A warming world can widen reach and accelerate evolution of the virus
In less than a month, Kanpur in Uttar Pradesh recorded 123 cases of Zika infection, according to health officials. The state recorded its first case on October 23, 2021, around four months after outbreaks in Kerala and Maharashtra.
The recent surge in cases, the biggest since the outbreak in 2016, brings back focus on the role of climate change in the spread of the infection.
In the years that immediately followed the first case of Zika infection in 1952, the disease was endemic to tropical and subtropical regions. The global rise in surface temperature due to climate change pushed the virus into colder reaches of the country.
The vectors of the virus — Aedes aegypti and Aedes albopictus mosquitoes — are ectotherms. This means the body temperature is regulated by their habitat — the warmer the surroundings, the warmer it is within the host body.
The suitable range for the transmission of the Zika virus is 23.9-34 degrees Celsius, according to a study that tried to map the imminent threat from the infection due to thermal variations.
In a worst-case global warming scenario, where greenhouse gas (GHG) emissions are not reduced, over 1.3 billion new people will be exposed to Zika by 2050, the study showed. Of this, 1.17 billion will be outside Latin America and Caribbean, which saw the worst outbreak in 2015-16.
As many as 737 million people worldwide, mostly in southern and eastern Asia and sub-Saharan Africa, will be in regions suitable for year-round Zika transmission.
Moderate-case and worst-case scenario for 2050
The number of months suitable for Zika transmission in 2050 if climate change is moderate (a) and if it is extreme (b). Source: Global Change Biology
Even in a more moderate climate change scenario, the number of people vulnerable to Zika infection will go up by 2.50 billion, the paper showed.
The study is based on two Representative Concentration Pathway (RCP) outcomes designed by the United Nations Intergovernmental Panel on Climate Change of reducing GHG emissions in varying degrees: RCP8.5, the worst-case or ‘business-as-usual’ scenario with no significant emission reductions, and RCP4.5, where there is moderate mitigation of climate change.
The world is not on track to keep warming within 1.5°C over pre-industrial levels, and global temperature rise can cross 2.4°C by the end of this century despite pledges made at the 26th Conference of Parties (CoP26) to the United Nations Framework Convention on Climate Change.
The higher global temperatures can accelerate evolution of the pathogen and vector, apart from widening the range of transmission, according to a paper in the journal Nature. “Such evolution can play a significant role in vector-borne disease emergence, re-emergence and spread (for example, through pathogens evolving resistance to treatment or vectors evolving resistance to pesticides),” the scientists wrote in the paper.
Aedes mosquitoes breed in small pools of water. Thus, heavy and frequent rainfall due to climate change can result in higher transmission rates.
But the relationship between rainfall and Zika infection isn’t as simple. The virus can also flourish during droughts in water storage containers — a correlation seen during the outbreak in Latin America.
Precipitation patterns upset by climate change thus fuels the spread of the Zika virus previously untouched by the infection.
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