Distribution of dengue virus types in Aedes aegypti in dengue endemic districts of Rajasthan, India.

Background & objectives: Dengue haemorrhagic fever (DHF) is the major cause of sustained morbidity/ mortality among human cases of dengue in dengue endemic areas of Rajasthan. Screening of mosquitoes collected from disease endemic settings and typing the virus could provide signifi cant epidemiological information for prospective risk of DHF. We therefore carried out a study on different dengue virus types as occurring in fi eld collected Aedes aegypti mosquitoes from four dengue endemic districts of Rajasthan, India. Methods: Adult Ae. aegypti were collected from the human dwellings of urban, peri-were collected from the human dwellings of urban, urban and rural settings of four dengue endemic districts of Rajasthan, India. The fi eld collected adults were fed on 4 per cent glucose solution and kept in the laboratory for 3-4 days. The adult fi eld collected Ae. aegypti, were subjected to indirect fl uorescence antibody test (IFAT) following standard procedure. Commercially acquired monoclonal antibodies against DEN types 1, 2, 3 and 4 were used. The remnants of IFA test subjected mosquitoes were made into viral suspension which was inoculated into the cell culture medium and mouse brain to confi rm the presence of virus as shown by IFA test. Results: Of the 498 adult Ae. aegypti tested, 78 (15.6%) were positive by IFA test. Among urban areas, desert area (Jodhpur) showed highest (21.6%) mosquito infectivity followed by 7.1 per cent in forest and river area (Kota) and least (3.2%) in semi-arid area (Jaipur). Among rural settings also, desert area showed maximum (25.0%) natural infection in mosquitoes followed by rural setting-1 of semi-arid area (24.1%). Among urban setting of desert area, all the four dengue types viz., DEN-1, 2, 3 and 4 were detected. In semi-arid area, urban settings showed presence of DEN-3, whereas among rural settings, rural-1 showed all the four DEN types, rural-2 showed DEN-1 and DEN-3, rural-4 showed DEN-3 and DEN-4, and rural-3 showed no mosquito infections. In forest and river area, among urban settings only, three DEN types, 1, 2 and 4 were observed. Interpretation & conclusion: In desert and semi-arid areas of Rajasthan, where people possess tendency of over- and sustained storage of domestic water, present observations on occurrence of all four dengue virus types may have important bearing on the epidemiology of DHF in the area.

Entomological studies for surveillance and prevention of dengue in arid and semi-arid districts of Rajasthan, India.

BACKGROUND & OBJECTIVES: Rajasthan is one of the dengue endemic states of India. Very few studies have been published on entomological aspects of dengue in this state. Owing to water scarcity, inhabitants in desert areas overstore domestic water which leads to the persistence of dengue vectors within the domestic premises. Area specific knowledge on breeding, key containers and seasonal rhythms of vector population is essential for preparing an effective prevention plan against dengue. Present paper reports results of entomological investigations on dengue vectors in arid and semi-arid districts of Rajasthan. METHODS: Longitudinal studies were undertaken during 2004-06 in one arid and two semi-arid dengue endemic districts of Rajasthan. Adult and larval Aedes were collected from the randomly selected houses in representative towns and villages with associated details of container types and water storage practices of inhabitants. RESULTS: In urban areas during all the seasons adult house index (AHI) of Aedes aegypti was maximum in desert zone (25) and least in semi-arid area with saline river III (1). The difference of AHI during three seasons was statistically significant (chi2 = 16.1, p < 0.01 for urban; and chi2 = 50.71, p < 0.001 for rural). Breeding of Ae. aegypti among urban settings was maximum in desert zone. During all the seasons cement tanks were the key breeding habitats for Ae. aegypti in desert as well as semi-arid areas. INTERPRETATION & CONCLUSION: Water storage habits during summer season emerged to be the risk factor of vector abundance in urban areas of arid and semi-arid settings. A carefully designed study of key containers targeting cement tanks as the primary habitats of mosquito control may lead to commendable results for dengue prevention.

Distribution and seasonality of vertically transmitted dengue viruses in Aedes mosquitoes in arid and semi-arid areas of Rajasthan, India.

BACKGROUND & OBJECTIVES: Transovarial transmission of dengue virus is a crucial etiological phenomenon responsible for persistence of virus during inter-epidemic period of the disease. Distribution and seasonality of this phenomenon in disease endemic areas may contribute to explain emergence of dengue and its subsequent prevention. The study on seasonal and area distribution of transovarial transmission of dengue virus in Aedes aegypti, Ae. albopictus and Ae. vittatus has been made in desert and non-desert districts of Rajasthan, India from 2006 to 2007. The observations revealed role of different Aedes species in transmission and retention of dengue virus. METHODS: The larvae of Ae. aegypti, Ae. albopictus and Ae. vittatus were collected during each of the study seasons from rural and urban areas of three districts-Jodhpur, Jaipur and Kota. The larvae were collected from domestic and peri-domestic containers and from tree holes of peri-urban foci such as gardens and parks and were reared into adults in the laboratory at room temperature. The laboratory reared adults were subjected to Indirect fluorescence antibody test (IFAT). The laboratory-reared adult mosquitoes showing positive IFA were treated as the sample showing vertically transmitted dengue virus. RESULTS: Pooled data for all the four seasons revealed maximum (15.7%) mosquito infectivity in Ae. albopictus followed by Ae. aegypti (12.6%) in Jodhpur district. In Jaipur district, Ae. vittatus showed highest infection (20%) of vertically transmitted virus followed by Ae. albopictus (18.7%) and least in Ae. aegypti (13.3%). In Kota district, pooled data for all the four seasons showed maximum vertical infection of mosquitoes in Ae. albopictus (14.2%). INTERPRETATION & CONCLUSION: Transovarial transmission of dengue virus by available vector species in a dengue endemic setting could be the key etiological phenomenon responsible for re-emergence of the disease from inter-epidemic to epidemic phase of disease onset. The observations in the present study suggest that during winter season which is not the active transmission season of dengue in Rajasthan, Ae. albopictus has shown maximum percentage of vertically transmitted virus. Our observation substantiates with the earlier studies that how Ae. albopictus is horbouring virus during inter-epidemic period of dengue. Another important lead emerging through present study is the high mosquito infectivity of Ae. aegypti during summer and rainy seasons especially from desert districts, Jodhpur and semi district. This observation suggests that in Rajasthan, owing to tendency of overstorage of domestic water by the inhabitants, mosquito and vertically transmitted virus get pronounced during summer season which could precedes the active transmission season of dengue during following rainy season.