Dengue-related mortality could rise by 13% in Pune, Maharashtra, in the near future due to changes in the monsoon from global warming, a study by researchers from the Indian Institute of Tropical Meteorology has found. The study is among the first to do a regional analysis of dengue-related mortality by incorporating the latest set of climate models in its projections.
India is among the world’s dengue hotspots, reporting between 1,90,000 and 2,89,000 cases annually since 2021, according to data from the National Centre for Vector Borne Disease Control. Cases of dengue begin to rise at the onset of monsoon season, when the conditions for the spread of the disease are apt.
The study modelled future instances of dengue mortality in the city of Pune. Dengue mortality is projected to rise even in low emission scenarios where global warming is limited to 1.5 degrees above pre-industrial levels. As per the Intergovernmental Panel on Climate Change, global warming has already reached 1.1 degrees above pre-industrial levels. In the near term, dengue mortality will increase by 12%-13% by 2040 in the city “regardless of emissions pathways”, says the study.
High emissions scenarios that result in surface temperatures above 1.5 degrees celsius could push dengue-related mortalities 23% to 40% higher by the mid-term (2041-2060) and 30% to 112% higher in the long term (2081-2100), depending on the extent of emissions.
“The climate-dengue relation is region specific. However, the methodology and model framework used in this study can be adapted to develop region-specific dengue models for other areas, as long as the city or district shares the local health data with us,” Sophia Yacob, a researcher from IITM and lead author of the study, said in an email to Mongabay India.
The lag time between the onset of meteorological conditions enabling dengue occurrence and mortality is sufficient for early warning systems in dengue management, the study says.
Early warning for dengue
Dengue occurrence is sensitive to changes in rainfall, humidity, and temperature. Each of these meteorological factors influence the life cycle of Aedes aegypti and Aedes albopictus mosquitoes, which are responsible for the spread of dengue.
“Rainfall creates breeding sites for mosquito eggs and larvae, but this does not immediately result in increased dengue risk,” explained Yacob, adding, “It takes time for these larvae to mature into adult mosquitoes capable of transmitting the virus. When these adult mosquitoes become infected and begin spreading dengue weeks later, the impact of the initial rainfall on dengue cases becomes apparent.”
Assessing data from 2004 to 2015 in Pune, the researchers determined that wet weeks with cumulative rainfall between 0.5 mm and 150 mm led to an increase in dengue mortality, while rainfall events of more than 150 mm reduced dengue mortality risk, likely because heavy rain flushed out mosquito eggs and larvae. Similarly, humidity levels of 60%-78% and temperatures between 27 degrees celsius and 35 degrees celsius were optimum for high rates of dengue mortality.
Incorporating the lag between favourable weather conditions and dengue mortality into the dengue model “is crucial,” the study states, in order to account for the multiple processes involved in transmission and infection. “Altogether, from the availability of aquatic habitats for mosquito breeding to a dengue mortality [in Pune], it takes approximately 1-3 months,” the study says, adding that the lag in climate-dengue associations “provides early warnings with a sufficient lead time to prepare and respond effectively to dengue outbreaks”.
An Aedes albopictus mosquito bites a human. Experts say that rainfall creates breeding sites for mosquito eggs and larvae. Weeks later, the developed adult mosquitoes begin transmitting dengue. Image by James Gathany, Centers for Disease Control and Prevention, United States Department of Health and Human Services.
Future mortality
The study modelled future projections of dengue in altered climates using the Coupled Model Intercomparison Projects 6, or CMIP6, a set of climate models widely used by scientists and forums like the Intergovernmental Panel on Climate Change. The CMIP6 models consider a range of socioeconomic pathways for development with different emissions scenarios.
Across high and low emissions scenarios, increases in dengue mortality are “predominantly driven by the increase in temperature and changes in rainfall patterns,” says the study. Changes in temperature could contribute 12%-22% in the increase of dengue mortality while changes in rainfall patterns could offset dengue mortality by 3-4% (due to the flushing effect of extreme rainfall events which are projected to rise with higher levels of global warming).
“If future emissions can be controlled to keep the global average surface temperature changes below 1.5 degrees Celsius, the anticipated rise in dengue mortalities relative to the reference period will be below 13%. However, if emissions continue to increase and temperatures exceed the 1.5 degrees Celsius threshold, the corresponding increase in dengue mortality ranges from 23 to 40%,” the study says, adding, “Alternatively, when the global surface temperature crosses the 2 degrees Celsius threshold, dengue mortalities are projected to increase by 40%-112% relative to the reference period.”
This article was first published on Mongabay.
