Nipah, Chandipura are just tip of the iceberg

Jul 31, 2024 08:42 PM IST

Why are so many of the virus outbreaks zoonotic (transmitted to us from animal hosts)? It is not because animals have suddenly turned malevolent towards humans, determined to win back their violated territories through microbial weapons of mass destruction. It is because humans have created conveyor belts for viral transmission from forest-dwelling wildlife to free-living or captive-bred animal populations and then on to humans. Extensive deforestation, large-scale livestock breeding for meat and dairy consumption, and export of exotic animals and pets, coupled with extensive human travel within and between countries, create opportunities for viruses to leap into vulnerable human bodies anywhere in the world. Their ability, especially notable among respiratory viruses, to speedily mutate means that records of infectivity and immune escape will keep getting broken.

The climate crisis is now accelerating the process by which viral zoonoses spread across the world. With rising temperatures and depleting freshwater supplies, animals and humans are migrating to lands outside their usual habitat. This creates scenarios of unfamiliar encounters between species that hitherto were strangers to each other. This mingling opens up opportunities for virus spillover to new animal hosts that may offer shelter or suffer serious illness because of these pathogens. Either way, the chain of transmission extends further to humans. As science writer Ed Yong said recently in the pages of The Atlantic, we have created the age of “Pandemicene” as we transitioned from the Holocene to the Anthropocene.

A team of scientists led by Colin Carlson, a global change biologist at Georgetown University, recently profiled climate crisis-propelled flight paths of potential virus spillovers across species as we journey into the future. Their seminal publication, in Nature, warns us that at least 10,000 species of viruses have the ability to infect humans. At present, the majority of them are circulating only amongst wild animals. Disruption of natural ecological barriers opens many gates for their transmission to other species, including humans. Moreover, the climate crisis is leading to animal and human migration on a large scale, creating unforeseen encounters between many species across multiple geographies. Viruses clearly love the emergence of such novel social platforms connecting animal species that were strangers to each other till the climate crisis drove them close.

Carlson’s group created a phylogeographical model of interactions between displaced mammals and viruses in different climate crisis and land-use scenarios. They project a geographical range shift for 3,139 mammalian species by 2070 and predict that “species will aggregate in new combinations at high elevations, biodiversity hotspots and in areas of high human population density in Asia and Africa”. They forecast that this will result in cross-species transfer of viruses an estimated 4,000 times. Ebola, which was previously confined to West Africa, is likely to become pan-African. Similarly, South Asia will see zoonotic viruses moving well beyond their previously defined zones.

The study predicts that, even under optimistic scenarios for the climate crisis, there will be around 300,000 first encounters between species that never interacted before, leading to about 15,000 spillovers during which viruses will gain entry into new mammalian hosts that were naive to them. Carlson and colleagues caution us that these ecological transitions are already underway, even before we have reached or breached the barrier of 2°C rise in global temperatures that the Paris Agreement set for 2100. Carlson and colleagues nicknamed their research The Iceberg Study, indicating that the present surge of zoonotic infections is a mere fraction of the dangers that the world will face in the future from species jumping viruses.

Among mammalian species, bats (flying mammals) are bandmasters of the zoonotic virus parade. They will account for the majority of virus dispersal pathways that lead to spillovers. SARS CoV-2 was first isolated from bats living in caves around Wuhan. The long-distance flight capacity of bats enables them to respond quickly to the climate crisis and makes them an efficient courier service for virus transport. Their feeding habits result in fruits being contaminated by their secretions, as in the case of the Nipah virus. The highly diverse bat populations of Southeast Asia will remain a major source of pandemic threats.

Carlson and colleagues mapped out threats related to the migration of terrestrial mammals. But, spillovers of viruses and other microbes occur in birds and aquatic species, too. Birds harbour and transport several species of influenza viruses. Patterns of seasonal avian migration are changing as familiar nesting grounds are less welcoming. Insects, which feed on the blood of mammals, also move to new habitats to seek suitable levels of temperature and humidity. The climate crisis is stirring up all of those cauldrons.

The Iceberg Study communicates the urgency of putting the brakes on the unfolding of the climate crisis. Simultaneously, we must strengthen One Health microbial surveillance systems, linking information across multiple species and multiple locations. We should use Artificial Intelligence to predict potential mammalian and avian migration pathways and identify probable microbial spillover events. We must prepare our health systems to ably anticipate, abort and attenuate new pandemic threats. Having sighted the iceberg, we need to steer around it.

K Srinath Reddy is distinguished professor of public health and past president, Public Health Foundation of India.The views expressed are personal