Climate Change, Animal Movements, And Virus Transmissions

Climate change has many consequences, most of which are negative, and many of which are already happening. These include increased frequency and severity of storms, heat waves, and droughts, as well as the extinction of some species. 

Due to global warming and land use change (such as deforestation or urbanization), some habitats are degraded or destroyed. As a result, the animals who live there are forced to move elsewhere or expand their range to survive, up to several hundred kilometers away. However, these animals don’t move alone: they bring with them their parasites and pathogens that were previously restricted to limited geographic areas. Thus, if climate change and land degradation cause massive territory shifts, many wild animals will share the same habitat for the first time. The risk of viral transmission between species is therefore likely to increase. 

This lesser-known consequence of climate and land use change poses a public health problem by increasing the risk of zoonoses — that is, diseases transmitted from animals to humans. But it also poses a problem for the protection of biodiversity, by increasing diseases transmitted between wild species. This could further endanger them, leading to the suffering and death of many animals. 

In this study, the authors simulated the movement of animal populations to new habitats under different global circumstances. They wanted to determine which species will encounter each other for the first time, and thus estimate the risk of viral transmission (which is notably increased by the phylogenetic similarity between species). They explored several models taking into account 3,139 mammal species. They considered different scenarios related to climate change (below 2°C or above 4°C) and land use change (sustainable use or rapid deterioration) by 2070. These models accounted for variables such as population growth, greenhouse gas emissions, and forest cover, among other things. 

According to their models, and regardless of the scenario, 300,000 “first encounters” between species that don’t normally share a territory could occur due to shifting animal populations. This amounts to twice as many encounters as today. For the scenario with global warming below 2°C and sustainable land use, these first encounters would result in at least 15,000 viral transmissions. 

However, for this same scenario, when the migration limitations due to animals’ biological characteristics were taken into account, the number of first encounters and transmissions decreased by 61% and 70%, respectively. Among all mammals studied, bats were responsible for 90% of first encounters. The reason is that they are able to fly very long distances and thus disperse much more than other flightless species.

All regions of the world are expected to be affected, but some more than others, such as the high-elevation areas of Africa and Asia with rich ecosystems and biodiversity. In addition, areas with high human density, such as croplands, are expected to be particularly impacted.

Surprisingly, the models also found that a moderate climate increase of less than 2°C by 2070 could create more wild animal encounters and disease transmissions than a drastic increase of more than 4°C. The authors explain that animal species would be more successful at moving with the temperature change if it’s smaller. Finally, they note that at least since 2011, many projected encounters and transmissions may have already taken place. 

Although their models showed that even reduced global warming is likely to become an important factor in viral transmission, the authors emphasized the importance of fighting against rising temperatures. This is because the consequences of an increase of more than 2°C would be much worse in terms of biodiversity loss, human migration, and other global impacts. 

Unfortunately, as the results suggest, it may be too late to prevent wild animals from crossing paths and spreading diseases. As a result, advocates should push experts to strengthen disease surveillance, both for interspecies and animal-to-human transmissions. In addition, governments need to improve public health services in preparation for new diseases, especially in regions that will be most affected. Finally, experts must continue to develop and share data on this issue so that they can develop proactive responses to future zoonotic diseases.