Vector borne diseases and climate change in Nepal

[PHP Nepal Vol 3 Issue 8 August 2013] | Nepal is vulnerable to climate change. Many of the common diseases found in Nepal are climate related and a changing climatic pattern would make the subtropical and warm temperate regions of Nepal more vulnerable to Vector Borne Diseases (VBDs) such as Malaria, Dengue, Japanese Encephalitis (JE), and Visceral Leishmaniasis (VL).

Climate change modifies the transmission pattern of VBDs. Temperature, precipitation, humidity, and other climatic factors are known to affect the reproduction, development, behavior, and population dynamics of the arthropod vectors of these diseases.

Mosquitoes of the genus Anopheles and Plasmodium are sensitive to environmental conditions. Due to the complexities of their respective life cycles and the many uncertainties regarding their interaction with environmental parameters, reliable modeling of their behavior under different climate conditions is fraught.  

Dengue and JE are caused by two important flaviviruses and the former has now increasingly becoming an emerging disease since the first case report in 2004. A series of dengue fever outbreaks in several districts has been seen in different time intervals. Changes in rainfall pattern, particularly longer rainy seasons, will increase the sites where Aedes sp of mosquitoes (the Aedes aegypti mosquito can transmit the viruses that cause dengue fever) will breed. Extreme weather events will disrupt health systems, and the capacity to perform surveillance of, and manage dengue outbreaks. 

Climate change impact on JE vector proliferation might be influenced in a similar way predicted for dengue vectors but the potential impact of climate change on JE remains to be investigated. On a global scale, however, JE incidence may decline as a result of large-scale vaccination programs but due to its zoonotic nature, outbreaks might occur anytime. 

Climate change indirectly increases JE vector proliferation, longevity and may also increase disease risk because of changing patterns of agricultural practices such as irrigation allowing the mosquitoes to breed. It also influences migration patterns of birds. Since large numbers of mosquitoes feed on ardeid birds their migration between rural and urban ecosystems during spring may result in JE vectors being introduced into new areas. 

JE was introduced in 1978 in Nepal. Mostly JE cases in Nepal are confirmed in low land plains of Terai but reporting of JE cases from non endemic hill region of Kathmandu valley is evidence that JE virus can cause the encephalitis even in hilly districts. The mosquito vectors have also been reported in the hill and mountain areas of Nepal which can survive at the temperature of 200C that can be experience in the Kathmandu valley. But additional studies are needed in high-altitude mountain districts to confirm the presence of environmental and ecological conditions that promote JE virus transmission. 

Chikungunya is another serious vector borne viral disease. There are many global evidences of climate change impact on chikungunya epidemiology. The research published in Science daily on 17 December 2012 linked chickungunya outbreak in Atlanta, Miami to dramatic increase in temperature due to climate change. The modeling dynamic of chickungunya in UK by Diego Ruiz-Moreno confirmed that like other vector borne diseases the magnitude of outbreaks is related with strong seasonal temperature patterns. 

In Nepal, three laboratory confirmed cases of chikungunya fever has been recently reported for the first time by Sukraraj Tropical and Infectious Disease Hospital, Teku, Kathmandu. The cases were reported from Dhading district. It indicates new appearance of VBDs is also common in Nepal. Although patients coming from endemic areas at an early stage of infection may have imported the virus and was responsible for cases of chikungunya it would be a possible future threat to public health with the changing climate in Nepal.

Visceral Leishmaniasis (Kala-azar) first reported in Nepal in 1980s is serious VBDs of the Terai region. An outbreak of VL was reported during 2003-2006 one/two months after heavy rainfall. Catastrophic flooding may cause people to migrate to non-flood areas and increase the population density which would significantly increase the risk of infectious diseases including Kala azar. However, the impact if climate change on Kala-Azar is still unclear. This is because of the absence of studies on sand fly (Phlebotomus argentipes) ecology in relation to climate change.

In most Asia Pacific countries many researches and activities have been conducted on how climatic variations influence human health. However, in Nepal, research on human health impacts of climate change is in a budding stage. At least, if we have time series data of climate and diseases of all regions or districts, we can correlate reported incidence of VBDs and the changing climatic patterns. This will also help identify the climate-attributable part of disease and the long term trends throughout the nation. Development of adequate response plans, effective and locally appropriate strategies, and enhancement of surveillance systems are also needed to reduce vulnerability to VBDs that are spread due to changing climatic pattern.

Yadav Prasad Joshi is a PhD Student at School of Medicine in Sungkyunkwan University, South Korea.

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