Climate change creates new uncertainties for human exposure to vector-borne diseases, and this is especially true for dengue fever and malaria.
The changing weather patterns affect the development and dynamics of the disease vectors and the pathogens they carry. The range of Aedes mosquitos is changing, with dengue fever and the chikungunya virus emerging in areas where they were previously unknown. There is evidence that the geographic range of ticks and mosquitoes that carry disease has changed in response to climate change.
As temperatures warm, mosquitoes and other warm-weather vectors can move into higher altitudes and new regions farther from the equator. Increased rainfall, flooding, and humidity creates more viable areas for vector breeding and allows breeding to occur more quickly, as eggs hatch faster in hotter climates.
According to data from the World Health Organization, deaths related to vector-borne diseases such as malaria and dengue may also increase. The number of malaria incidences has dropped significantly in many Asian countries, but it remains a major health challenge for the region. Dengue has dramatically grown over recent decades and is now the leading cause of hospitalization and death among children in many Asian countries.
Based on the geographical distribution of malaria, WHO projects that by the 2030s, up to 50 additional malaria-related deaths a year in Oceania, up to 500 a year in Southeast Asia, and up to 2,000 a year in South Asia will be attributable to climate change. By the 2050s, this number rises to almost 10,000 attributable deaths per year in South Asia.
Besides climate, the recent spread of dengue appears to be dependent on rapid urbanization rates and expanding global transport networks. According to the geographical distribution of dengue, by the 2030s, climate change will cause approximately 30 additional deaths related to dengue per year in East Asia and approximately 200 additional deaths in South Asia.
An ADB study on the possible impacts of climate change on malaria and dengue has relied on estimated changes in the population shifts, the geographic disease distributions, and current national mortality estimates.
In much of South Asia and Southeast Asia, a sharp increase in dengue cases occurred in 2015. The Philippines, for example, reported more than 169,000 cases, representing almost a 60% increase in case numbers compared to the previous year. Large outbreaks have also been reported for Malaysia, Thailand, and Viet Nam.
The geographic distribution of malaria is expected to change as climate change continues, and may expand into new regions. Despite the strong connection between malaria and climate, there is still quite a bit of uncertainty about future malaria transmission rates worldwide, mainly because there are many other factors that affect the spread of the disease, including socioeconomic development, drug resistance, and immunity.
However, the effect of future climate change on the rates of dengue transmission is complex. On the one hand, areas with higher rainfall and higher temperatures can expect higher rates of dengue transmission because the mosquitoes thrive in warm, moist environments.
In this backdrop, rates of dengue transmission may actually increase in regions that are projected to become more prone to drought. This is because the Aedes mosquitoes often carry dengue breed in containers used for household water storage, and because the need for such water storage containers will increase in areas projected to be more prone to drought as the climate continues to change. Thus, there may likely be more habitats for dengue vectors in areas projected to become drier.
By Zulker Naeen
The writer is a South Asian Fellow at Climate Tracker. He is a communication graduate of the University of Liberal Arts Bangladesh (ULAB).