The first case of the novel coronavirus (CoV) was reported in the Chinese city of Wuhan in late December 2019. Since then, the deadly CoV has spread rapidly throughout the world, with 107,252,265 confirmed cases, including 23,55,339 deaths reported to WHO as of February 12, 2021. The total death toll in India is reported to be 155k, with 776 deaths in Goa.
The worst affected is the US followed by Europe. As the health and human toll grows, the economic damage, due to lockdown measures, is already evident and represents the largest economic shock the world has experienced in decades.
Over 60% of emerging infectious diseases worldwide have been traced to zoonoses (an infectious disease caused by a pathogen), most of which originate in wildlife. Amongst the animal-pathogen hosts of all mammalian orders, bats carry the highest proportion of zoonotic viruses. According to published research, CoVs account for over a third of the sequenced bat virome, corresponding to an estimated more than 3000 different CoVs carried by the world’s bats.
Most coronaviruses carried by bats cannot jump into humans. Several CoVs known to infect humans are very likely originated in bats, including the three types associated with human fatalities: the Middle East respiratory syndrome (MERS) CoV and severe acute respiratory syndrome (SARS) CoV-1 and CoV-2. Spill-over of CoVs and other zoonoses to humans are closely linked to an increase in contact with pathogen-carrying wildlife, driven by the expansion and intensification of agriculture, and infrastructural development. Strains of CoV found in bats in the southern Chinese Yunnan province currently most closely resemble both SARS-CoV-1 and SARS-CoV-2, suggesting this or neighbouring regions in Myanmar and Laos as plausible places of origin of the bat-borne ancestors. These regions also comprise the native habitat of masked palm civets (Paguma larvata) and Sunda pangolins (Manis javanica), which are assumed to have acted as intermediate hosts that eventually transmitted SARS-CoV-1 and SARS-CoV-2, respectively, to humans. Captured civets and pangolins infected by these viruses were transported to wildlife markets in Guangdong and Wuhan, respectively, where the initial outbreaks in human populations occurred.
The number of CoVs present in an area is strongly correlated with local bat species. An increase in local bat population may therefore increase the probability that a CoV with potentially harmful properties for human life. Climate change, which drives the geographic distributions of species by altering the appropriateness of ecological habitats, forces species to leave some areas and move into others taking their viruses with them. This not only altered the regions where viruses are present but most likely allowed for new interactions between animals and viruses, causing more harmful viruses to be transmitted.
A study in the journal Science of the Total Environment provides the first evidence of a mechanism by which climate change could have played a direct role in the emergence of SARS-CoV-2, the virus that caused the Covid-19 pandemic. The study has revealed large-scale changes in the type of vegetation in the southern Chinese Yunnan province, and adjacent regions in Myanmar and Laos, over the last century.
Climatic changes characterized by higher atmospheric carbon dioxide levels, increased temperature, altered precipitation patterns, and decreased cloud cover, which affect the growth of plants and trees ‒ have changed natural habitats from tropical shrub-land to tropical savannah and deciduous woodland. This process created a suitable environment for many bat species found in the region that predominantly require forest type habitats, explaining the marked increase in the bat population.
The study also revealed that an additional forty bat species have moved into the southern Chinese Yunnan province in the past century, accommodating around 100 more types of bat-borne coronavirus, making it the global hotspot. The genetic data suggests that SARS-CoV-2 may have arisen in this area.
To reinforce their findings, the researchers constructed a map of the world’s vegetation as it was a century ago by using different environmental parameters. Then they used information on the vegetation requirements of the world’s bat species to create the global distribution of each species in the early 1900s. Comparing this to current distributions allowed them to see how the number of different species has changed across the globe over the last century due to climate change.
The estimated climate change-driven increase of around 40 bat species in the Yunnan province corresponds to a rise in the local number of bat-borne CoV in the order of 100 viruses, given that each bat species carries on average 2.67 CoVs. This process likely created significant opportunities for cross-species viral transmission that may have facilitated the eventual spill-over to humans.
Other biotic and abiotic factors not considered in the above study, such as hunting, invasive species, and pollution, can also play significant roles in determining bat habitat appropriateness. The estimated climate change-driven shifts in the geographical distributions of pathogen-carrying species will need to be integrated into epidemiological models to quantitatively assess the role of climate change and the contribution of specific climatic variables on viral-sharing networks.
To reduce the risk of future zoonotic spill-overs, the study underlines measures to protect natural habitats, impose strong regulations on wildlife hunting and trade, establish appropriate animal welfare standards on farms, markets, and transport vehicles, and discourage high-zoonotic-risk dietary and medicinal customs, whilst accommodating the socio-economic needs that drive current patterns.
(The writer is a scientist and researches global climate change issues)
