aedes aegypti mosquito vector was found to be prevalent in the western, northern, IndoGangetic and eastern plains, Assam valley and the coastal areas of Orissa state in India. The species was non-existent in the Himalayan region. In north-central highlands, the species showed low-to-moderate prevalence, while in south-central highlands, the mountainous areas were largely free but high populations of the vector were encountered in the valleys. Similarly, the eastern plateau, including the eastern ghats were comparatively free of the vector except large towns in the Mahanadi basin. The Satpura ranges of north Deccan were also found to be free of Ae. aegypti. The elevation, type of relief, terrain, density of population, water storage practices in droughtprone regions and high rainfall leading to formation of secondary foci had direct relationship with the prevalence of the species. Altitudes above 1000 metres were found to be unfavourable for the species. Being hygroscopic, the species depicted a phenomenon of ‘annual pulsation’. It tends to move to ‘mother foci’ in the central parts of cities which are humid during the dry season and spreads out during the wet season. Aedes albopictus was encountered in the peripheral areas of towns where it replaced the Ae. aegypti populations. However, in the eastern plateau, the species penetrated upto the central parts, probably due to lack of intra-species competition from aegypti which is very scanty in the region. The information proved to be of immense value in delimiting areas which were prone to DF/DHF epidemics. The internal dynamics provided useful information for developing control strategies. * This study was funded by the Indian Council of Medical Research, New Delhi Prevalence of Aedes aegypti and Aedes albopictus–Vectors of Dengue and DHF in India Dengue Bulletin – Vol 21, 1997 85 Introduction Dengue epidemics have been known to occur over the last two centuries in tropical and subtropical areas of the world. However, the role of the mosquito Aedes aegypti as the vector of this arbivirus has been known only during the past 70 years. In 1953-54 a new disease syndrome associated with dengue appeared in the Philippines, which later spread throughout south-east Asia and the Western and South Pacific. Unlike classical dengue which causes only morbidity, this disease entity affected young children and caused severe illness with haemorrhage and shock, resulting in high mortality, and earning a name for itself as dengue haemorrhagic fever/ dengue shock syndrome or DHF/DSS. In India, the association of haemorrhagic manifestations were noticed for the first time in an outbreak in Calcutta in 1963. In this outbreak both the viruses, i.e. dengue and Chikungunya, were found to be circulating together. Since then the country has reported several dengue outbreaks in different parts of the country with manifestations of haemorrhagic symptoms in varying degrees. To assess the receptivity of different geographical areas of the country to this infection, an attempt was made to determine the distribution of Aedes aegypti in 1968, based upon the museum collection of the National Institute of Communicable Diseases. Since this collection was not represen-tative of the whole country, a compre-hensive study was launched in 1969 to assess the extent and intensity of the prevalence of Aedes aegypti, the known vector of DF/DHF, and its associated species Aedes albopictus, the suspected vector in the country. This paper high-lights the results of five years of inquiry.