Date Published: June 18, 2015
Publisher: Public Library of Science
Author(s): Meghnath Dhimal, Bodo Ahrens, Ulrich Kuch, Matthew Baylis.
Despite its largely mountainous terrain for which this Himalayan country is a popular tourist destination, Nepal is now endemic for five major vector-borne diseases (VBDs), namely malaria, lymphatic filariasis, Japanese encephalitis, visceral leishmaniasis and dengue fever. There is increasing evidence about the impacts of climate change on VBDs especially in tropical highlands and temperate regions. Our aim is to explore whether the observed spatiotemporal distributions of VBDs in Nepal can be related to climate change.
A systematic literature search was performed and summarized information on climate change and the spatiotemporal distribution of VBDs in Nepal from the published literature until December2014 following providing items for systematic review and meta-analysis (PRISMA) guidelines.
We found 12 studies that analysed the trend of climatic data and are relevant for the study of VBDs, 38 studies that dealt with the spatial and temporal distribution of disease vectors and disease transmission. Among 38 studies, only eight studies assessed the association of VBDs with climatic variables. Our review highlights a pronounced warming in the mountains and an expansion of autochthonous cases of VBDs to non-endemic areas including mountain regions (i.e., at least 2,000 m above sea level). Furthermore, significant relationships between climatic variables and VBDs and their vectors are found in short-term studies.
Taking into account the weak health care systems and difficult geographic terrain of Nepal, increasing trade and movements of people, a lack of vector control interventions, observed relationships between climatic variables and VBDs and their vectors and the establishment of relevant disease vectors already at least 2,000 m above sea level, we conclude that climate change can intensify the risk of VBD epidemics in the mountain regions of Nepal if other non-climatic drivers of VBDs remain constant.
According to the latest report of the Intergovernmental Panel on Climate Change (IPCC), the average warming of the global mean surface temperature was 0.85°C [0.65–1.06°C] over the period of 1880 to 2012 . Importantly, different trends of surface temperature warming at the regional scale and the highest increase has been recorded over the last three decades in mountains and mid-high latitudes of the northern hemisphere. For example, the rate of warming in the Himalayas has been reported to have been much greater (0.06°C/year) than the global average in the last three decades indicating that the Himalayas are more sensitive and vulnerable to global climate change than other areas of the earth . Climate change affects human health mainly by three pathways: (1) direct impacts by increasing the frequency of extreme events such as heat, drought and heavy rainfall, (2) effects mediated through natural systems such as disease vectors, water-borne diseases and air pollution, and (3) effects that are heavily mediated by human systems such as occupational impacts, under-nutrition, and psycho-social problems [3,4]. As the published literature continues to focus on the effects of climate change in developed countries, the effects on the most-vulnerable populations residing in least developed and developing countries are underreported . These poor or developing countries are historically least responsible for greenhouse gas (GHG) emissions but most vulnerable to climate change impacts, and are also currently suffering the heaviest disease burden indicating an “ethical crisis” . Several challenges for conducting climate change and health research in developing mountainous countries have been reported. These include a lack of trained human resources, financial resources, long-term data and information, and suitable methods that are applicable to the local context . Furthermore, the largest health risks will occur in populations that are most affected by climate sensitive diseases such as vector-borne diseases (VBDs) and in those left behind by the economic growth .
A systematic literature search was performed and summarized information on climate change and the spatiotemporal distribution of VBDs in Nepal following providing items for systematic review and meta-analysis(PRISMA) guidelines. We searched for peer-reviewed articles published in English language before December2014 in the PubMed and Web of Science databases. Besides, we searched for relevant journal articles in Google Scholar and retrieved government reports from their web sites. We used the following search terms in title, abstract and keywords:
Nepal andClimate, climate change, temperature, rainfall, precipitation, relative humidity, weather, Aedes, Anopheles, Culex, Phlebotomus, dengue, malaria, kala-azar, Japanese encephalitis, visceral leishmaniasis, lymphatic filariasis, mosquito-borne diseases, VBDs.
The review of observed and future projections of climatic data show a conducive environment for the transmission of VBDs in Nepal, especially in the highlands (mountains) which had been assumed to be free from these diseases. Despite a decade-long armed conflict and political instability in Nepal, there has been a substantial decline in the incidence of all major VBDs except DF which has only emerged in Nepal since 2004. The presence of disease vectors and reports of series of autochthonous cases of VBDs in hill and mountain regions of Nepal that had previously been considered to be non-endemic suggests that the local transmission of VBDs might be favoured by rising temperatures. However, the transmission of VBDs among humans is more complex than mere temperature changes, and this fact has been extensively reviewed [9,28,33,34,39,108,109,110,111]. One may also hypothesize that improvements in diesease surveillance and health care services, land use changes, population growth, globalization in general, in particular international trade, tourism and travel, migration and other movement of people, the expansion of road networks and the shipment of goods, unplanned urbanization and improvements in livelihood and access to health care services, etc., could be responsible for an increased detection of VBD cases in new areas. However, the presence of vectors and local transmission of VBDs at altitudes above 2,000 m, which clearly stands against the conventional logic that high altitude regions are free of VBDs because of cold temperature, strongly suggests that global warming is playing a role in the observed transmission. Therefore, we discuss below climate change and the spatio-temporal distribution of VBDs in Nepal in comparison with the results of studies from other mountainous countries around the world.
The studies reviewed here suggest that both the observed and projected climate are conducive for the transmission of VBDs in the mountain regions of Nepal which had previously been considered non-endemic for these diseases. The short-term data shows a clear association between climatic factors and VBDs, but it is complex and difficult to project long-term effects of climate change in the face of rapid environmental and socio-economic changes and attribution to climate change is not determined in the existing studies. Despite continuous efforts of the government to control them and their declining incidence over the last decade (except for DF), VBDs have over the years been expanding their geographical ranges especially in mountain regions of the country. This might be attributed to environmental changes, in particular climate change, along with socio-economic factors. However, the observed spatial expansion of VBDs in new areas, especially in cool margins of mountain regions, that is correlated with the observed warming climate does not necessarily show a causal relationship. As VBDs show a heterogeneous distribution and spatiotemporal variation in the trends of climatic variables across the country, well-designed long-term local studies are needed to determine attribution of climate change to the observed transmission and distribution of VBDs in new areas. Therefore, VBD monitoring, surveillance and research should be strengthened in areas where risk of VBD is not yet determinedand VBD control programmes are not yet focused. Moreover, tourists and returning migrant workers coming to Nepal from disease endemic regions (including the country’s own lowlands) should be made aware about VBDs, their responsibility and potential role in spreading infections especially when travelling in mountain regions, and should be encouraged to engage in reasonable preventive and prophylactic measures including vaccination.