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I remember standing in my kitchen over a year ago, listening to a news podcast after a long day on my Neurology rotation as a second-year medical student. The episode shared updates on Covid-19, which in February 2020, felt like whispers of a far-off problem rather than the devastating worldwide pandemic it would soon become. I remember the podcast host and guest speculating on the origins of this new virus. Bats and wet markets were mentioned, but I was surprised when the reporters discussed other circulating theories, including a conspiracy that Covid-19 was engineered in a virology lab. These reporters were not alone in considering fantastical and xenophobic theories, and after a year of lockdowns, facemasks, and social distancing, many are still wondering how and why this international disaster could have possibly occurred.

For public health experts though, the answers to these questions have been relatively clear for months. Covid-19 is an example of an “emerging zoonotic disease,” or a new disease that originates in animals. When multiple different viruses infect the same animal, they can share and change their genetic codes, producing novel viruses that did not previously exist and occasionally enabling viruses to gain the capability to infect humans. This process is responsible for a number of recent disease outbreaks including HIV, Ebola, H1N1 (Swine Flu), and Covid-19.1,2 Each of these viruses resulted in very serious outbreaks because our immune systems have not adapted to fight “novel” zoonotic infections, making them ideal pathogens for epidemics and pandemics.

Concerningly, the number of known “zoonotic spillover” events where an animal virus infects humans has been increasing since the 1940s.1,3 While this may be in part due to technological and research advancements improving identification of new viruses, public health experts warn that the probability of zoonotic spillover will likely continue to rise, potentially increasing the frequency of future epidemics and pandemics.

Why is zoonotic spillover becoming more common? Well, climate and environmental scientists have a pretty good idea. Zoonotic spillover occurs when different animal species live in close quarters with one another and with humans. As we continue to cut down forests and use natural lands to expand our cities, farms, and industries, we encroach on natural habits, forcing animals to live in new environments with unfamiliar co-inhabitants.1,2,5,6 Extreme weather patterns and global warming related to climate change also impact the types of environments various animal populations can habitate.1,7 This disruption of ecosystems leads to increased sharing of viruses between animals that would not naturally have close contact. Large, crowded farms similarly promote the sharing of infectious agents between various animal and humans.1,3 While it is difficult to say that destruction of natural habits, global warming, and agriculture practices directly caused one specific pandemic, like Covid-19, each of these factors increases the probability that a virus will be shared amongst animals, gain the ability to infect humans, and cause severe, easily transmissible disease leading to an epidemic or pandemic.

Given the disastrous global impacts of Covid-19, preventing future pandemics should be a top priority, and we must start by targeting their root causes: unsustainable land use, global warming, and modern agriculture practices. Climate and environmental scientists are already working on solutions to address these problems, including drastically reducing greenhouse gas emissions contributing to global warming, regulating deforestation and human encroachment on natural ecosystems, and minimizing interactions between animals and humans on farms and in nature.

Instituting new policies aimed at improving our climate and environment will admittedly require widespread restructuring of our economy and industries, but the benefits of reducing future pandemics outweigh any obstacles, especially considering we have seen how a pandemic can bring the entire world economy and our daily lives to a halt like nothing else in modern history. While some may argue our pandemic mitigation efforts should only focus on early detection, surveillance, and control of novel viruses, targeting the upstream causes of zoonotic spillover—human impact on climate and environment—will help reduce future pandemics altogether, before a single human becomes infected.4, 8

So, let’s get one thing straight about Covid-19: human impact on the environment likely created the conditions under which the virus developed and will likely contribute to future pandemics if we do not take drastic measures to improve the health of our climate and planet. Only when we understand how and why pandemics occur can we take actionable steps towards preventing another one, and I for one, have lived through enough pandemics for a lifetime.

— Berman is a third-year medical student at the University of Wisconsin School of Medicine and Public Health. 

Works cited

  1. Ka-Wai Hui E. Reasons for the increase in emerging and re-emerging viral infectious diseases.Microbes Infect.2006; 8(3): 905-916.
  2. Plowright R, Parrish C, McCallum H. et al. Pathways to zoonotic spillover.Nat Rev Microbiol. 2017; 15: 502–510.
  3. Sanyaolu A, Okorie C, Mehraban N et al. Epidemiology of Zoonotic Diseases in the United States: A Comprehensive Review. JInfect Dis and Epidemiol. 2016; 2:021.Doi:10.23937/2474-3658/1510021.
  4. Madhav N, Oppenheim B, Gallivan M, et al. Pandemics: Risks, Impacts, and Mitigation. In: Jamison DT, Gelband H, Horton S, et al., editors. Disease Control Priorities: Improving Health and Reducing Poverty. 3rd edition. Washington (DC): The International Bank for Reconstruction and Development / The World Bank; 2017 Nov 27. Chapter 17.
  5. Afelt A, Frutos R, Devaux C. Bats, Coronaviruses, and Deforestation: Towards the Emergence of Novel Infectious Diseases? 2018. Frontl Microbiol. 2018; 9:702.
  6. Sokolow S, Nova N, Pepin K et al. Ecological interventions to prevent and manage zoonotic pathogen spillover.Phil. Trans. R. Soc. 2019;374(1782): 20180342.
  7. Hueffer K, Parkinson AJ, Gerlach R, Berner J. Zoonotic infections in Alaska: disease prevalence, potential impact of climate change and recommended actions for earlier disease detection, research, prevention, and control. Int J Circumpolar Health. 2013; 72.
  8. Plowright RK, Reaser JK, Locke H, et al. Land use-induced spillover: a call to action to safeguard environmental, animal, and human health. The Lancet. 2020. Doi: 10.1016/S2542-5196(21)00031-0.



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