Analysis of Down to Earth Magazine: ''Mosquito Borne Diseases due to Global Warming''

Table of Contents

Context

Aedes mosquitoes that are responsible for Mosquito-borne diseases are transmitting a range of diseases that are particularly becoming invasive in a rapidly warming world. They are also no longer restricted to the monsoon season.

Background

Nationwide vector surveillance published in 2021 by the Indian Council of Medical Research(ICMR), New Delhi, showed that mosquitoes in three of the 6,492 pools assessed were positive for the Zika virus. Aedes aegypti sampled during the 2018 Zika outbreak in Jaipur, Rajasthan, showed the presence of Asian lineage of the virus and not the American lineage that is linked to the pandemic.

About A aegypti

An A aegypti is native to sub-Saharan Africa, and in its native environment, it lives in tree holes and small pools of water and bites non-human primates.
It is believed that these mosquitoes first moved to nearby human habitations during droughts when the tree holes dried up.
The mosquitoes then moved out of Africa during the transatlantic slave trade.
The first case of yellow fever was reported outside Africa in Yucatan, Central America, in 1648. Similarly, A albopictus is native to tropical Southeast Asia, where it was originally a forest species that fed on wild animals.
It first spread to the islands in the Indian and Pacific Oceans and then during the 1980s extended its range across temperate regions in Europe, Africa and the Americas.

About ZIKA Virus

Since 2007, there have been multiple outbreaks of the Zika virus across the world.
ZIKA WAS first isolated from a rhesus monkey in Uganda’s Zika forest in 1947 and in humans in 1952.
Over the next five decades, only 15 cases were reported from Africa and Southeast Asia.
There was a massive spread of this zoonotic disease (infections that emerge in animals and jump to humans) in Yap, an island group in the Western Pacific, in 2007.
The virus then spread to other Pacific Islands before reaching Brazil.
It then spread rapidly to other parts of Latin America, Central America, Mexico and the Caribbean.
The virus usually causes mild fever, aches, rashes and conjunctivitis.
The Zika virus is spread by Aedes aegypti and Aedes albopictus mosquitoes. It can also be transmitted through blood transfusions, sexual contact and via exposure to urine. There is no vaccine.

Why matter of Concern?

The spread of mosquito-borne diseases such as Zika to new geographic regions and populations is a cause of worry on several grounds.
As no specific treatment is available for most such diseases, be it Zika fever, dengue or chikungunya.
Their symptoms closely resemble covid-19 and other viral infections that spread through the bite of insects. This might lead to delayed diagnosis or misdiagnosis of the disease.
Retrospective testing in Kerala found that before the pregnant woman was identified as the first case of Zika fever on July 8, some 13 of 19 staff in the hospital where she was undergoing treatment were also positive for the virus. This indicates cryptic transmission of zikv [Zika virus] in Kerala state since May 2021
Erratic weather patterns due to climate change have added an uncertain, surprise dimension to the battle against mosquitoes, particularly the species of Aedes that are the most common carrier of infections like Zika, dengue, chikungunya and haemorrhagic yellow fever.
Originally found in tropical and sub-tropical regions, Aedes mosquitoes are now present on all continents except Antarctica.
If left unchecked, rare diseases like Zika might become endemic to regions that have remained unaffected by it.
With distinct white bands on their bodies and legs, Aedes mosquitoes are originally found in tropical and subtropical parts of the world.
Though most serious diseases such as dengue, chikungunya, yellow fever and Zika are transmitted by just two species— Aedes aegypti and Aedes albopictus—these are fast emerging throughout the world as a public health threat.

Global Warming and Mosquito-Borne Diseases

While human-made conditions have made it easy for the mosquitoes to spread over larger areas, global warming has further aided in their proliferation as warm and wet environments are excellent places for mosquitoes to breed.
A modelling study published in Nature Communications on May 1, 2020, shows that the world became about 1.5 per cent more suitable per decade for the development of aegypti during 1950-2000. It predicts that the trend is likely to increase to 3.2-4.4 per cent per decade by 2050.
By 2050, 49 per cent of the world’s population will live in places where Aegypti and Albopictus are present if greenhouse gas emissions continue at the current rates.
This was explained by researchers in Nature Microbiology on March 4, 2019, using historic data on the distributions of these two mosquito species over time in Europe and the US.
Under current climate conditions and population densities, both mosquito species will continue to spread globally over the coming decades.
A aegypti is predicted to spread within its current tropical range, but also in new temperate areas in the US and China, reaching as far north as Chicago and Shanghai, respectively.
An albopictus is forecast to spread widely throughout Europe and reach large areas of France and Germany over the next 30 years. It would establish itself in the northern US and the highlands of Latin America and East Africa.
The researchers point out the importance of curbing climate change considering that after the next five to 15 years, expansion of these mosquito species will be driven by changes in climate, temperature and urbanisation that create new and favourable habitats for them to breed.
Climate change increases the vector population too, as mosquitoes are now able to breed through the year.
For instance, at present most of the predicted transmission risk of the Zika virus occurs in the tropics.
A study published in Global Change Biology on October 9, 2020, shows that if climate change remains unmitigated, as many as 1.3 billion new people could be living in areas with temperatures suitable for Zika transmission by 2050.
Most regions in South and East Asia and sub-Saharan Africa could face year-round outbreaks of Zika infection.
A mathematical model published in Frontiers in Public Health on June 12, 2019, indicates that while the global abundance of A aegypti over the last century till now increased by 9.5 per cent, it is expected to increase by 20 per cent under low carbon emission and 30 per cent under high carbon emission scenarios by the end of this century.
The breeding behaviour of the mosquitoes has also changed. Traditionally, it is believed that the mosquito species breed in clean water, but now studies suggest that they can also lay eggs in polluted water.
There is also evidence that Aedes mosquitoes can breed in brackish water.

Viruses too are changing with Climate Change

Climate change has also affected the disease-carrying capacity of mosquitoes.
A review paper published in The Lancet on October 30, 2021, assessed the influence of temperature and rainfall on the capacity of the mosquito to spread the disease. The researchers overlaid it with human population density data to estimate the reproductive number (R0; the expected number of secondary infections resulting from one infection).
Their findings show that the R0 for all arboviral diseases tracked has increased since 1950-54; in 2020, number of infections transmitted by A aegypti was 13 per cent higher than the 1950-54 baseline years and those spread by A albopictus was 7 per cent higher.

Way Forward

While curbing climate change seems to be a solution to escape the mosquitoes, it may not be as easy a task.
An aegypti is heat-tolerant and an A albopictus is heat-limited.
In a warming world, year-round transmission potential from A aegypti is likely to expand, particularly in South Asia and sub-Saharan Africa, transmission potential of A albopictus is likely to decline substantially in the tropics as the tropics eventually become too hot for it.
This suggests that while complete mitigation of climate change to a pre-industrial baseline may protect almost a billion people from arbovirus range expansions, middle-of-the-road mitigation could produce the greatest expansion in the potential for viral transmission by A albopictus.
The worrisome part of this is mitigating climate change would shift the burden of Aedes transmitted viruses from higher-income regions back onto the tropics, where the transmission would have otherwise begun to decline due to rising temperatures.