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Though global research points to the link between climate change and dengue, there are no such studies in India where cases of the mosquito-borne disease have surged and spread to new regions

Several global studies have pointed to the connect between dengue and climate change. (Image by jon hayes)

Several global studies have pointed to the connect between dengue and climate change. (Image by jon hayes)

Dengue cases are soaring in India, especially its capital Delhi and its suburbs, which alone have reported more than 3,700 cases and 17 fatalities so far this year , compared to around 995 cases in 2014. Hospitals and clinics across the city have been inundated with patients, putting a strain on emergency services.

The season for the dengue virus is also lengthening, starting earlier and continuing even after winter sets in.

This surge tallies with recent research that indicates a link between climate change and dengue. This has also been backed by an Intergovernmental Panel on Climate Change (IPCC) report that mentions projected changes in dengue transmission due to changing climate.

Despite the spiralling cases, however, India lacks studies on establishing a link between changing climate and dengue spread. The state of studies on climate change and dengue in India is “negligible”, says Ramesh Dhiman, scientist at New Delhi’s National Institute of Malaria Research.

“In addition to climatic conditions, local conditions like water availability, water storage practices and the lifestyle of communities are also important determinants,” Dhiman told India Climate Dialogue.

Dhiman’s comments reflect wider concern among a section of scientists that while India has, by now, considerable climate data, there is no matching data from the health sector to draw definite conclusions about links between climate change and changes in disease patterns in India.

“We do not have enough health data. We request the health authorities to come forward to work with climate scientists,” Madhavan Rajeevan, director of Indian Institute of Tropical Meteorology (IITM), Pune, said at a workshop in Delhi organised by the Indian Meteorological Society, University of California, Berkeley, and the Delhi-based NGO TARU for scientists from the climate and health sectors to design mechanisms for coordination.

Dengue is spread by two mosquito species – Aedes aegypti and to a lesser extentAedes albopictus – both of which breed in stagnant water pools, including water tanks, drains and pot plants.

Both mosquitoes are being increasingly reported from many areas, particularly from southern and eastern India, Dhiman pointed out.

Health ministry figures indicate that dengue cases in India this year have already crossed 27,000 and an estimated 60 people killed across the country.

The spatial distribution and the annual incidence of dengue is increasing in India, Dhiman said. “It used to be a disease of urban areas. But more and more rural areas are also reporting dengue due to water storage practices.” Delhi, he added, especially witnessed a spurt due to advanced and good rains from July to the third week of August.

Pointers to climate change-dengue links

Though India is yet to conduct a country-specific study on the mosquito borne disease, several global studies have pointed to the connect between dengue and climate change.

The IPCC’s report on climate impacts and adaptation said models “suggest that a warming projection of 2°C by 2100 will result in a net increase in the potential latitudinal and altitudinal range of dengue and an increase in duration of the transmission season in temperate locations”.

But it also cautioned that these models do not include the impact of population changes and public health factors, which could limit some of the potential increase.

A December 2014 United Nations University report, which mapped the global vulnerability to dengue, using a water-associated disease risk index, a tool to assess water-associated disease hotspots over area and time, states, “overall, highest levels of vulnerability to dengue are observed in South Asia and Southeast Asia”.

Also, countries close the equator, including southern India, Colombia, Indonesia, Nigeria and Sierra Leone, tend to be vulnerable to dengue throughout the year because they experience consistently favourable exposure conditions.

Another contributing factor is high population density. Highly-populated Asian countries, such as India, Bangladesh, China and Indonesia are also highly vulnerable to dengue, it said.

In 2014, researchers from Australia surveyed major online peer-reviewed research on the impact of climate factors such as temperature, rainfall and humidity on dengue transmission. They searched reports published from January 1991, when dengue cases started emerging in Australia, to October 2012.  Their findings, published in BMC Infectious Diseases, have implications globally.

Climatic factors, in particular temperature, rainfall and relative humidity, play a significant — and sometimes contrasting – roles in mosquito biology, affecting the virus transmission, the report in the journal observed.

Higher temperatures increase the rate of larval development and shorten the emergence of adult mosquitoes; increase the biting rate of mosquito and reduce the time required for virus replication within the mosquito.  Extreme higher temperatures, however, reduce mosquito survival time.

Similarly, heavy rainfall may flush away eggs, larvae, and pupae from containers in the short term; but residual water creates breeding sites in the longer term.

Relative humidity is another key factor that influences mosquito lifecycles at different stages. The combined effect of temperature and humidity significantly influences the number of blood meals, (the blood the mosquito feeds on while biting a person), and can also affect the survival rate of the mosquito, and the probability that it will become infected and transmit dengue, the journal summarised.

The exhaustive review concluded that dengue transmission is sensitive to climate variability and change. “We believe that it is important to develop, employ and integrate different quantitative modelling approaches for dengue transmission compatible with long-term data on climate and other socio-ecological changes and this would advance projections of the impact of climate on dengue transmission,” it recommends.

This adds to detailed estimations from researchers in Mexico, published in 2013 in the journal PLoS Neglected Diseases, based on exhaustive datasets from 23 years of province-specific dengue reports across nine climatic regions.

Using modelling studies, they estimated the impact of weather on dengue, accounting for the effects of non-climatic factors in Mexico.  Their model included the effects of some non-climatic factors that  also influence the disease , such as water supply.

The report also noted that higher temperatures lead to unreliably water supply and water shortages which, in turn, lead to higher water storage. Hence dengue incidence “may increase to about 40% by 2080 due to climate change”.

“This increase in dengue incidence may be aggravated by a rising access to piped water if it leads to domestic water storage, although any adaptation measures to rising dengue may also affect the risk,” it says.  “Our results contribute to a better overall understanding of the epidemiology of dengue.”

Their analysis “indicate that weather significantly influences dengue incidence in Mexico….These findings highlight the importance of using flexible model specifications when analysing weather–health interactions.”

The researchers recommend the need for flexible modelling approaches to analyse links between weather and diseases, especially those that do not show a linear relationship; every rise in temperature directly correlated with rise in new cases.

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