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Prevalence of Aedes Aegypti and Aedes Albopictus– Vectors of Dengue and Dengue Haemorrhagic Fever in North, North-East and Central India*

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Prevalence of Aedes Aegypti and Aedes Albopictus– Vectors of Dengue and Dengue Haemorrhagic Fever in North, North-East and Central India* Prevalence of Aedes aegypti and Aedes albopictus– Vectors of Dengue and Dengue haemorrhagic fever in * North, North-East and Central India By N.L. Kalra Consultant (Malaria Research Centre), 20 Madhuban, Vikas Marg, Delhi – 110092 S.M. Kaul Joint Director, National Malaria Eradication Programme, 22 Sham Nath Marg, Delhi – 110054 and R.M. Rastogi Malaria Unit, WHO/SEARO, New Delhi – 110002 Abstract The Aedes aegypti mosquito vector was found to be prevalent in the western, northern, Indo- Gangetic 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 drought- prone 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 84 Dengue Bulletin – Vol 21, 1997 Prevalence of Aedes aegypti and Aedes albopictus–Vectors of Dengue and DHF in India suspected vector in the country. This Introduction paper high-lights the results of five years Dengue epidemics have been known to of inquiry. occur over the last two centuries in tropical and subtropical areas of the world. However, the role of the mosquito Aedes Study areas aegypti as the vector of this arbivirus has been known only during the past 70 The study covered ten physiographical years(1). In 1953-54 a new disease regions of India, viz. (i) Himalayan region; syndrome associated with dengue (ii) Western plains; (iii) Northern plains; appeared in the Philippines, which later (iv) Indo-Gangetic plains; (v) Eastern spread throughout south-east Asia and the plains; (vi) Central highlands; (vii) North Western and South Pacific. Unlike classical Deccan; (viii) Eastern plateau; (ix) Coastal dengue which causes only morbidity, this plains of Orissa, and (x) Assam valley and disease entity affected young children and the states of Jammu & Kashmir, Himachal caused severe illness with haemorrhage Pradesh, Punjab, Haryana, Chandigarh, and shock, resulting in high mortality, and Rajasthan, Uttar Pradesh, Madhya earning a name for itself as dengue Pradesh, Bihar, Orissa, and West Bengal haemorrhagic fever/ dengue shock (Fig.1). syndrome or DHF/DSS. In India, the association of Material and methods haemorrhagic manifestations were noticed for the first time in an outbreak in Calcutta In the present study the conventional in 1963. In this outbreak both the viruses, methods of Aedes survey as adopted by i.e. dengue and Chikungunya, were found the National Institute of Communicable (2) Diseases (NICD) for outbreak studies were to be circulating together . Since then the (4) country has reported several dengue followed . As the scope of the study was outbreaks in different parts of the country limited to determining the distribution of the with manifestations of haemorrhagic species, the information was collected on symptoms in varying degrees. To assess the following aspects: the receptivity of different geographical areas of the country to this infection, an (i) Occurrence (as evinced by attempt was made to determine the indigenous breeding), and distribution of Aedes aegypti in 1968, (ii) Intensity of infestation (No. of based upon the museum collection of the houses found positive per ward) National Institute of Communicable Diseases(3). Since this collection was not House (premises) Index : represen-tative of the whole country, a No. of houses positive for compre-hensive study was launched in Aedes larvae 1969 to assess the extent and intensity of _________________________ X 100 the prevalence of Aedes aegypti, the No. of houses examined known vector of DF/DHF, and its associated species Aedes albopictus, the Dengue Bulletin – Vol 21, 1997 85 Prevalence of Aedes aegypti and Aedes albopictus–Vectors of Dengue and DHF in India Figure 1. Pysiographical Regions of India Surveyed for Aedes aegypti and Aedes albopictus For information collection, 10 to 40 Observations towns, depending on size and population, were selected in each region. Towns/cities During the period 1969-1973, a total of were divided into wards/ localities and 50 203 towns/cities covering ten physiogra- houses selected at random were phical regions corresponding to dry and wet seasons were surveyed, except examined in each ward. Wherever the Assam, Kashmir valley and towns in West aegypti population reached the peripheral Bengal which were surveyed during the limit, the searches were extended to the dry season only. The houses in towns/ adjoining rural areas as well. Each region cities investigated during dry and wet was surveyed twice corresponding to dry seasons for Aedes aegypti and Aedes season (March to June) and wet season albopictus are included in Inserts B and C. (July to October). An analysis of the data indicated the following patterns of distribution. Searches were carried out in domestic, peridomestic and extra-domestic habitats. 1. Himalayan region Tree holes were the principal habitats A total of 20 towns spread over south examined under extra-domestic situations. Kashmir Himalayas, Punjab Himalayas, Kumaon hills and eastern Himalayas were 86 Dengue Bulletin – Vol 21, 1997 Prevalence of Aedes aegypti and Aedes albopictus–Vectors of Dengue and DHF in India investigated. The majority of the areas The Aedes aegypti population depicted investigated had an elevation above 500 low-to-moderate rates of positivity, except metres. The region was found to be in water-scarce areas of Haryana completely free of Aedes aegypti. Aedes (Ambala, Panipat, Rewari and Rohtak), albopictus was found to be the where a large population builds up predominant species in Punjab Himalayas, particularly during the wet season. while in south Kashmir Himalayas and Kumaon hills, its prevalence was scanty. 4. Indo-Gangetic plains 2. Western plains The Indo-Gangetic plains comprise The area includes the arid zone of moving Ganga-Jamuna doab (which literally sand dunes commonly known as “Thar means area lying between two rivers), desert” situated between the Indus plain Rohilkhand plains and Avadh plains. Of dunes on the west and Rajasthan upland the doabs of India, the Ganga-Jamuna in the south-east. The area is very thinly doab is by far the largest and most fertile populated due to extreme scarcity of water. and densely populated area. The elevation and character of the flood plains The Aedes aegypti population seems change within the doabs. On the east are to be fully entrenched in the area and the low-lying Rohilkhand and Avadh shows perennial prevalence. In Sikar, it plains. These plains are seamed with was found to infiltrate into rural areas deserted rivers. The Ramganga and during the wet season. Water scarcity and Sarda rivers meander through the the resulting water storage practices were Rohilkhand plains and lower reaches of determined as the main factor for the high Gomti and Ghaghra flow through the build-up of Aedes population. Avadh plains. On the north, these plains are bordered by the narrow waterless Aedes albopictus was encountered sandy belts of Bhabar and the swampy both during dry and wet seasons in the belt of alluvial soil (Terai), supporting vast peripheral areas of towns/cities. expanses of marshy land with luxuriant growth of vegetation. 3. Northern plains The Aedes aegypti population seems The region includes the plains of Punjab to have achieved ecological stability in the and Haryana. Major rivers in the Punjab Ganga-Jamuna doab areas. Most of the region are Sutlej and Beas, which have towns were found to be positive both long mountainous courses and provide a during dry and wet seasons, indicating a large network of canals for irrigation perennial prevalence of the species. purposes. The south-western parts of Proximity of the towns near the two great Haryana face an acute shortage of water. rivers, which provide riverine routes for the The whole area is densely populated and dispersal of the species and high density is one of the richest wheat-growing areas of human population, seems to have of the country. Dengue Bulletin – Vol 21, 1997 87 Prevalence of Aedes aegypti and Aedes albopictus–Vectors of Dengue and DHF in India provided a foothold to the species in this The Aedes aegypti populations region. showed a definite relationship with the terrain. These were found to be quite Both in Rohilkhand and Avadh plains, stable both in the south Bihar plains and the aegypti population were detected in the Bengal basin. Both the regions are wet season only, except in some large vulnerable to repeated introduction of the towns (e.g. Lucknow and Jaunpur) or species through the Ganga, which has a towns which are situated on the bank of lot of riverine traffic.
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