Changing climatic conditions, particularly temperature and moisture variations following events such as extreme rainfall in some places and drought in others, will lead to a surge in the spread of vector-borne and infectious diseases across India, say scientists.
As concerns mount over the recent increase in respiratory viral infections, including H2N3, adenoviruses and swine flu, in many parts of India, the scientists said it might be too early to attribute it to climate change. But is definitely plausible.
The prospect of climate change leading to an increased burden with the spread of diseases such as dengue, chikungunya and malaria looms large.
According to public health expert Poornima Prabhakaran, steadily rising temperatures affect the pattern of transmission of disease agents like viruses as well their vectors through a number of pathways.
“These include changes in the incubation period, the transmission potential and the duration of transmission - all of which can impact the trends of diseases," Prabhakaran, director, Centre for Environmental Health at Public Health Foundation of India (PHFI), told PTI.
The incubation period is the time between exposure to a pathogenic organism and when symptoms and signs are first apparent.
Changing climatic conditions, Prabhakaran noted, also become more favourable for the spread and disease transmission potential of viruses and their vectors.
"Hot and humid conditions can both impact the disease transmission pathways, frequency of disease occurrence and severity of disease," she explained.
Ecologist Abi T Vanak added that changes in climate will also result in the shift of habitat for species, thereby introducing new vectors to some areas, or making some species more susceptible to new viruses that may have the potential to transmit to humans.
“For example, extreme rainfall and flooding in the drier parts of the country can result in outbreaks of diseases that are typically associated with the wetter parts," Vanak, interim director, Centre for Policy Design, Ashoka Trust for Research in Ecology and the Environment (ATREE), Bangalore, told PTI.
"This is applicable to both water-borne diseases such as cholera and dysentery, as well as vector-borne diseases such as malaria, dengue, and chikungunya," he explained.
Extreme weather events such as heatwaves can also cause high stress of animals, making them more vulnerable to disease prevalence and outbreaks of potentially zoonotic diseases, he said.
Prabhakaran’s team is involved in a collaborative research effort that aims to demonstrate the links between changing climate conditions and patterns of vector-borne diseases like dengue and malaria.
The CHARISMa project aims to build a dashboard of climate-health information services that may aid city-level officials in decision-making for timely and effective interventions with a focus on hotspots.
What could be the way out?
Modelling future scenarios using state-of-the-art techniques that allow predictive future disease patterns or hotspots can be a useful tool to aid decision-makers in planning suitable and timely interventions, researchers have said.
A study published last year in the journal Nature predicted that climate change will vastly increase the risk of new viruses infecting humans.
There are currently at least 10,000 viruses “circulating silently” among wild mammals and climate change may trigger them to cross over into humans, it concluded. The finding holds particularly true for countries such as India, Indonesia, China and the Philippines, and some African regions that have been hotspots for deadly diseases spread from animals to humans over the last several decades, including flu, SARS, HIV, Ebola and COVID-19, the researchers said.
Global warming is linked to the risk of new emerging viruses, agreed Debapriyo Chakraborty, postdoctoral researcher, unit on infectious diseases and vectors-ecology, genetics, evolution and control (MIVEGEC) Research Institute for Development (IRD), Montpellier, France.
"India, as part of the global south, is thought to see a rise in certain vector-borne viral diseases, for example, dengue, following global warming," Chakraborty told PTI.
“It's also speculated that those viruses are now spreading to newer places such as mountains, which used to be colder for mosquitoes to breed just a few decades ago," he added.
Global warming, Chakraborty believes, will also lead to increased floods, which may trigger important water-borne viral diseases such as Hepatitis A and Norovirus, a very contagious virus that causes vomiting and diarrhoea.
"Global warming is also predicted to increase the spreading of emerging viruses by causing increased movement of environmental refugees. These are some of the risk drivers,” he said.
There are several perspective research papers on the risks between climate change and disease risk in India. However, much of the primary literature with robust scientific studies is still from the global north.
Chakraborty said the rise in cases of respiratory viruses in India to climate change is possible but requires scientific research to establish a clear link.
“For instance, we know that the traditional emergence of bird flu viruses during winter months is linked to winter migratory ducks. It is now speculated that climate change is disrupting both their behaviour and migratory routes causing emergence during warmer months too,” he said.
Many respiratory viruses are of wildlife origin and climate change can impact the emergence of new viruses by changing the ecology and behaviour of those wild animals.
“Also, human behavioural and demographic change (e.g., increased use of AC, changed crop cycle, mass migration) to climate change can change the epidemiology of viruses,” Chakraborty said.
There is another worry that climate change poses — the rise in the frequency of otherwise rare diseases spreading to new areas and the possible emergence of new hotspots for certain existing diseases like scrub typhus and leptospirosis.
"There is also likely to be high inter-year variability in the emergence of such diseases, thereby making preventive measures and preparedness difficult because the infrastructure and training needs for such outcomes are likely to be spatially and temporally highly variable year on year,” Chakraborty added.