Explainer: What Is 'Viral Spillover Risk,' And How Can It Lead To New Pandemics

The effects of climate change can be seen in a variety of environments, ranging from changes in crop yields due to unreliable weather conditions to species extinction. Another effect, according to new research, could be an increased risk of "viral spillover" in some regions, which could lead to new pandemics in the coming years.

Climate change may shift the range of certain viral vectors and reservoirs northward, and the High Arctic zone may become a breeding ground for emerging pandemics.

What is viral spillover?

Viruses are among the most abundant entities on the planet, but in order to replicate, they must infect a host's cell. According to the findings, these virus/host relationships appear to be relatively stable within superkingdoms, which are large groups of organisms. However, viruses below this rank may infect a new host from a reservoir host (where it usually resides) by being able to transmit sustainably in a novel host - a process known as 'viral spillover.'

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A spillover event caused the COVID-19 causative virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), to spread from bats to humans.

Spillover events occur when a virus overcomes several obstacles to become viable in another species. The majority of spillover events follow the same pattern. To avoid destroying the species' viability, the virus should not be overly effective in its primary host species, which also serves as a reservoir for the virus.

Close contact between the primary and secondary host species is required for transmission. The virus must also overcome the barriers that would normally prevent spillover events, such as circumventing the inherent incompatibility between the virus and its new host and overcoming the new host's immune response.

What are the chances of a spillover?

Spillover events are extremely rare, but their likelihood varies depending on the virus and host. This includes how frequently the secondary host species interacts with the primary host species, population connectivity, and population density.

In general, the likelihood of a spillover event occurring can be divided into three categories: pathogen pressure, human and vector behaviour, and host characteristics. All of these phases are accompanied by significant barriers to the occurrence of spillover events, which the virus must overcome.

Pathogen pressure is the amount of pathogen exposed to the new host at any given time and location. This is determined by the pathogen's prevalence and release from the reservoir host, as well as its subsequent survival and spread in the new host.

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What does the study say?

Researchers from the University of Ottawa collected sediment and soil samples from Lake Hazen in Canada, the world's largest High Arctic lake by volume and the region's largest freshwater ecosystem, to investigate the possibility of viral spillover.

They then used DNA and RNA sequencing to reconstruct the virus composition of the lake area. They calculated the spillover risk and discovered that the likelihood of a virus spreading to a new host increases with runoff from glacier melt, which they used as a proxy for climate change. As temperatures rise, glaciers melt, making it easier for previously ice-trapped viruses and bacteria to find new hosts.

What was the result?

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While it was discovered in this study that the risk of viral spillovers increases with changes in the environment at a specific location caused by global warming, this does not guarantee that a pandemic caused by viruses will occur here. ¡°Altogether, we provided here a novel approach to assessing spillover risk¡­This is not the same as predicting spillovers or even pandemics,¡± said the authors, as cited by the Indian Express.

This is due to the presence of another critical link in the process. As long as viruses and their 'bridge vectors,' which act as hosts and aid in their spread, are not present in the environment at the same time, the likelihood of dramatic events remains low. However, this does not necessarily imply relief.

Previous examples of spillover events

The flu is one of the most well-known and common examples of a spillover event. The influenza virus originated in migratory birds, with spillover events affecting other birds, pigs, and humans. Humans, on the other hand, are now a primary reservoir host species for influenza A.

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Bats, like birds, are a relatively common source of spillover events. The Ebola virus, for example, originated in bats. Most researchers believe that the COVID-19 causative virus came from bats, though it is still unknown if there was an intermediate species. Spillover events from wild animals can be more difficult to track, both in terms of locating the original reservoir host species and comprehending the factors that influence a spillover event.

Strikingly, a September 2019 study looked at the spread of SARS-CoV from bats to humans in China. This study found no direct link between contact with bars and coronavirus in humans, but it did discover that frequent contact with domestic animals and livestock was common in communities where coronavirus was present.