A hot topic at the environment–health nexus: investigating the impact of climate change on infectious diseases

Climate change — the ultimate challenge of our time COVID-19 pandemic aside, climate change is the ultimate challenge of our time. However, to date, there has been insufficient political thrust to make that much-needed climate action a reality. Climate change and infectious diseases Infectious diseases represent only one facet of the threats arising from climate change. Direct impacts from climate change include the more frequent occurrence and increased magnitude of extreme weather events, as well as changing temperatures and precipitation patterns. For climate-sensitive infectious diseases, these changes implicate a shift in geographical and temporal distribution, seasonality, and transmission intensity. Sizing up the problem Susceptibility to the deleterious effects of climate change is a net result of the interplay of not only environmental factors, but also human, societal, and economic factors, with social inequalities being a major determinant of vulnerability. The global South is already disproportionately affected by the climate crisis. The financial capacity to pursue adaptation options is also limited and unevenly distributed. Conclusions Climate change-induced mortality and morbidity from both infectious and non-infectious diseases, among other adverse scenarios, are expected to rise globally in the future. The coming decade will be crucial for using all remaining opportunities to develop and implement adequate mitigation and adaptation strategies.


Climate change, the IPCC report and COP26
COVID-19 pandemic aside, climate change is the ultimate challenge of our time. However, to date, there has been insufficient political thrust to make that much-needed climate action a reality. In light of world leaders recently convening in Glasgow for the 2021 United Nations Climate Change Conference (COP26) (COP26 2021), civil society, NGOs and scientists around the world are alerting policymakers to all that is at stake and urging them to take more action to tackle the climate crisis. In a 'call for emergency action' published in the run-up to the conference by more than 200 medical journals, health professionals stress that 'the greatest threat to global public health is the continued failure of world leaders to keep the global temperature rise below 1.5°C and to restore nature' (Atwoli et al., 2021). One of many predicted adverse effects of a global temperature rise above 1.5°C (as compared to 1990 levels) that Atwoli and colleagues (Atwoli et al., 2021) identify in their emergency call is an overall increase in infectious diseases. Specifically, climate change is projected to impact both on the endemicity and prevalence of infectious diseases (Lafferty, 2009). These changes are predicted to occur at varying extents in different localities around the globe, which is why a global perspective together with local awareness of and action according to the changes and challenges that lie ahead are warranted.
In the 'Special Report on Global Warming of 1.5°C', the Intergovernmental Panel on Climate Change (IPCC) (IPPC Report. 2021) reports that climate change will affect human health by causing 'changes in the distribution and abundance of vector-borne diseases such as dengue fever and malaria, which are projected to increase with warming of 1.5°C and further at 2°C in most regions' (Hoegh-Guldberg et al., 2019). Climate change impacts on vector and pathogen distribution but goes mostly synergistically hand in hand with other ecological challengesamongst them overcrowding; agricultural and industrial land use and excess consumption of natural resources; deforestation; the pollution of water, air and soil; altogether in the extreme resulting in ecosystem degradation and biodiversity loss (Hess et al., 2020). In the Amazon basin, as one of the extreme examples, deforestation has been shown to contribute to the spread of disease vectors (Ellwanger et al., 2020).

Climate Change and Infectious Diseases
Climate change is predicted to alter the incidence, prevalence, and distribution of infectious diseases because it drives changes in temperature and precipitation (Semenza & Paz, 2021;Panic & Ford, 2013). Direct impacts from these changes include the more frequent occurrence and increased magnitude of extreme weather events such as floods, droughts and heatwaves. For climate-sensitive infectious diseases, these changes implicate a shift in geographic and temporal distribution, seasonality, and transmission intensity (Hoegh-Gueldberg et al. 2019;Semenza & Paz, 2021).
Climate (and climate change, since the beginning of times) is one of many discreet drivers of human, animal (and plant!) diseases (Nazarov et al. 2020;Heffernan, 2013); one amongst multiple driving forces, and modulators of infectious diseases processes, in the sense of providing context rather than being a major driver on its own behind human and animal disease evolution (Heffernan, 2013). This reminds us that interventions focusing predominantly on climate change as root cause of changes in infectious diseases processes might be falling short of acknowledging the full dimension of the complexities of the interplay of both initiating and impacting forces (Heffernan, 2013). As Funari and colleagues (Funari et al., 2012) emphasize, the simultaneous influence of other causes cannot be understated. Such causes include the destruction of natural habitats, drug resistance, increasing urbanization, rising population density, health service availability, poverty, and human migration and travel (Funari et al. 2012;Heffernan, 2018), which themselves are all closely intertwined with climate change. 6

Diverse effects of climate changeexamples malaria and dengue
Across sub-Saharan Africa, for example, warmer temperatures and changing rainfall patterns are likely to increase the suitable breeding habitat for several Anopheles species, leading to expansions of the suitable geographic range for malaria in some areas, an increasing likelihood leading to predictions of malaria endemicity expanding and intensifying in tropical highlands in Africa; as well as in the Eastern Mediterranean region, and in the Americas Colón-Gonzalez et al. 2021) -with malaria and dengue only being two 'protoype' diseases in terms of the dimension of the problem; but others of course also being involved. Of interest, the multifactorial complexity of mosquito populations and associated disease dynamics waxing (and waning!) is highlighted by the sometimes forgotten fact that the emergence of dengue and its vectors in mediterranean and more northern parts of Europe would rather be a re-emergence than an emergence (Schaffner & Mathis 2014); highlighting once more that climate is only one factor amongst others.
Firstly, not all geographical expansion of vectors, pathogens and increases in infectious diseases incidence or prevalence occurring can simply be allotted to climate change. For example, the recent spread of A. stephensi from Asia into Africa highlights the complexity of vector and disease expansion which in this case is better explained by urbanisation and migration phenomena (Sinka et al., 2020) rather than climate change and global warming.
Secondly, climate change and global warming do not necessarily work unidirectionally in favour of geographical spread or increase of an infectious disease activity. Indeed, again, particularly for A. stephensi, it has been in fact shown that increasing temperatures, in warmer regions, can affect replication and survival as fact as the propensity to transmit Plasmodium falciparum rather negativelyand thus positively from a human and malaria control point of view (Murdock et al. 2016;Agyekum et al., 2021).
The secondary implications of climate change, with one example being floodings following torrential rains, as well as on the other hand increasingly depleted and polluted global freshwater resources, exacerbate existing vulnerabilities of health systems and human and animal morbidity and mortality caused by vector-borne, food-borne and water-borne infectious diseases (Funari et al., 2012;Sarkodie 2019;Ellwanger et al., 2020).

Factors governing climate changes, and how to size-up the problem
Susceptibility to deleterious effects of climate change is a result not only of the interplay of environmental factors but also governed by human, societal, and economic factors, with social inequalities being another major determinant (Semenza & Paz 2021). The global South is already disproportionately affected by the climate crisis. This is partly due to increased social vulnerability in low-income contexts where disease risk is amplified by poverty and lack of healthcare services (Heffernan, 2018). The financial capacity to pursue adaptation options is also often limited, and unevenly distributed.
How can the full dimension of the problem be sized-up as correctly as possible; which models of thinking are most appropriate, and what practical solutions are at hand? Adaptation to and reaction towards one of those aspects (magnifying infectious diseases distribution and impact) is an important part of the picture, but can that be to any avail if the root causes are not tackled altogether?
Sizing-up climate effects, for example on infectious diseases, however, remains challenging for a number of reasons. Metcalf and colleagues (Metcalf et al., 2017) point out that some infectious disease processes occur hidden from plain view; and that most infections lead to at 8 least partial immunity; possibly depleting the susceptible individuals pool, resulting in transmission dynamics alterations which can obstruct the view on the proportion of the climate signature as individual contributor to the overall problem. Modelling approaches to simulate climate-disease correlations as they occur under natural conditions remain complex and thus error-prone, as some variables might be difficult to factor-in (Metcalf et al., 2017).
Ostfeld (Ostfeld 2009) emphasizes that both extrinsic variables such as climatic effects, as well as intrinsic effects such as the build-up of immunity in host populations should be included in models. Whereas other models make the assumption that climate is the only factor influencing the distribution of infectious diseases such as falciparum malaria (Ostfeld, 2009). Given that interventions need to be financially affordable, implementation is less feasible in low-resource settings than in developed countries (Ostfeld, 2009). Rohr and Cohen (Rohr & Cohen 2020) report on a wealth of progress which has been made over the past years in the understanding how climate and climate change influence disease dynamics.
However, whilst the concept is easy to comprehend and endorse as it seems all logical, the science behind understanding how climate change impacts on infectious disease remains complex.

Mitigation and adaptation
Deliberations on how to tackle both the spread and (re)emergence of infectious diseases under climate change would be incomplete without a discussion of the critical role that climate change mitigation can and needs to play in doing damage control for the future. To date, mitigation efforts have been insufficient. If business-as-usual and the current levels of inaction continue, climate models place us on trajectories of much more than 1.5°C warming by the end of the century (Hoegh-Gueldberg et al., 2019). Intensified mitigation efforts within a very tight timeframe of a couple of years only are thus mandatory to decelerate and ultimately terminate further global warming. The discourse about climate change mitigation still needs to be steeply stepped up, in order to trigger the drastic measures required. Given the IPCC's aforementioned finding that any further increases in temperature will have negative consequences for human, animal and plant health (Hoegh-Gueldberg et al. 2019), although we should not forget that the opposite, i.e. climate change in the other direction, would have similar negative effects (Wan et al. 2019;Cui et al. 2019), it is absolutely critical that urgent action is taken to limit the global mean temperature increase to below 1.5°C. This will require carbon emission reductions clearly beyond the current trajectory, also in the health sector which accounts for 4% of the world's greenhouse gas emissions (Hess et al., 2020). Quite simply, the extent to which mitigation efforts are going to be implemented within the next decade will determine the future of the planet as we know it.
There are some concrete actions that can be undertaken to reduce, and adapt, to infectious disease and other health risks that will be exacerbated by climate change. First of all, adopting a One-Health approach will help to understand and address the interactions between the environment, human health and animal health (Ellwanger et al., 2020). Building on that, there should be increased funding for tackling climate change related health risks (Hess et al., 2020) as well as pandemic preparedness. Enhanced surveillance is another strategy; intensifying sentinel surveillance (Grobusch et al., 2021) and the development and expansion of overarching strategies linking surveillance activities better together on national and supranational levels is another one, such as recent WHO (WHO Pandemic and Epidemic Intelligence Hub, 2021) and European Union (HERA, 2021) initiatives, amongst others.

Conclusions
Climate change-induced mortality and morbidity from both infectious and non-infectious diseases, amongst other adverse scenarios, is expected to rise globally in the future.
Infectious diseases are only one facet of the threats arising from climate change. Whilst some infectious diseases will be reduced, mostly climate change effects will propagate infectious diseases. The coming decade will be crucial for using all opportunities left to develop and implement adequate mitigation-and adaptation strategies.

Contributors
LCG and MPG jointly conceived and wrote the manuscript, and approved its final version as submitted.

Funding Source
No funding received.

Declaration of Competing Interests
Both authors declare no competing interests.

Ethical Approval
Not applicable.