Environmental Degradation, Climate Change and Infectious Diseases: What’s the Link?

Anthropogenic changes in global landscapes and the climate increase the risk of infectious diseases by altering the environmental niche of the disease vector or animal reservoir hosts and the adaptation of some pathogens to a warming planet.

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The One Health approach, developed by the World Health Organization, recognizes the intricate interplay between human, animal and environmental health. This initiative is of utmost importance, considering the rapid spread of infectious diseases associated with environmental issues, particularly deforestation and climate change. Indeed, as the global landscape and climate are drastically changing, the emergence of zoonotic and vector-borne diseases has become a primary health concern. 

Understanding the different environmental factors for various infectious diseases is paramount to ensure health security in the foreseeable future.

Deforestation, Biodiversity and Disease Outbreaks

In 2018, researchers from Poland and France published an opinion article that highlights the potential emergence of novel coronavirus infections due to deforestation, particularly in Southeast Asia. The researchers argue that virus-bearing bat populations have settled in close proximity to human dwellings due to the loss of habitat and the compatibility of bat lifestyles with certain human living environments (for example, houses and barns for shelter). This increases the risk of disease transmission from these bats to humans, epitomizing the issue of zoonotic spillovers as a result of anthropogenic environmental degradation.

Zoonotic spillovers, also known as spillover infections, are the transmission of pathogens from animals – typically vertebrates – to humans. This transmission route comprises around 60% of emerging infectious diseases. One major example of zoonotic spillover is the emergence of human immunodeficiency viruses (HIV) from lentiviruses originating in African non-human primates, which have caused the global acquired immunodeficiency syndrome (AIDS) pandemic. Similarly, the potential transmission of Ebolaviruses from fruit bats to humans subsequently resulted in the Ebola outbreaks that peaked in Western Africa. Hence, as global land use is changing, particularly the expansion of human populations into forest areas, there is a concomitant risk of exposure of humans to pathogens or their carrier animals. In addition, deforestation could alter the abundance and distribution of pathogen-carrying animals or vectors.

The transmission of pathogens from vertebrate animals to humans could also be facilitated by vectors (typically arthropods), such as mosquitoes, ticks, flies and fleas. Examples of vector-borne diseases include dengue, malaria, Zika and chikungunya. Similar to direct zoonotic spillovers, these vector-borne diseases are affected by deforestation. One study assessing the effect of deforestation on the abundance of mosquitoes in 12 countries across five continents found that out of 87 mosquito species analyzed, over half (52.9%) were positively affected by deforestation, including those that are vectors for human pathogens (for example, Aedes or Anopheles mosquitoes). While the reason is yet to be completely understood, the availability of human-made water bodies for breeding sites and agricultural animals as alternative hosts are important factors. Another study shows the correlations between reduced forest areas – particularly due to palm oil expansion – and zoonotic or vector-borne disease outbreaks across the globe from the years 1990 to 2016 (Fig. 1), emphasizing the threat of infectious diseases as a by-product of environmental degradation.

Another aspect to consider is biodiversity. High biodiversity of vertebrate animals could reduce the chance of a vector to feed on disease-carrying animal hosts, a phenomenon called dilution effect. In other words, as a vector has more available animal hosts to feed upon, it is less likely to rely on the ones that carry human pathogens. Hence, the loss of biodiversity – an inevitable consequence of deforestation – may entail an increase in vector-borne infections within the affected areas, as has been demonstrated for Lyme disease and West Nile virus infections in the United States.

Climate Change and Spread of Pathogens

An interactive map developed by researchers at University of Hawaiʻi at Mānoa summarizes the impacts of different climate hazards on the transmission of several human pathogens. Indeed, ample evidence has shown that climatic changes could influence the spread of various infectious diseases.

Vector-borne diseases

The main factor driving climate-related spread of infectious diseases is the alteration of vector availability and infectivity. As vectors are cold-blooded animals (ectotherm), vector abundance, survival and feeding activity are expected to increase at higher temperatures. So is also the rate of pathogen development within these vectors (extrinsic incubation). Hence, as the global climate is rapidly changing – particularly global surface temperature – some pathogens would shift or expand to different geographical areas across the world. 

A study has shown that the distribution of malaria shifted to higher altitudes in Ethiopia and Colombia in warmer years. Another example is the effects of temperature, precipitation, relative humidity and wind on vector mosquitoes capable of transmitting West Nile virus, with specific climatic events associated with West Nile virus infections having been observed in Europe/Eurasia, North America and South America. 

The mosquito Aedes albopictus, a vector for chikungunya and dengue viruses, has also been established in 13 European Union/European Economic Area countries in 2023 – an increase from eight countries in 2013. A separate study predicted the expansion of dengue into countries currently considered low-risk or dengue-free, with 2.25 billion more people estimated to be at risk of dengue in the year 2080 compared to 2015. Taken together, these studies highlight the profound effects of climate change on the spread of vector-borne infectious diseases. 

Nevertheless, the relationship between climate effects and vector-borne diseases is multifactorial, hence no single factor could be used to accurately predict disease occurrence. Other factors associated with the interactions between humans, vectors and the environment should also be considered.

Zoonotic diseases

Beyond vector-borne diseases, climatic events could also induce the expansion of zoonotic diseases. In particular, extreme seasonal variations in temperature or rainfall could lead to a temporary increase in the availability of food supply to certain reservoir animals, resulting in population explosion. This has been demonstrated in a Hantavirus epidemic in southwestern United States, in which climatic conditions led to a 10-fold increase in the disease-carrying mouse populations related to food availability and reduction in the number of predators. Other Hantavirus pulmonary or cardiopulmonary outbreaks in Panama or the United States, respectively, have also been attributed to an increased exposure of humans to the rodent reservoir host as a result of atypically heavy rainfalls. In Australia, the climate change-related migration of black flying foxes, a key reservoir for Hendra virus, has been identified as a likely cause of spillover events in southern horse populations that led to human infections.

Water-borne diseases

Extreme weather events related to climate change include an increased prevalence of devastating cyclones, storm surges and flooding. These climatic events facilitate the spread of water-borne diseases by mobilizing pathogens in the environment and compromising wastewater management systems. In particular, climate-vulnerable countries are susceptible to disease outbreaks during extreme weathers, primarily due to the lack of clean water for drinking and washing, as it has been reported for cholera, infant diarrhea, pneumoniae, dengue and malaria in Bangladesh. Cholera epidemics have also been reported in East Africa in the wake of the 2015-2016 El Niño Southern Oscillation. Another study by researchers at Columbia University showed that tropical cyclones in the US between the years 1996 and 2018 were associated with an increase in Shiga toxin-producing Escherichia coli infections, Legionnaires’ disease and Cryptosporidiosis. Global leptospirosis outbreaks have also been associated with floods and heavy rainfalls.

Rising sea temperatures also provide conducive conditions for bacterial growth and expansion. A major concern is the climate change-induced spread of Vibrio cholerae, a bacterial species capable of causing cholera pandemics. Viability, growth and survival of V. cholerae as well as its interactions with the copepod animal host are increasing with the warming of brackish and marine environments. In Denmark, a study has shown the correlation between Vibrio and Shewanella infections and coastal summer temperatures. In addition, the pathogenicity of some water-borne bacteria, such as Shigella, V. cholera and Salmonella, is regulated by temperature, typically increasing at elevated temperatures (particularly at around 37C).

Solutions and Outlook

Global collaborative efforts are required to tackle the spread of infectious diseases, particularly from policy and institutional domains. An example of a global collaborative initiative is the establishment of One Health High-Level Expert Panel (OHHLEP), an advisory group of experts that aims to identify factors that link human health with those of animals and the environment. This approach will allow for the development of timely interventions of various infections. Indeed, early warnings are essential for efficient prevention and mitigation of infectious diseases. To this end, novel technologies are available for the detection and monitoring of various pathogens, including a repertoire of genomic technologies, point-of-care tests and computer-based analytical tools – among others (Fig. 2).

Open-access databases for systematic storage of health data are also an integral aspect of the One Health approach to tackling the expansion of infectious diseases. Vibrio MAP Viewer or Surveillance Atlas of Infectious Diseases established by European Centre for Disease Prevention and Control (ECDC) are examples of databases needed for devising effective strategies to control the spread of infectious diseases. 

Most importantly, as with any global health issues, the fight against infectious diseases depends on firm policies that ensure equal access to nourishment, clean water and healthcare – no one is safe, until everyone is safe.