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The Climate Crisis: Unveiling the Nexus between Climate Change and Infectious Diseases

Edited by Ryan Jien.


Background

As defined by the United Nations (n.d.), climate change is the long-term shifts in temperature and weather patterns, either due to natural processes or human activities. According to climate scientists, humans are responsible for the acceleration of global warming over the past few decades (United Nations n.d.). In comparison to what it was before the industrial revolution, the Earth is now 1.1°C warmer, indicating that we are currently not on track to meet the Paris Agreement target at COP21 in 2015, which is to limit global temperature from exceeding 1.5°C above pre-industrial levels (United Nations n.d.; COP28 UAE 2023). Indeed, climate change impacts the world in diverse ways, ranging from the biodiversity in our nature to our health and safety. In terms of health, climate change not only contributes to problems such as heat-related illnesses and mental health issues, but also infectious diseases, (Ebi et al. 2021). There is evidence that climate change is likely associated with the new threat of infectious diseases by contributing to the emergence and re-emergence of infections, making this an alarming issue which requires collaborative research from multiple disciplines. This article aims to delve into the impact of climate change from a global health perspective in terms of infectious disease.


Linking climate change and infectious disease

Referring to the chain of infection, climate change can have direct effects on pathogens, hosts, reservoirs and vectors while indirectly affecting the epidemiological dynamics. 


Figure 1: Chain of infection. Source: Centers for Disease Control and Prevention (CDC) (2022)

In terms of infectious agents, climate change might drive pathogen evolution as pathogens adapt to the increasingly warmer weather (Baker et al. 2022). A compelling example of this evolution is the enhanced thermotolerance in fungi, which potentially contributes to the emergence of new fungal diseases (Casadevall 2023). This could happen as the increased thermotolerance will enable opportunistic pathogenic fungi such as Candida auris to jump from environmental habitats to intermediary avian hosts which act as reservoirs before their subsequent transmission to humans who are in close contact with birds (Casadevall et al. 2019). Moreover, the heightened thermotolerance in fungi may compromise the primary mammalian immune defence against pathogenic fungi – that being warm blood – thus exposing humans to a greater risk of fungal disease (Casadevall 2023).


Figure 2: Proposed scheme for the emergence of C. auris. Source: Casadevall et al. (2019)

Furthermore, global warming, a key aspect of climate change, is anticipated to accelerate the rate of development of certain pathogens, resulting in a larger pathogen population size and increased infections (Baylis and Risley 2023). An instance of this phenomenon is an upsurge in Salmonellosis, a food-borne bacterial disease which is normally associated with an increase in the ambient temperature, indicating the effect of climate on the replication rate of the bacteria (Semenza et al. 2022). Also, it is estimated that global warming will extend the warm season and possibly increase the frequency of infection of warm-associated diseases due to longer periods conducive for the development of certain pathogens within a year (Baylis and Risley 2023).


In relation to vectors, climate change might increase the transmission of vector-borne diseases due to several reasons. To begin with, the rise in ambient temperature will increase the rate of mosquito larval development, accelerating the speed of virus replication, thus causing the incidence of mosquito-borne diseases such as Zika, dengue and malaria to escalate (Baker et al. 2022, Ryan et al. 2021, Hales et al. 2002). Besides, the changing wind pattern is implicated in affecting the distribution of certain pathogens and vectors such as Culex tritaeniorhynchus mosquitoes, the main vectors of Japanese encephalitis (Baylis and Risley 2023, El-Sayed and Kamel 2020). According to El-Sayed and Kamel (2020), the wind has been found accountable for the emergence of Culex tritaeniorhynchus mosquitoes in several regions in China. Additionally, human response to climate change has also influenced the vector distribution and the pattern of vector-host interactions. As an illustration, in response to drought, human behaviour of storing water has created conducive breeding spots for Aedes albopictus, a well-known container breeder, leading to the emergence of Chikungunya virus (Baylis and Risley 2023).


Regarding the host, climate change could potentially impact the host immune response to pathogens. To illustrate, increased exposure to ultraviolet B (UV-B) radiation due to ozone depletion is implied to cause a depression in the number of Type I T-helper lymphocytes (Th1), a group of lymphocytes responsible for cell-mediated immune response and clearance of intracellular pathogens (Baylis and Risley 2023). As stated by Baylis and Risley (2023), this immunosuppression might render humans more susceptible to intracellular pathogens, namely virus, intracellular bacteria such as Mycobacterium tuberculosis, Listeria monocytogenes and Rickettsia as well as protozoa such as Toxoplasma gondii and Leishmania. These intracellular pathogens often pose significant threats to public health as they are more challenging to manage due to their ability to evade the host immune system and their locations within the host cells which are more difficult for the drugs to reach (Thakur et al. 2019, Kamaruzzaman et al. 2017).


With respect to epidemiological dynamics, climate change is likely to intensify the frequency of human-animal interactions, increasing the risk of emergence of zoonotic pathogens into human populations (Baylis and Risley 2023, Baker et al. 2022). An example of pathogen spillover associated with climate change events is the Hendra virus, a significant emerging zoonoses in Australia. Hendra virus is a biosafety level 4 organism in close relationship with Nipah virus which causes respiratory and neurologic diseases in humans with a high mortality rate (Yuen et al. 2021). As per Yuen et al. (2021), spillover of Hendra virus into horse populations which subsequently infected humans in the southern part of Australia was observed following the 100km southward migration of black flying foxes (a common natural reservoir of Hendra virus) due to a lack of food sources as a result of climate change. Apart from that, alterations in human demography, housing and movement owing to climate change might influence the pattern of international trade, local animal transportation and farm size, thereby affecting the chances of contact between an infected human or animal with a vulnerable one (Baylis and Risley 2023). 


Outlook in the new era of infectious disease

While mortality and morbidity due to infectious diseases have declined over the years, possibly attributed to the development of vaccines and improved sanitation, the new threats of infectious disease are likely arising from emerging and re-emerging infections, driven by several factors, particularly climate change (Baker et al. 2022).


To deal with the challenges in this new era of infectious disease, adoption of a One Health approach by viewing the issue with a transdisciplinary lens is advisable as climate change has amplified the chances of human-animal interactions and thus the transmission of zoonotic diseases (Hess et al. 2020). For this reason, collaboration across multiple sectors and disciplines becomes imperative to understand and address the increasingly significant interconnection between the environment, humans and animals (Grobusch and Grobusch 2022). As highlighted by Zhang et al. (2022), it is crucial to proactively employ a One Health approach as effective prevention of emerging infections instead of only applying it during the outbreak. The shift from crisis response to effective prevention holds promise in safeguarding public health by ensuring food security, particularly animal-source food, controlling the transmission of antimicrobial resistance and prioritizing environmental sanitation (Zinsstag et al. 2018).


In the domain of pandemic preparedness, the use of predictive modelling and early warning systems that incorporate seasonal climate forecasts could potentially assist in understanding the emergence and transmission of future outbreaks (Hess et al. 2020). Technologies like artificial intelligence and machine learning could serve as promising tools for pandemic response and public health security (Jordan and Mitchell 2015). Besides, an enhanced surveillance system that integrates earth observations from satellites, weather stations or drones and local environmental observations is essential in the estimation and early detection of possible outbreaks as well (Hess et al. 2020). Genomic surveillance systems which utilize progressively advanced sequencing technologies such as nanopore sequencing technologies could potentially enable rapid characterization and tracing of novel pathogen variants and provide a more detailed picture of the transmission dynamics when coupled with epidemiological information (Wang et al. 2021, Baker et al. 2022). 


Conclusion

In conclusion, the relationship between climate change and infectious disease is evident. This is a critical issue currently impacting the achievement of the United Nations Sustainable Development Goals (SDGs), specifically SDG 3 (Good Health and Well-being) and SDG 13 (Climate Action). This underscores the urgent need for effective measures to address this issue. Indeed, supranational and interdisciplinary efforts are necessary as the issue of climate change and infectious disease is a worldwide challenge not just affecting public health, but also global health and planetary health.

 

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