Date Published: March 16, 2015
Publisher: Public Library of Science
Author(s): Meghnath Dhimal, Ishan Gautam, Hari Datt Joshi, Robert B. O’Hara, Bodo Ahrens, Ulrich Kuch, Michael J Turell. http://doi.org/10.1371/journal.pntd.0003545
Abstract: BackgroundThe presence of the recently introduced primary dengue virus vector mosquito Aedes aegypti in Nepal, in association with the likely indigenous secondary vector Aedes albopictus, raises public health concerns. Chikungunya fever cases have also been reported in Nepal, and the virus causing this disease is also transmitted by these mosquito species. Here we report the results of a study on the risk factors for the presence of chikungunya and dengue virus vectors, their elevational ceiling of distribution, and climatic determinants of their abundance in central Nepal.Methodology/Principal FindingsWe collected immature stages of mosquitoes during six monthly cross-sectional surveys covering six administrative districts along an altitudinal transect in central Nepal that extended from Birgunj (80 m above sea level [asl]) to Dhunche (highest altitude sampled: 2,100 m asl). The dengue vectors Ae. aegypti and Ae. albopictus were commonly found up to 1,350 m asl in Kathmandu valley and were present but rarely found from 1,750 to 2,100 m asl in Dhunche. The lymphatic filariasis vector Culex quinquefasciatus was commonly found throughout the study transect. Physiographic region, month of collection, collection station and container type were significant predictors of the occurrence and co-occurrence of Ae. aegypti and Ae. albopictus. The climatic variables rainfall, temperature, and relative humidity were significant predictors of chikungunya and dengue virus vectors abundance.Conclusions/SignificanceWe conclude that chikungunya and dengue virus vectors have already established their populations up to the High Mountain region of Nepal and that this may be attributed to the environmental and climate change that has been observed over the decades in Nepal. The rapid expansion of the distribution of these important disease vectors in the High Mountain region, previously considered to be non-endemic for dengue and chikungunya fever, calls for urgent actions to protect the health of local people and tourists travelling in the central Himalayas.
Partial Text: Aedes (Stegomyia) aegypti (L.) and Aedes (Stegomyia) albopictus (Skuse) are known primary and secondary vectors of dengue virus (DENV) and several other arboviruses [1–4]. These species are also the known principal vectors of chikungunya virus (CHIKV) transmission [4–6]. Aedes albopictus has been recorded for Nepal since the 1950s [7,8], but the introduction of Ae. aegypti and its involvement in dengue fever (DF) outbreaks in Nepal are very recent events with Ae. aegypti first detected and dengue first outbreak reported in 2006 [9–12]. The presence of immature stages of Ae. aegypti and Ae. albopictus has already been reported from different areas of the Middle Mountain region, including Kathmandu Valley, average altitude 1,350 m above sea level[asl] [9,10,13,14]. Autochthonous cases of DF along with the circulation of all four DENV serotypes were first reported in 2006 during an outbreak that mostly affected urban areas of the Terai lowlands , and an autochthonous case from Nepal’s capital city Kathmandu was first reported during an epidemic in 2010 . Although chikungunya fever (CHIK) cases have not previously been reported in Nepal, the first local transmission of CHIKV was recently reported in the Middle Mountain region of central Nepal in 2013 . Moreover, a simultaneous circulation of DENV and CHIKV has been reported in many recent studies [6,17]. These findings along with reports of DF cases from different districts of the Middle Mountain region, with the majority of cases from central Nepal [9,14], suggest that CHIKV and DENV have already expanded their distribution into higher altitudes of Nepal. Despite frequent outbreaks of DF, no previous record of CHIK cases can be attributed to a lack of differential diagnosis in Nepal . The expansion of CHIKV and DENV vectors into higher altitudes may accelerate because the observed warming is more pronounced in the mountain regions of Nepal compared to the lowlands of the Terai and Siwalik hill regions [18–20].
We determined the presence and abundance of the CHIKV and DENV vectors Ae. aegypti and Ae. albopictus along an altitudinal transect from Birgunj (∼85 m asl) to Dhunche (∼2,100 m asl) in central Nepal. During the collection periods, Ae. aegypti and Ae. albopictus were commonly found up to 1,350 m asl in the Kathmandu valley and rarely found from 1,750 to 2,010 m asl in Ranipauwa and Dhunche, respectively. We also recorded Cx. quinquefasciatus as the second most abundant species throughout this study transect. This species is the principal vector of lymphatic filariasis in Nepal. The results of our physiographical, seasonal and habitat sampling allowed us to identify significant predictors of the presence of these CHIKV and DENV vectors and their co-occurrence. The abundance of these mosquito species was significantly affected by the climatic variables temperature, rainfall and relative humidity.