You read it right here just the other day: "Three out of four newly described diseases in humans have a zoonotic origin". Over the past few years, we have learned this lesson the hard way: AIDS, bird flu, Ebola and, of course, COVID-19. All of these diseases are caused by pathogens that initially affected other species and managed to jump to humans and cause epidemics. This lethal jump is the result of a process known as zoonotic spillover.
As we continue to watch events unfold more than a year into the COVID-19 pandemic—a possibility that scientists warned us about—our attention inevitably turns to yet another scientific warning, on which our future unquestionably depends: climate change. For quite some time now, climate change has been more than just a prediction. In addition to anticipating its possible future consequences and figuring out how to mitigate them, we must also identify its influence on the present.
The journal Nature Medicine recently published a comment in which members of ISGlobal’s Climate and Health programme, in collaboration with a researcher from the University of São Paulo, do precisely that: explore the possible links between the COVID-19 pandemic and climate change .
The journal 'Nature Medicine' recently published a comment in which members of ISGlobal’s Climate and Health programme, in collaboration with a researcher from the University of São Paulo, do precisely that: explore the possible links between the COVID-19 pandemic and climate change
“Climate change can both facilitate zoonotic spillovers and have an effect on transmission chains”, write Xavier Rodó, Adrià San José, Karin Kirchgatter and Leonardo López . “These effects alongside human behavior and awareness should be integrated in pandemic forecasting models.”
Although it is too early to say whether climate has played a role in the spread of SARS-CoV-2, the authors argue that climate change could directly or indirectly help pathogens make the leap to other species. On the direct side, climate can facilitate a pathogen’s survival, development and spread; however, the authors suggest that this is “ unlikely” to occur on a massive scale. On the indirect side, however, “climate effects are much wider and far more complex”.
Although it is too early to say whether climate has played a role in the spread of SARS-CoV-2, the authors argue that climate change could directly or indirectly help pathogens make the leap to other species
“Climate change superimposed to a dramatic anthropogenic alteration of ecosystems is leading to a gradual substitution of species, shrinking of ecosystems and decreases in species diversity,” they argue. “Biodiversity is a protective barrier against zoonotic spillover. If we destroy it, we would also be lowering our protection”. This trend could lead to greater contact between humans and wildlife and facilitate zoonotic outbreaks.
Extreme events such as forest fires, droughts, floods, famines and migrations are another key factor, since they can place additional pressure on animals and bring them into contact with humans.
Possible Impact on Transmission Chains
The authors argue that climate can “dramatically” contribute to human-to-human transmission of infectious diseases. “While climate stability seems to be appropriate for pathogen evolutionary differentiation enhancing continued transmission, climate extremes may also promote strain selection by exposing new niches for colonisation,” they write.
The authors argue that climate can “dramatically” contribute to human-to-human transmission of infectious diseases
However, an analysis of the current effects of climate change paints a mixed picture. The impact on indicators such as absolute humidity, precipitation and average temperature varies considerably across different regions of the planet.
The same is true of the distribution and magnitude of zoonotic outbreaks since 1940. In terms of disease incidence, the largest outbreaks have occurred in tropical Africa and Southeast Asia and have not been evenly distributed across all tropical or biodiverse regions.
“There is a non-random distribution of outbreak types by latitude and continent,” explain the authors. “Thus, it is not the case that zoonotic spillover events such as respiratory virus outbreaks are seen more frequently in places with a larger number of virus-carrying species (e.g. bats) or a greater degree of ecosystem alteration (e.g. the Amazon rainforest).” Further research is needed to determine whether this is the result of selection pressure over many years.
What does seem clear, according to the authors, is that “outbreaks of respiratory zoonoses tend to occur much more often in association with the only biodiversity hotspot for mammals, etc., that is not located in the tropics but rather in contact with the subtropical and temperate climate region of the Northern Hemisphere”. In Southeastern China —the only such region subject to the climate seasonality of the Northern Hemisphere winter—outbreaks of respiratory zoonoses appear to occur with a frequency proportional to the diversity and abundance of zoonotic hosts. The authors argue that cold temperatures and low humidity in winter are necessary for a zoonotic spillover event to evolve into a respiratory pandemic , as occurred with SARS-CoV-2. “Hence the need to closely monitor these regions and the climate, carry out preventive studies, and deploy early control tools,” conclude the authors.
Many questions remain unanswered. For example, why have certain tropical countries seen “limited” spread of COVID-19? According to the authors, the high relative humidity of the tropics may hinder aerosol transmission. They therefore believe that tropical countries should focus especially on preventing transmission via fomites and droplets.
Many questions remain unanswered. For example, why have certain tropical countries seen “limited” spread of COVID-19? According to the authors, the high relative humidity of the tropics may hinder aerosol transmission
To add another level of complexity, the authors mention a mathematical model that predicted Japan’s COVID-19 peak in the winter of 2020-2021 thanks to the inclusion of temperature variables. With the same lockdown measures but without the effect of cold temperatures, “ Japan would not likely have experienced a third wave”, the authors note.
Science is still a long way from closing the case on this extraordinarily complex issue, in which human behaviour plays a fundamental role. According to Rodó et al., understanding all these phenomena and integrating them into predictive models is essential to gaining the upper hand in this global challenge. A prerequisite for achieving this, in the authors’ view, is to bridge the “traditional discipline divide among epidemiologists and climatologists”.
Xavier Rodó, Adrià San José, Karin Kirchgatter and Leonardo López: Changing climate and the Covid-19 pandemics: more than just heads or tails. Nature Medicine. April 2021. https://doi.org/10.1038/s41591-021-01303-y.