Research Article: Spatio-Temporal Distribution of Dengue and Lymphatic Filariasis Vectors along an Altitudinal Transect in Central Nepal

Date Published: July 31, 2014

Publisher: Public Library of Science

Author(s): Meghnath Dhimal, Ishan Gautam, Aljoscha Kreß, Ruth Müller, Ulrich Kuch, Patrick J. Lammie.

Abstract: BackgroundRapidly increasing temperatures in the mountain region of Nepal and recent reports of dengue fever and lymphatic filariasis cases from mountainous areas of central Nepal prompted us to study the spatio-temporal distribution of the vectors of these two diseases along an altitudinal transect in central Nepal.Methodology/Principal FindingsWe conducted a longitudinal study in four distinct physiographical regions of central Nepal from September 2011 to February 2012. We used BG-Sentinel and CDC light traps to capture adult mosquitoes. We found the geographical distribution of the dengue virus vectors Aedes aegypti and Aedes albopictus along our study transect to extend up to 1,310 m altitude in the Middle Mountain region (Kathmandu). The distribution of the lymphatic filariasis vector Culex quinquefasciatus extended up to at least 2,100 m in the High Mountain region (Dhunche). Statistical analysis showed a significant effect of the physiographical region and month of collection on the abundance of A. aegypti and C. quinquefasciatus only. BG-Sentinel traps captured significantly higher numbers of A. aegypti than CDC light traps. The meteorological factors temperature, rainfall and relative humidity had significant effects on the mean number of A. aegypti per BG-Sentinel trap. Temperature and relative humidity were significant predictors of the number of C. quinquefasciatus per CDC light trap. Dengue fever and lymphatic filariasis cases had previously been reported from all vector positive areas except Dhunche which was free of known lymphatic filariasis cases.Conclusions/SignificanceWe conclude that dengue virus vectors have already established stable populations up to the Middle Mountains of Nepal, supporting previous studies, and report for the first time the distribution of lymphatic filariasis vectors up to the High Mountain region of this country. The findings of our study should contribute to a better planning and scaling-up of mosquito-borne disease control programmes in the mountainous areas of Nepal.

Partial Text: Dengue fever (DF) is a mosquito-borne viral disease which has become a major international public health concern in recent years. Dengue virus (DENV), the causative agent of this disease, belongs to the genus Flavivirus, family Flaviviridae, and is transmitted by Aedes mosquitoes, especially by the yellow fever mosquito (Aedes [Stegomyia] aegypti) and the Asian tiger mosquito (Aedes [Stegomyia] albopictus) which are respectively considered to be its primary and secondary vectors in Southeast Asia [1], [2]. In the last five decades, the incidence of DF has increased 30-fold, and geographical expansions to new countries and, in the present decade, from urban to rural settings have occurred [3]. For example, in the World Health Organization (WHO) South-east Asia Region (SEARO), the area with autochthonous DENV transmission has extended to the sub-Himalayan foothills of Bhutan and Nepal since 2004 and 2006, respectively [3], [4]. In the past, DF had often been considered a public health problem of lesser concern because of its low mortality rate and an infrequent occurrence of epidemics [5]. However, rapid economic development and urban growth in developing countries with a lack of careful planning of housing, water resources, sewage and waste management, along with the globalization of trade and travel, have since contributed to rendering DF the most important mosquito-borne viral disease of humans [5]. Despite progress with the development and clinical trials of vaccines against DENV infection, no such vaccine is available on the market yet [6], and there is no specific antiviral treatment either. Thus, controlling the population of dengue vector mosquitoes, especially A. aegypti and A. albopictus, and limiting their dispersal to new regions remains crucial for the prevention and control of DENV transmission [4].

The present entomological study of DF and LF vectors in four distinct physiographical regions of central Nepal from September 2011 to February 2012 documented differences in their spatial and temporal dynamics. The DENV vectors A. aegypti and A. albopictus were found from the lowlands up to the Middle Mountain region (1,310 m asl) in central Nepal. Culex quinquefasciatus, the principal vector of W. bancrofti filariasis in Nepal [12], [43], was found throughout the studied longitudinal gradient from 80 to 2,100 m asl and was constantly abundant in the six-month trapping period. The presence of these vectors and reports of cases of DF up to the Middle Mountain region and LF up to the High Mountain region suggest that DENV and the LF parasite W. bancrofti are potentially established in central Nepal.