Impact of Climate on Dengue Virus: The increasing incidences of dengue across several parts of India has become a cause of concern. Earlier this month, the Health Ministry informed the parliament that the country recorded 32,091 dengue cases, 75 percent higher than the 18,391 cases recorded in the first six months in 2023 till June this year.
In a statement, Union Health Secretary Apurva Chandra said that till the month of July, India had seen 50 percent more cases as compared to the corresponding period last year.
According to a report by The Print, the highest number of dengue cases, which can be fatal in severe cases, has been in Kerala, Karnataka, Maharashtra, Tamil Nadu and Andhra Pradesh.
Dengue is a viral infection transmitted to humans through the bite of infected mosquitoes. Often called, break-bone fever, the vector-borne disease is transmitted to humans through the bites of infected female mosquitoes, primarily the Aedes aegypti mosquito.
According to the World Health Organization (WHO), about half of the world’s population is now at risk of dengue with an estimated 100–400 million infections occurring each year. Dengue is found in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas.
In recent decades, the incidence of dengue has grown dramatically around the world, with cases reported to WHO increasing from 505 430 cases in 2000 to 5.2 million in 2019. The global health agency maintains that a vast majority of cases are asymptomatic or mild and self-managed, and hence the actual numbers of dengue cases are under-reported. Many cases are also misdiagnosed as other febrile illnesses.
Climate Change and Dengue
Over the recent years, WHO revealed that several factors are identified that are associated with the increasing risk of spread of the dengue epidemic: the changing distribution of the vectors (chiefly Aedes aegypti and Aedes albopictus mosquitoes), especially in previously dengue naïve countries; the consequences of El Niño phenomena in 2023 and climate change leading to increasing temperatures and high rainfall and humidity; fragile health systems in the midst of the COVID-19 pandemic; and political and financial instabilities in countries facing complex humanitarian crises and high population movements.
According to a 2024 study published in BMJ, climate change poses significant threats to public health, with dengue representing a growing concern due to its high existing burden and sensitivity to climatic conditions.
“Climate change causes an increase in both rains and dry spells. Both of these are known to increase the risk of infections by making the climatic conditions favourable for certain vector borne infections,” Dr Anita Mathew, Director-Internal Medicine. Consultant Infectious Disease, Fortis Hospital, Mulund told Financial Express.com.
Dr. Mathew also highlighted that the prevalence of dengue has essentially increased worldwide due to international travel as well as climate change in tropical regions. Hence, post monsoon almost for a period of four to six months, there is a risk of increase in mosquito breeding sites, he said.
The UN health body highlighted that dengue is now endemic in more than 100 countries in the WHO Regions of Africa, the Americas, the Eastern Mediterranean, South-East Asia and the Western Pacific. The Americas, South-East Asia and Western Pacific regions are the most seriously affected, with Asia representing around 70% of the global disease burden.
Internationally, studies have shown the dengue’s prevalence has increased by 18% between 1995 – 2014. Estimates suggest that this number is likely to increase to 2.5 billion people at risk by 2080 based on climate estimates, with most of these risks coming from countries like India where the disease is already endemic.
Michael Dawson, Head of strategy, impact & partnerships at the Balipara Foundation told Financial Express.com that the relationship with rising temperatures and rising dengue is linear has been found to be largely linear until it crosses a 27-30 degree Celsius threshold. After this point, heat affects the mosquitoes and their populations begin to decline, he said.
“In India, the most critical change has been an increase in the period within which the dengue virus can be spread by the mosquito species Aedes aegypti and Aedes albopictus. This window has steadily been increasing since the 1950s at a rate of 1.69% and is currently 5-6 months. As temperatures warm and as the monsoons continue even into November in certain parts of the country, this period is likely to increase further. Similarly, the range of Aedes aegypti is likely to increase, reaching into parts of the Himalaya where the mosquitoes were once absent,” Dawson revealed.
Dengue is spreading to new areas in Europe, the Eastern Mediterranean and South America. The largest number of dengue cases reported was in 2023. The WHO Region of the Americas reported 4.5 million cases, with 2300 deaths. A high number of cases were reported in Asia: Bangladesh (321 000), Malaysia (111 400), Thailand (150 000), and Viet Nam (369 000).
It is noteworthy that climate change is already affecting human health in a number of different ways. Increasing heat waves and overall rising temperatures is increasing the prevalence of heat related deaths, strokes and illnesses.
“In riverine communities and areas, the prevalence and risk of water-borne diseases has been on the rise as both the number of floods as well as their extreme nature increases, disrupting access to clean water during the monsoons. Changing temperatures and weather patterns have also fundamentally transformed the incidence, range and prevalence of various vector-borne diseases – malaria and dengue being the most striking examples of this phenomenon,” Dawson told Financial Express.com.
‘Increased overall prevalence of dengue’s four serotypes’
While there is no current statistical evidence demonstrating a direct linkage between climate change and dengue complications, the increased overall prevalence of dengue’s four serotypes is possibly increasing reinfection risk – with reinfection generally having worse outcomes on the whole for individuals, Dawson highlighted.
There are four known serotypes of dengue virus—DENV-1, 2, 3, and 4—with co-circulation of all four becoming a common feature in most tropical countries in recent years.
Additionally, increased prevalence and spread of dengue at a rapid rate is likely to provide the conditions for evolutions in the virus and the possibility of the emergence of new serotypes, he told Financial Express.com.
A 2023 BJM study revealed that the transmitted to humans by infected female Aedes aegypti mosquitoes, dengue control focuses on managing mosquito populations—making the disease one of the most prominent examples of how climate change translates into mortality. Moreover, ambient temperature and rainfall are two critical factors that regulate the environmental suitability for the transmission of most mosquito-borne infectious diseases, such as Zika and dengue fever.
How the status can be improved?
Global warming, as a result of climate change, is likely to develop a more favourable living environment for the dengue fever vector, a 2023 study published in Infectious Disease Modelling journal revealed.
According to Dawson, the most important structural action the world can undertake to deal with the spread of diseases like dengue is to reduce emissions and therefore, keep the world on track for lower increases in temperature due to warming.
“At a national level, dengue outbreaks need to be controlled through better management of water, infrastructure and waste, particularly as pesticide use outgrows its usefulness with the emergence of resistant mosquito species. Stagnant water in waste sites, in construction sites and open-air reservoirs can all become breeding sites for mosquitoes. As rainwater harvesting becomes a pressing necessity in India, care needs to be taken to ensure that water that is harvested is stored appropriately to minimise the risk of it becoming a breeding ground for mosquitoes. In cities across the country, as construction expands rapidly, sites in close proximity to residential areas need to either be appropriately covered or drained or regularly cleaned to reduce stagnation and risk of becoming breeding grounds,” he told Financial Express.com.
He also highlighted that effective disposal of waste and waste management systems – especially a reduction in open air waste, whether landfills or dustbins – is also critical.
“Healthcare systems and diagnostics need to be strengthened, with awareness for early detection in areas where dengue has previously not been prevalent. Modelling of projections on the basis of rainfall and population have also been suggested as a means of developing early warning systems for outbreaks,” Dawson told Financial Express.com.
Ongoing and continued support into research for a vaccine is also vital and necessary – and any vaccine created needs to be made accessible to populations most likely to be affected by dengue, he added.