Climate change and the emergence and exacerbation of infectious diseases: A review

Abstract

Experts expressed severe concerns over the possibility of increasing burden of infectious diseases as the planet’s climate began to change years ago. There have been increased rates of climate-related catastrophes and as global temperatures rise, emergence of certain viruses has become a serious concern. Vectors are susceptible to changing temperatures as they exhibit innate responses to thermal stress to increase survivability. Climate change impacts virus reservoirs, increasing transmission rates of vectors. Vector-borne diseases have already witnessed increasing numbers compared to before. Certain non-endemic areas are encountering their first-ever infectious disease cases due to increasing temperatures. Tick-borne diseases are undergoing transformations provoking a heightened prevalence. Food-borne illnesses are expected to increase owing to warmer temperatures. It is important to recognize that climate change has a multivariable impact on the transmission of viruses. With climate change comes the potential of increasing interspecies interactions promoting jumps. These factors must be considered, and an informed strategy must be formulated. Adaptation and mitigation strategies are required to curb these diseases from spreading. Despite significant evidence that climate change affects infectious diseases, gaps in research exist. We conducted this review to identify the potential role climate change plays in the emergence of new viruses.

Key Words: Climate change; Infectious disease; Emergence; Catastrophe; Vector-borne disease

Core Tip: Changing core temperatures and increased incidence of climate change catastrophes have led to certain infectious disease outbreaks across the globe. As the climate continues to change and leads to the destruction of local biodiversity, the consequent spread of vectors is predicted to lead to further escalation in vector-borne diseases. The global community has also expressed significant concern regarding the spread of known lethal pathogens such as malaria and the West Nile viruses. Development and implementation of National Health Adaptation Plans is recommended to predict and tackle these emerging threats effectively.

INTRODUCTION

Climate change is one of the most predictable and detrimental global hazards with significant long-term consequences on human health and well-being. Health impacts are visible on every continent as temperatures increase, sea levels rise, and a greater incidence of flooding and drought is observed[1,2]. These changes are significant as they threaten individual health crises and present a prominent threat to the public health sector. While a myriad of adverse health complaints, such as respiratory diseases, malnutrition, heat-related morbidity, and mortality, are escalated due to climate change, the emergence and dissemination of infectious diseases is of particular concern[2].

The relationship between climate change and infectious diseases is intricate, requiring views from multiple standpoints. As the climate warms and biodiversity is lost, the spread of vectors and animal species contributes to the emergence of diseases[3]. While the direct effects of the changing climate on circulation and incidence of infectious diseases are evident, one needs to recognize the indirect effects. Increased flooding in certain areas of the world prompts climate-driven migration into overcrowded shelters, increasing the burden of communicable diseases[4]. Similarly, harsher climates induce food and water insecurity, contributing to malnutrition and predisposing individuals to infectious diseases. Keeping in mind recent global health crises, especially the coronavirus disease 2019 (COVID-19) pandemic, it is imperative to have a broader understanding of the emergence of infectious diseases influenced by climate change worldwide.

This review aims to explore climate change's role in surfacing of infectious diseases through direct effects. By exploring this multifaceted relationship between climate change and infectious disease, this review aims to identify significant trends, highlight the ramifications, provide adaptation and mitigation strategies, and analyze the existing gaps. Through this, effective policy change decisions can be taken to prevent public health crises related to climate change.

IMPACT OF CLIMATE CHANGE ON INFECTIOUS DISEASES

Vector-borne disease

Diseases transmitted through infected arthropod species, such as mosquitoes, sandflies, and ticks, are classified as vector-borne diseases. Climate readily affects the biological traits of both the vector and pathogen. Temperature variations can reshape the emergence of zoonotic diseases through changes in the number of vectors, transmission cycles, and the contact between species[5]. Water availability is a determinant in oviposition patterns of vectors. As precipitation increases, vectors are more likely to find suitable habitats and mature faster with increased breeding.

Mosquito-borne disease

Dengue: Dengue, transmitted by Aedes mosquitoes, is among the most important vector-borne diseases worldwide with a high disease burden. Increasing temperatures exhibit an almost exponential pattern on the species’ physiological traits, such as biting and development rates for vectors up to a certain level before tapering off. Similarly, increased rainfall facilitates the development of mosquito breeding sites up to a certain threshold, after which flooding demolishes them[6]. The primary vectors for dengue Aedes aegyptii and Ae. Albopictus has increased in transmission by 10% and 15% since 1950 due to greater climate suitability[7]. Increased precipitation and temperatures due to climate change are projected to intensify dengue by 2050 in various parts of the world, including western Africa, the southeastern United States, and inland areas of Australia. These favorable climate conditions for the vector-borne virus will also prolong the transmission period by four months, specifically in the Eastern Mediterranean and Western Pacific regions[6,8].

Moreover, long-term weather and population estimations predict 50%-60% of the population to be at threat of dengue virus by 2085 compared to 35% of the population at risk if climate change did not ensue[9]. These projections are alarming as they emphasize the ramifications of unmitigated climate change on disease burden. Moreover, it highlights how dengue fever may not be limited to specific parts of the world but become a global health issue if necessary actions are not taken to minimize climate change.

Malaria: Malaria is a life-threatening disease caused by the Plasmodium parasites transmitted by the female Anopheles mosquitoes. Although the past five years have witnessed a decrease in the incidence of this disease owing to a range of human interventions, climate change can still cause increased transmission[6]. Spread of malaria is affected by ecological factors such as temperatures, precipitation, and topography[10]. As a result, equatorial regions with lower altitudes, increased rainfall, warmer temperatures, and higher humidity are favorable for the vector’s reproduction and longevity. Studies demonstrate that based on the projected climate change scenarios of longer monsoon periods and increased temperatures in tropical areas of Africa, the Anopheles vector is expected to widen its distribution area[8]. Similarly, geographical expansion of the virus is predicted in various other parts of the world, such as in Southern Europe, South Africa, China, and the highland areas of Columbia and Ethiopia[8,9]. These predicted projections underscore the global impact of climate change on public health and serve as a reminder to adopt enhanced policies for vector control and slowing down climate change to mitigate the risks of infectious diseases.

West Nile virus: West Nile virus (WNV), primarily transmitted by the Culex species of mosquitoes, can cause serious health infections ranging from mild febrile illness to severe meningitis or encephalitis. The principal hosts for these mosquitoes are wild migratory birds, but there have been cases of human-to-human transmission (through blood or transplacental transfusions)[10]. Temperature appears to be the predominant factor in WNV transmission. Optimum temperatures regulate vector growth rate, mosquito survival, and external incubation time period[11]. As temperatures increase (up to a certain threshold), the virus develops within vectors, and their transmission becomes increasingly efficient. These findings are particularly important as vectors can circulate WNV to further geographical regions. Germany, previously considered a non-endemic region for WNV, witnessed its first appearance of the vector-borne virus in 2018, which was the country’s warmest year[12]. As a result, human infections have been recorded in all the following years demonstrating constant dissemination in the transformed environment of Central-East Germany[13]. Furthermore, in areas with previously established presence of the virus, studies demonstrate an increasing likelihood of WNV infections by 2025 based on climate change projections[14]. This phenomenon not only highlights the potential role climate change plays in expanding suitable habitats for vectors to grow and transmit but also the strengthening of the virus in already existing territorial areas. As a result, regions already susceptible to the disease are at risk of increasing their disease burden.

Tick-borne

Lyme disease is the most common vector-borne disease in North America and Europe. It is caused by the Borrelia burgdorferi spherocyte bacteria transmitted mainly by Ixodes pacificus and I. scapularis ticks[15]. Like mosquitoes, ticks are affected by weather conditions, and the changing climate can be responsible for the frequency and severity of Lyme disease. Elevated temperatures affect egg development, population density, growth cycle, and ticks' dispersal. Studies demonstrate that temperature is the most important factor of tick colony establishment in Canada, considering the increase in disease incidence in the past few years and a predicted increase of 213% of suitable habitat by 2080s[9,16]. Presuming a 2°C increase in the average temperature, a 20% increase in disease prevalence is estimated in the coming decades. Increased suitable habitat directly influences the prevalence of the disease as enhanced environmental suitability encourages tick breeding, consequently increasing the tick population and the likelihood of disease transmission. In the United States, a 2-4°C anticipatory increase in the average temperature is speculated to cause a 20% increase in Lyme disease cases in the coming decades[17].

Similarly, a 10% increase in infection susceptibility at the end of the century is predicted for Slovenia, a country that is already vulnerable to Lyme disease[17]. These alarming projections raise concerns 1about the pervasive influence of climate change on disease proliferation. Without proper mitigation strategies to minimize climate change, increasing suitable habitats and disease prevalence run the risk of aggravating public health catastrophes.

Food-borne diseases

Food-borne diseases are those transmitted by eating food infected with pathogens or toxins. The risk of foodborne illness can be directly modified through high temperatures, causing amplification of replication cycles and growth, survival, and easy transfer of the pathogens. Drastic heat and precipitation can influence pathogens, worsening the risk of gastrointestinal and diarrheal diseases. The likelihood of mean weekly temperatures is directly associated with increased susceptibility to campylobacteriosis in the European Union[9]. Climate change projections indicate an increase of 6000 cases of Campylobacter per year in Norway, Sweden, Denmark, and Finland by the end of the century, translating to a 200% rise[18]. Massachusetts witnessed an increase in the incidence of the disease, with a lag of 2-14 days following a climax in annual temperature. Similarly, Canada detected a growth in cases of Campylobacter (along with Salmonella and pathogenic Escherichia coli) by 2.2% for every degree of increase in temperature[19].

Salmonella is another food-borne pathogen causing global outbreaks. Increase in temperatures directly influences Salmonella’s reproduction rates as an upsurge in ambient temperature correlates with incidence spikes. In Maryland, the likelihood of infection was related to extreme heat events between 2002-2012[20]. According to studies, a 5.6% and 8.8% increase in the incidence of cases was anticipated with a 1°C increase in mean weekly minimum and maximum temperatures, respectively[21].

Air-borne diseases

Meningococcal meningitis is an infection of meninges, caused by the bacteria Neisseria meningitidis, that causes high death rates in developing countries. It is believed that increased concentration of dust, high winds, elevated temperatures, and low humidity may cause damage to nasopharyngeal mucosa leading to increased susceptibility to meningitis[22].

COVID-19 cases and temperature have also shown a positive correlation[23]. It is possible that extreme heat forces people indoors which can increase the risk of virus transmission.

IMPLICATED MECHANISMS AND TRENDS IN INFECTIOUS DISEASES AS INFLUENCED BY CLIMATE CHANGE

A detailed review published in Nature expanded upon how often, and through which mechanisms, climate change aggravates infectious diseases. It found that more than 50% of pathogenic diseases have, at some point, been exacerbated directly by climate change with most cases being impacted by warming, precipitation, and flooding leading to the aggravation in vector-borne, waterborne and airborne diseases, respectively[24].

Some pragmatic mechanisms that have been identified include increase in proximity of pathogens and human beings through these changes in climate. Wildlife migration due to changing temperatures and habitability of regions[25] coupled with directly changing human societal behavior, such as being involved in more recreational activities involving water bodies and the outdoors in general, are two common mechanisms identified. Additionally, specific changes such as decreased salination levels of certain water bodies, increased stagnation of water, changing season length prolonging pathogen and vector lifecycles are common mechanisms which have increased disease transmissibility and strengthened pathogen adaptability and resilience[26,27]. Interestingly, in a detailed statistical analysis, temperature itself was found to positively correlate with the number of patients with infectious diseases[28].

In addition to this, the World Health Organization, more than a decade ago, found that climate change will cause an additional burden of certain diseases, such as diarrheal disease, directly due to the impact of climate change. It also suggested increased rates of malaria by up to 5% due to temperature surges[29].

The Center for Disease Control reports that infectious diseases have been on an upward trajectory. For example, cases of tick-borne diseases have increased from 10000 cases per year to 60000 cases per year from 2001 to 2019 in the United States[30]. Furthermore, currently it is assumed that 25% of the global population consumes fecally contaminated water. As droughts worsen due to increased temperatures in certain parts of the world and access to clean drinking water is further restricted, these rates will likely rise in certain vulnerable regions leading to worsening of water-borne diseases in these geographies[31]. Certain viruses such as Zika and Chikungunya have also been demonstrating increased geographical and seasonal range[32,33].

ADAPTATION AND MITIGATION STRATEGIES

Adaption and mitigation strategies are crucial in diminishing climate change's ramifications on infectious diseases. Adaptation strategies focus on tackling climate change[33]. Key strategies include modeling climate resilient health systems that include a (1) Well-informed health workforce equipped with the necessary mechanisms to assist climate resilience; (2) Health information systems that facilitate skillful handling of health risks arising from extreme events caused by climate change; (3) Efficient service distribution including necessary foundations in cases of emergencies; and (4) Sufficient financing[34] and improving education[35]. However, in order to build these climate-resilient health systems, comprehensive knowledge about current consequences and expected risks is required in the form of a Health National Adaptation Plan (HNAP). Developing an HNAP helps certify that the consequences of climate change on human health are taken into account from community to national levels[36]. An important component of establishing a HNAP is administering vulnerability and adaptation assessments.

Vulnerability and adaptation assessments should inspect the capabilities of health systems and services to resist extreme events caused by climate change. Stress tests can provide scenario-based evaluations to complement these assessments through recognition of circumstances under which health systems would struggle to manage climate-dependent health outcomes[37,38]. A summary of these key adaptation strategies is illustrated below in Figure 1.

Figure 1  Health system adaptation to climate change.

Adaptation strategies go in junction with mitigation measures that are also important in controlling the ramifications of climate change on health systems. Mitigation denotes to pursuits that curb greenhouse gas emissions or remove them from the atmosphere[33]. Afforestation and tree-planting programs, intensifying energy effectiveness in operations, and greening vehicle fleets are all helpful mitigation strategies for reducing the impact of climate change[33,37]. A multidisciplinary collaboration is required between multiple organizations, including healthcare workers, environmental scientists, researchers, policy-makers, and government bodies, to effectively address the complex challenges climate change poses on public health.

GAPS IN RESEARCH AND FUTURE DIRECTIONS

Despite the strong relationship between climate change and infectious diseases, significant limitations persist in research. While the effects of climate change and infectious diseases are commonly studied in developed countries, underdeveloped countries, which often face stronger consequences of increased disease burden, need to be considered. There is a growing need to study the intricate interlinkage between other climate variables and the different vectors and pathogens in relation to their effect on human health. Longitudinal studies are required to better understand climate change's sustained impact on human health. Additionally, research is particularly lacking in evaluating the effectiveness of adaption and mitigation strategies that can help alleviate the burden of diseases greatly impacted by climate change. Current studies have focused on trends and have attempted to predict how the burden of infectious disease will escalate in the future owing to climate change. However, very few studies the intricate impact and evolutionary force exerted on pathogens to adapt. Laboratory studies evaluating how these pressure force pathogens to adapt could be very useful. Furthermore, evaluating pragmatic approaches such as increased access to antibiotics and antiviral medications, role of socio-economic status, and precisely evaluating how a shift in human behavior makes us vulnerable to pathogens is also very important. By incorporating these limitations, evidence-based interventions can be integrated to curb the effects climate change potentially has on infectious diseases.

CONCLUSION

Infectious diseases related to climate change can be prevented. This review highlights sufficient evidence to emphasize climate change's role in exacerbating and developing infectious diseases, particularly vector-borne and food-borne, in regions where they did not exist previously. Without proper adaptations and mitigation strategies, the projections provided are likely to worsen over time, causing frequent worldwide pandemics.