The recent coronavirus outbreak (i.e., nCovID-19; Fig. 1) has caused global panic, along with widespread travel bans, home quarantines and country-wide lockdowns. The World Health Organization (WHO) declared nCoVID-19 as a pandemic as of March 11th, 2020 (WHO, 2020). To tackle this global health crisis, scientists are attempting to synthesize a vaccine, while countries are trying to mitigate the number of infections by isolating infected patients and vulnerable populations. However, nCovID-19 is not the only zoonotic disease that has caused a major epidemic and pandemic over the past few decades. Famous examples include the Spanish flu, Zika virus, Severe Acute Respiratory Syndrome (SARS), human immunodeficiency viruses (HIV), Ebola, and Lyme disease. A natural question to ask is: Are the risks of zoonotic diseases rising in the Anthropocene due to human-induced climate change?
What is a zoonotic disease?
Zoonotic diseases are a class of common infectious diseases that spread from animals to humans, either by bacteria, viruses, parasites or fungi. As seen in Figure 2, there are numerous ways in which animal-circulated pathogens can be passed to humans. Airborne diseases, like influenza, are pathogens that can be transmitted by close proximity to animals. Vector-born diseases, such as malaria or Lyme disease, can be transmitted by insects, e.g. mosquitos or ticks. Food-borne diseases, such as salmonella, can be transmitted via consumption of animal products, e.g. chicken or fish. A final transmission pathway is through direct contact with animals, such as animal bites or touching of infected animals (including wildlife) e.g. to rabies, HIV, Ebola, SARS, nCovID-19 (?). Thus, paths of zoonotic diseases are multi-faceted. A list of zoonotic diseases can be found in Table 1.
It is becoming clear that humans are inducing significant changes to the global environment and the ecosystems within it (Watson et al., 2002; Boillat and Ifejika Speranza, 2019). The emergence of zoonotic diseases in the past century is partly due to the rise in human population along with increases in both travel and trade (Fig. 3). These changes have prompted the re-distribution and increased trafficking of animal species which has led to an increase in the frequencies of human and wild animal contact (Jones et al., 2013; Pecl et al., 2017; Fig. 3A and 3B). Increases to human population also bears additional consequences, such as urbanization, which can drastically change the landscape of human and wild animal interactions. In the following section, we will be discussing how the effects of anthropogenic climate change can manifest in the emergence of zoonotic diseases.
Climate change and the emergence of zoonotic diseases
The most recent Intergovernmental Panel on Climate Change (IPCC) report outlines that Earth could see global mean surface temperatures rise by up to 4 ˚C by the end of this century (RCP 8.5 scenario). Such warming may not seem so significant, but it is becoming clear that even 1 °C above pre-industrial levels implies large environmental changes: from melting ice-sheets to changes in the patterns of precipitation and drought (e.g. Wilhite, 2000; Pollard et al., 2015; IPCC AR5, 2018; Myhre et al., 2019). Gradually, scientists have been working to understand the effects of climate change on elements beyond the conventionally-defined climate system, such as vegetation, marine ecosystems or land surface (Fig. 4).
One of the potential effects of climate, which is not often advertised, is its role in the emergence or re-emergence of zoonotic diseases (Fig. 5) and over the past decade, epidemiologists and climate scientists have been exploring such links (Morse, 2001; McMichael, 2004; Harper and Armelagos, 2010; Goodwin et al., 2012; Jones et al., 2013). There has been a rise in the distances species typically migrate due to climate change (animals and human), and it is argued that this trend will continue over time (Bates, 2002; Besthorn and Meyers, 2010; Pecl et al., 2017; IPCC AR5 WGII, 2014). Changes in migratory patterns can result in modified interactions between wildlife and humans (Despommier et al., 2006; Jones et al., 2013). Furthermore, these trends are predicted to be compounded when coupled to extreme weather events that are a consequence of anthropogenic climate change (IPCC AR5 WGII Ch. 11, 2014).
Despite uncertainties in predicting the future effects of anthropogenic climate change on human and animal health, many epidemiologists agree that the changes in climate and ecotones, a transitional area between two organisms, has a crucial role to play in regulating the emergence and re-emergence of zoonotic diseases. This argument is well supported from looking back over the past few decades where: the influence of climate through temperature changes has been shown to increase malaria transmission (Paaijmans et al., 2010), changes in annual temperature and precipitation patterns have been shown to have links to the preponderance of dengue fever through the rise in Asian tiger mosquito populations (Ogden et al., 2014), reports have linked climate change to a rise in tick-born disease, such as Lyme disease (e.g. Okuthe and Buyu, 2006; Lukan et al., 2010; Tokarevich et al., 2011; Andreassen et al., 2012; Estrada-Peña et al., 2012; Jaenson et al., 2012), and finally, the Hanta virus has been tied to seasonal and inter-annual variability in climate, as well as a rise in temperature, precipitation and humidity (Klempa, 2009; Fang et al., 2010; Liu et al., 2011).
Earth has seen dramatic changes in climate over 4 billion years. The question remains: How will climate change affect humanity in the near future?
The United States department of defense has classified climate change as a threat multiplier (U.S. department of defense, 2014). Furthermore, the cost and death toll from natural disasters and pandemics are very high and can impact the global economy and increase socioeconomic divides (Fig. 5 and 6). The problem is not only that anthropogenic climate change will induce dramatic changes in the Earth’s biogeochemical cycles, but also that humans’ ability to adapt to the rapid changes that they have imposed will be incredibly costly.
Research on the relationship between climate and diseases has seen an exponential growth over the past few decades (Fig. 7), reflecting the urgency to understand the potential links between climate change and zoonotic diseases, and this trend will likely increase in the future. While climate scientists and epidemiologists face many uncertainties in understanding climate patterns and how diseases emerge and spread, links and feedbacks between these two seemingly disparate fields are emerging. It is often difficult to link a specific extreme weather event, such as a hurricane, to the effects of anthropogenic climate change. However, the use of numerical models and multi-decadal data allows us to determine with a higher degree of confidence the link between a class of extremes, such as category 4 and 5 hurricanes, and climate change. The same can be said about zoonotic diseases. We may struggle to tell whether the outbreak of nCovID-19 was accelerated by factors related to climate change or not, but by utilizing scientific tools and models along with interdisciplinary efforts, we may uncover critical links between anthropogenic climate change and zoonotic diseases. Thus, society should be mindful of the changes humans are making to the Earth system, and should prepare to address the potential effects of climate change and anthropogenic activities on the occurrence and frequency of pandemic-level events.
This post has been edited by J. Bösken, G. Messori, and C. Nehme.
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